[Federal Register Volume 74, Number 84 (Monday, May 4, 2009)]
[Proposed Rules]
[Pages 20431-20443]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E9-10162]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2007-27862; Directorate Identifier 2007-CE-036-AD]
RIN 2120-AA64


Airworthiness Directives; Thrush Aircraft, Inc. (Type Certificate 
Previously Held by Quality Aerospace, Inc. and Ayres Corporation) Model 
600 S2D and S2R (S-2R) Series Airplanes

AGENCY: Federal Aviation Administration (FAA), Department of 
Transportation (DOT).

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: We propose to supersede Airworthiness Directive (AD) 2006-07-
15, which applies to Thrush Aircraft, Inc. Model 600 S2D and S2R (S-2R) 
series airplanes (type certificate previously held by Quality 
Aerospace, Inc. and Ayres Corporation). AD 2006-07-15 currently 
requires repetitive inspections of the \1/4\-inch and \5/16\-inch bolt 
hole areas on the wing front lower spar caps for fatigue cracking; 
replacement or repair any wing front lower spar cap where fatigue 
cracks are found; and reporting of any fatigue cracks found to the FAA. 
AD 2006-07-15 also puts the affected airplanes into groups for 
compliance time and applicability purposes. Since we issued AD 2006-07-
15, FAA analysis reveals that inspections are not detecting all 
existing cracks and shows the incidences of undetected cracks will 
increase as the airplanes age. Consequently, this proposed AD would 
retain the actions of AD 2006-07-15 and impose a life limit on the wing 
front lower spar caps that requires replacement of the wing front lower 
spar caps when the life limit is reached. This proposed AD would also 
change the requirements and applicability of the groups discussed above 
and remove the ultrasonic inspection method. We are proposing this AD 
to prevent wing front lower spar cap failure caused by undetected 
fatigue cracks. Such failure could result in loss of a wing in flight.

DATES: We must receive comments on this proposed AD by July 6, 2009.

ADDRESSES: Use one of the following addresses to comment on this 
proposed AD:
     Federal eRulemaking Portal: Go to http://www.regulations.gov. Follow the instructions for submitting comments.
     Fax: (202) 493-2251.
     Mail: U.S. Department of Transportation, Docket 
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New 
Jersey Avenue, SE., Washington, DC 20590.
     Hand Delivery: U.S. Department of Transportation, Docket 
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New 
Jersey Avenue, SE., Washington, DC 20590, between 9 a.m. and 5 p.m., 
Monday through Friday, except Federal holidays.
    For service information identified in this proposed AD, contact 
Thrush Aircraft, Inc., 300 Old Pretoria Road, P.O. Box 3149, Albany, 
Georgia 31706-3149. The service information is also available on the 
Internet at www.thrushaircraft.com.
    For Further Information, Contact One of the Following:

--Cindy Lorenzen, Aerospace Engineer, ACE-115A, Atlanta Aircraft 
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite 450, 
Atlanta, Georgia 30349; telephone: (770) 703-6078; facsimile: (770) 
703-6097; e-mail: [email protected]; or
--Keith Noles, Aerospace Engineer, ACE-117A, Atlanta Aircraft 
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite 450, 
Atlanta, Georgia 30349; telephone: (770) 703-6085; facsimile: (770) 
703-6097; e-mail: [email protected].

SUPPLEMENTARY INFORMATION: 

Comments Invited

    We invite you to send any written relevant data, views, or 
arguments regarding this proposed AD. Send your comments to an address 
listed under the ADDRESSES section. Include the docket number, ``FAA-
2007-27862; Directorate Identifier 2007-CE-036-AD'' at the beginning of 
your comments. We specifically invite comments on the overall 
regulatory, economic, environmental, and energy aspects of the proposed 
AD. We will consider all

[[Page 20432]]

comments received by the closing date and may amend the proposed AD in 
light of those comments.
    We will post all comments we receive, without change, to http://www.regulations.gov, including any personal information you provide. We 
will also post a report summarizing each substantive verbal contact we 
receive concerning this proposed AD.

Discussion

History of AD Actions

    An accident in which the wing on a Thrush S2R series airplane 
separated from the airplane in flight prompted us to issue AD 97-13-11. 
The following presents the sequential AD history on this subject to 
date:
     AD 97-13-11, Amendment 39-10071 (62 FR 36978, July 10, 
1997), required (until superseded by AD 97-17-03) inspecting certain 
areas of the wing front lower spar caps for fatigue cracks, replacing 
any wing front lower spar cap where fatigue cracks were found, and 
reporting any fatigue cracks to the FAA.
     AD 97-17-03, Amendment 39-10195 (62 FR 43926, August 18, 
1997), superseded AD 97-13-11. AD 97-17-03 corrected a model 
designation and retained the actions of AD 97-13-11.
     AD 2000-11-16, Amendment 39-11764 (65 FR 36055, June 7, 
2000), superseded AD 97-17-03. AD 2000-11-16 changed the inspections 
required in AD 97-17-03 to repetitive, added airplanes to the 
Applicability section, changed the initial compliance time for all 
airplanes, and arranged the affected airplanes into six groups based on 
usage and configuration.
     AD 2003-07-01, Amendment 39-13097 (68 FR 15653, April 1, 
2003), superseded AD 2000-11-16. AD 2003-07-01 added airplanes 
manufactured with a similar design to the Applicability section and 
added an additional repair option.
     AD 2006-07-15, Amendment 39-14542 (71 FR 16691, April 4, 
2006), superseded AD 2003-07-01. AD 2006-07-15 increased the inspection 
frequency of Groups 1, 2, 3, and 6 airplanes and lowered the initial 
inspection time of Group 2 airplanes based on analysis of crack report 
data compiled from the previous ADs.

Events That Initiated This Proposed AD

    All of the ADs listed above required submitting reports to the FAA 
anytime a fatigue crack was found on a wing front lower spar cap. 
Recent analysis of the data from those reports and other historical and 
statistical data indicate the current inspections are not completely 
addressing the unsafe condition.
    Specifically, the data indicate a risk that some airplanes in the 
Thrush fleet may currently have undetected fatigue cracks in the steel 
spar cap using the existing inspection program. Airplanes with cracks 
in the wing front lower spar caps are unable to meet ultimate strength 
requirements, which could lead to a wing failure. As the incidences of 
cracking increase, which has occurred in the Thrush airplanes, the 
chance of an existing crack not being detected during an inspection 
increases.

FAA Analysis

    The FAA used a risk-based probability analysis to determine the 
risk of fatigue cracks occurring in the wing front lower spar cap on 
Model 600 S2D and S2R (S-2R) series airplanes. This analysis indicates 
the risk to the pilot and the public is too great to allow the 
continuation of the repetitive inspections as the only method to ensure 
the safety of these airplanes. The actions in this proposed AD are 
necessary to assure the continued airworthiness of these airplanes.
    We analyzed data obtained from reports of 117 fatigue cracks found 
on the wing front lower spar caps on these airplanes since 1997. The 
analysis of the crack reports led to our determination to consider 
imposing a life limit on the wing front lower spar caps. We have 
confidence in the accuracy of the reports submitted by the owner/
operators, Airframe and Powerplant (A&P) mechanics, and Level 2 and 3 
non-destructive inspectors. Anyone with documented evidence of owner/
operators, inspectors, or A&P mechanics on behalf of the owner 
submitting inaccurate crack reports or not submitting crack reports to 
the FAA should send that evidence to their local FAA Flight Standards 
District Office.
    We have a documented occurrence of a fatigue crack that went 
undetected for at least two inspection cycles. The crack grew until the 
wing front lower spar cap was completely severed, which is considered a 
failure even though the wing stayed attached to the airplane. The ``big 
butterfly'' plate and the lower splice plate, part numbers (P/Ns) 
20211-09 and 20211-11 respectively, installed on this airplane as an 
optional modification helped keep the wing together; however, the 
plates are not designed to carry all of the possible flight loads in 
the event a spar cap is severed.
    Installing stronger ``big butterfly'' plates is beneficial because 
it reduces stress in the wing front lower spar caps. The reduced stress 
slows the crack growth rate in the spar cap. This slower crack growth 
rate in airplanes equipped with ``big butterfly'' plates allows for 
less frequent inspections. Even though P/Ns 20211-09 and 20211-11 
reduce stress in the wing front lower spar caps and slow the crack 
growth rate, the plates will not handle all possible flight loads once 
the spar cap is severed. Any known cracks must still be repaired.
    Thrush Aircraft, Inc. has developed Custom Kit No. CK-AG-41, 
Revision A, dated March 8, 2007. This kit includes parts and procedures 
for replacing both wing front lower spar caps with new wing front lower 
spar caps, P/Ns 20207-15 and 20207-16, new inboard spar webs and 
doublers, and new, thicker ``big butterfly'' plate and lower splice 
plate, P/Ns 94418-5 and 94418-7 respectively.
    Airplanes that have Custom Kit No. CK-AG-41, Revision A, installed 
in its entirety will have lower stresses in the spar cap, which will 
delay the initiation of fatigue cracks and slow the fatigue crack 
growth rate allowing for less frequent inspections. A life limit would 
remain the same even after Custom Kit No. CK-AG-41, Revision A, is 
installed in its entirety. If additional fatigue testing and analysis 
is completed on this configuration in the future, a life limit may be 
adjusted.
    Our analysis showed the wing front lower spar caps will all crack 
due to fatigue. In determining the maximum time allowed for life 
limits, we gave consideration to the following:
     Reliability of the significant amount of crack data on the 
Thrush fleet;
     Existence of the on-going inspection program for the wing 
front lower spar caps; and
     Allowance of credit for time the airplanes operated with 
lower horsepower radial engines and were later modified by installing a 
turbine engine, a higher horsepower radial engine, or larger hopper.
    We could not consider the following when determining life limits:
     Individual airplanes operated at lower weights; and
     Individual airplanes operated at lower G loads.
    To consider these factors, individual airplanes would need to have 
recorded data for every flight since the wings were installed showing 
the weight and recorded Gs throughout each flight, along with fatigue 
analysis and tests using this data.
    In addition, we could not consider the effect of the following 
modifications when determining life limits:
     Kaplan splice blocks installed;
     ``Big butterfly'' plates and lower splice plates 
installed;

[[Page 20433]]

     Winglets installed; or
     Cold work process on the bolt holes performed.
    We do not have service information to calculate the effect of these 
modifications, and accurate fatigue test data or fatigue analysis data 
supported by tests has not been provided to us for these 
configurations. If we receive accurate fatigue substantiation data for 
airplanes with these modifications, we may allow changes to life limits 
by an alternative method of compliance.
    There is evidence of sharp, uneven edges on the spar cap bolt holes 
that resulted from the manufacturing process in Group 5 airplanes. Five 
fatigue cracks have been reported on Group 5 airplanes, and our 
analysis concludes fatigue cracks will occur on all these airplanes. 
Premature fatigue cracks begin when there is a crack starter, such as 
an uneven edge. At this time, there is no rework method to address the 
condition of these wing front lower spar caps with uneven bolt hole 
edges. Once the original wing front lower spar caps are replaced, a 
higher life limit for wing front lower spar caps without uneven bolt 
hole edges may be used.
    Initial compliance times for replacement of the wing front lower 
spar caps would be based on risk analysis that allows for compliance 
scheduling. For any of the affected airplanes that may exceed any life 
limits, the compliance time range would be based on total hours time-
in-service (TIS), which would address those high-usage airplanes first. 
Graduated compliance times would help alleviate grounding of airplanes 
due to the limited supply of wing front lower spar caps, while still 
addressing the increased risk for high-usage airplanes.

Long-Term Continued Operational Safety

    Repeated loads and the resulting stresses in the metal lead to 
fatigue. Over time, these stresses cause the metal to wear out and 
cracks will form in these airplanes even when operated within the 
approved limitations and envelope. Higher stresses in the wing front 
lower spar cap, caused by pulling excessive Gs and/or operating over 
the design weight of the airplane, will accelerate metal fatigue. Metal 
will also fatigue more quickly when operated in a wet or corrosive 
environment, which exists when dispensing agricultural chemicals or 
dropping fire retardants or water.
    Any type of inspection method may be affected by the reliability of 
the equipment used, the inspection procedure used, the environment in 
which the inspection is done, the quality of the calibration reference 
standard used, and various human factors, such as the knowledge, skill, 
experience, and dexterity of the inspector. Because of all these 
variables, most inspection results, while very good, are not always 
100-percent accurate. Over time, the probability of failing to detect a 
crack increases due to these variables, which increases the risk to the 
safety of these airplanes.
    Studies of the factors leading to inspection inaccuracy and their 
effect on a variety of inspection methods, including magnetic particle 
inspections and eddy current inspections, have been done by the 
National Aeronautics and Space Administration (since 1973 for the Space 
Shuttle design), the United States Air Force, and the FAA. These 
studies show variability in inspection results that are inherent to any 
measurement process.
    We received a report of cracks not being detected in the Thrush 
wing front lower spar cap using the ultrasonic method because of the 
configuration of the joint. Our records indicate that ultrasonic 
inspections are no longer being used in the field. This inspection 
method should be removed. If ultrasonic inspections are no longer 
allowed for these inspections, the availability of inspection 
facilities should not be affected because the two inspection facilities 
certified for ultrasonic inspections are also certified for eddy 
current inspections.
    As wing front lower spar caps accumulate hours TIS beyond the time 
when cracks have been found on other products of the same type design, 
the likelihood of fatigue cracks occurring in these wing front lower 
spar caps increases. Many of the affected airplanes have wing front 
lower spar caps that have been in service well past the number of hours 
TIS when cracks have been appearing on wing front lower spar caps in 
other products of the same type design. FAA statistical analysis of the 
crack data indicates the risk of a wing failure occurring is becoming 
very high for these airplanes.

Reclassification of Airplane Groups

    A recent review of the manufacturer's build record data shows some 
airplanes were placed in incorrect Groups and one airplane was 
inadvertently left out in the previous ADs. Our review shows that Model 
S2R-T34 airplanes, serial numbers (S/Ns) T34-147 through T34-167, were 
built with wing front lower spar caps identical to Group 2 airplanes; 
these airplanes should be reclassified from Group 1 to Group 2. Model 
S2R-G10 airplane, S/N G10-137, is currently included in Group 4 
airplanes but was built identical to Group 2; this airplane should be 
reclassified into Group 2. We inadvertently omitted Model S2R-T34 
airplane, S/N T34-170, from AD 2006-07-15; that airplane should be 
included in Group 2. We inadvertently listed Model S2R-T34 airplane, S/
N T34-225, in both Group 2 and Group 4 airplanes in AD 2006-07-15; it 
should be in Group 2 only. Model S2R-G1 airplane, S/Ns G1-107, G1-108, 
G1-109; Model S2R-G10 airplane, S/Ns G10-139 and G10-142; and Model 
S2R-T34 airplanes, S/Ns T34-236, T34-237, and T34-238, were built 
identical to Group 5; these airplanes should be in Group 5. No 
airplanes were built to the configuration previously identified as 
Group 4; Group 4 should be removed.

Relevant Service Information

    The following service information was included in AD 2006-07-15 and 
will be included in this proposed AD:

--Ayres Corporation Service Bulletin No. SB-AG-39, dated September 17, 
1996;
--Ayres Corporation Custom Kit No. CK-AG-29, dated December 23, 1997; 
and
--Quality Aerospace, Inc. Custom Kit No. CK-AG-30, dated December 6, 
2001.

    The new service information for this proposed AD is Thrush 
Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8, 
2007.

FAA's Determination and Requirements of the Proposed AD

    We are proposing this AD because we evaluated all information and 
determined the unsafe condition described previously is likely to exist 
or develop on other products of the same type design. This proposed AD 
would supersede AD 2006-07-15 with a new AD that would:
     Retain the actions of AD 2006-07-15;
     Add life limits for the wing front lower spar caps;
     Lower the initial and repetitive inspection times for 
Group 5 airplanes;
     Correct some airplane Group classifications;
     Add an airplane to the Applicability section; and
     Remove the use of ultrasonic inspection methods.
    The initial compliance time for all airplanes would be at least an 
additional 500 hours TIS after the effective date of the proposed AD 
for replacement of the wing front lower spar caps. Calculated from 
actual flight hour data from 285 S2R series airplanes, 500 hours TIS 
equates to the average yearly operational

[[Page 20434]]

time. The proposed compliance schedule should give owner/operators 
enough time to schedule the replacement of the wing front lower spar 
caps.
    Although not required in this proposed AD, we recommend installing 
``big butterfly'' and lower splice plates, P/Ns 20211-09 and P/N 20211-
11, or Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, since 
they increase the strength of the wing beyond the minimum safety 
standards.
    This proposed AD would require you to use the service information 
described previously to perform these actions.

Costs of Compliance

    We estimate that this proposed AD would affect 808 airplanes in the 
U.S. registry, including those airplanes affected by AD 2006-07-15.
    We estimate the following costs to do each proposed inspection:

----------------------------------------------------------------------------------------------------------------
                                                                        Total cost per       Total cost on U.S.
                 Labor cost                        Parts cost              airplane              operators
----------------------------------------------------------------------------------------------------------------
3 work-hours x $80 = $240..................                  $525                   $765               $618,120
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    We estimate the following costs to do cold work of bolt holes for 
the repair that may be required based on the results of the proposed 
inspection. We have no way of determining the number of airplanes that 
may need such repair:

----------------------------------------------------------------------------------------------------------------
                                                                                               Total cost per
                            Labor cost                                    Parts cost              airplane
----------------------------------------------------------------------------------------------------------------
1 work-hour x $80 = $80...........................................                  $100                   $180
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    We estimate the following costs to do any reaming of outer holes to 
\5/16\-inch diameter for the repair that may be required based on the 
results of the proposed inspection. We have no way of determining the 
number of airplanes that may need such repair:

------------------------------------------------------------------------
                                                       Total cost per
          Labor cost                Parts cost            airplane
------------------------------------------------------------------------
1 work-hour x $80 = $80.......  None.............                   $80
------------------------------------------------------------------------

    We estimate the following costs to do any drilling and reaming of 
outer holes and adding three holes to install a Kaplan splice block for 
the repair that may be required based on the results of the proposed 
inspection. We have no way of determining the number of airplanes that 
may need such modification:

------------------------------------------------------------------------
                                                       Total cost per
          Labor cost                Parts cost            airplane
------------------------------------------------------------------------
65 work-hours x $80 = $5,200..  $4,400 for splice               $10,200
                                 block and $600
                                 for hardware.
------------------------------------------------------------------------

    We estimate the following costs to do the proposed optional 
installation of Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision 
A, dated March 8, 2007. This kit may be used to do any necessary wing 
front lower spar cap replacement that would be required based on the 
results of the proposed inspection or that would be required based on 
reaching the proposed life limit:

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                                                                                               Total cost per
                            Labor cost                                    Parts cost              airplane
----------------------------------------------------------------------------------------------------------------
300 work-hours x $80 = $24,000....................................               $40,000                $64,000
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    We estimate the following costs to do any necessary wing front 
lower spar cap replacement that would be required based on the results 
of the proposed inspection or by the wing front lower spar cap reaching 
the proposed life limit:

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  Labor cost per wing front lower spar    Parts cost per wing                                Total cost on U.S.
                  cap                     front lower spar cap   Total cost per airplane         operators
----------------------------------------------------------------------------------------------------------------
200 work-hours x $80 = $16,000.........                $8,000   Each spar cap replacement           $38,784,000
                                                                 = $24,000.
                                                                Two spar caps per
                                                                 airplane = $48,000.
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Authority for This Rulemaking

    Title 49 of the United States Code specifies the FAA's authority to 
issue rules on aviation safety. Subtitle I, Section 106, describes the 
authority of the FAA Administrator. Subtitle VII, Aviation Programs, 
describes in more detail the scope of the Agency's authority.
    We are issuing this rulemaking under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701, ``General 
requirements.'' Under that section, Congress charges the FAA with 
promoting safe flight of civil aircraft in air commerce by prescribing 
regulations for practices, methods, and procedures the Administrator 
finds necessary for safety in air commerce. This regulation is within 
the scope of that authority

[[Page 20435]]

because it addresses an unsafe condition that is likely to exist or 
develop on products identified in this rulemaking action.

Initial Regulatory Flexibility Analysis

Introduction and Purpose of This Analysis

    The Regulatory Flexibility Act of 1980 (Pub. L. 96-354) (RFA) 
establishes ``as a principle of regulatory issuance that agencies shall 
endeavor, consistent with the objectives of the rule and of applicable 
statutes, to fit regulatory and informational requirements to the scale 
of the businesses, organizations, and governmental jurisdictions 
subject to regulation.'' To achieve this principle, the RFA requires 
agencies to solicit and consider flexible regulatory proposals and to 
explain the rationale for their actions to assure that such proposals 
are seriously considered.'' The RFA covers a wide-range of small 
entities, including small businesses, not-for-profit organizations, and 
small governmental jurisdictions.
    Unless the FAA can certify that a proposed rule will not have a 
significant economic impact on a substantial number of small entities, 
the FAA is required to prepare an initial regulatory flexibility 
analysis (IRFA) as described in Sec.  603 of the RFA. Such an analysis 
must include (1) a description of the reasons for the agency's action; 
(2) a statement regarding the objectives and legal basis for the 
proposed rule; (3) an estimate of the number of small entities that 
will be affected by the proposed rule; (4) a description of the 
projected recordkeeping, reporting, and other compliance costs; (5) a 
statement regarding any potential duplication, overlap, or conflict 
with all other relevant rules; and (6) a description of any significant 
alternatives that may minimize the significant economic impact of the 
proposed rule on small entities. Based on the following analysis, the 
FAA concludes that this proposed rule will have a significant economic 
impact on a substantial number of small entities.

Reasons Action by the FAA Is Being Considered

    A series of ADs, beginning in 1997 and culminating in AD 2006-07-15 
in 2006, addressed the issue of fatigue cracking of the wing front 
lower spar caps in Thrush Aircraft, Inc. (Thrush) Model 600 S2D and S2R 
(S-2R) series airplanes (type certificate previously held by Quality 
Aerospace, Inc. and Ayres Corporation). This type of fatigue cracking, 
if not addressed, could result in catastrophic wing failure. The 
original 1997 AD was issued after an accident on an S2R series airplane 
in which the wing separated from the airplane in flight. Requirements 
of inspection and possible replacement were changed in 2000 to 
repetitive inspections and possible replacement. In 2006, the 
inspection rate was doubled after a completely severed spar cap was 
found on one of the affected airplanes and the FAA noted that it was 
working with Thrush to develop a future terminating action. Analysis 
indicated that an undetected crack had existed during the previous two 
repetitive inspections of that spar cap.
    Subsequent FAA analysis has shown that spar cap fatigue cracking 
has increased as the fleet has aged, and will continue to increase. 
Consequently, the incidences of undetected cracks will increase, 
increasing the probability of catastrophic wing failure. The FAA has 
concluded that repetitive inspections, as required since the 2000 AD, 
are insufficient by themselves to ensure the safety of these airplanes 
and, accordingly, in this proposed AD the FAA proposes spar cap life 
limits to address this safety issue.

Objectives of, and Legal Basis for, the Proposed Rule

    The FAA is issuing this rulemaking under the authority set forth in 
49 U.S.C. 44701(a)(5), which mandates the Administrator prescribe 
regulations for practices, methods, and procedures necessary for safety 
in air commerce. This regulation is within the scope of that authority 
because it addresses an unsafe condition that is likely to exist or 
develop on the airplanes identified in this AD.

Description of the Small Entities That the Proposed Rule Will Apply and 
an Estimate of Their Number

    This proposed rule would potentially affect 808 U.S. registered and 
operated Thrush Model 600S2D and S2R (S-2R) series airplanes.\1\ In 
conducting this analysis, the FAA reviewed data from the FAA Registry 
(Registry) to determine how many of the affected Thrush airplanes are 
registered and operated by small entities. The Registry indicates that 
these 808 airplanes are owned by 546 separate entities in agricultural 
aviation. Although the Registry does not record financial or business 
data about the registered owners of aircraft, and such data for these 
entities are not readily available elsewhere, it appears that most, if 
not all, of the 546 entities are engaged in crop dusting, spraying, and 
seeding operations. These activities are classified in North American 
Industry Classification System (NAICS) industry, NAICS 115112--Soil 
Preparation, Planting, and Cultivating (including Crop Dusting, Crop 
Spraying). The concentration of these entities in a single NAICS 
industry reflects the specialized nature of agricultural airplanes with 
restricted airworthiness certificates. Furthermore, several of these 
entities were classified in the Standard Industrial Classification 
(SIC) equivalent of NAICS 115112 by http://www.manta.com. Although a 
few of these entities may also be engaged in firefighting, which is 
classified in NAICS 115310--Support Activities for Forestry (including 
Forest Fire Suppression), the FAA is unable to identify any of these 
entities as being principally engaged in firefighting. The Small 
Business Administration (SBA) small business classification for NAICS 
115112 is $6.5 million in business receipts, and $16.5 million in 
business receipts for NAICS 115310. Only one entity in this sample 
appears to have business receipts over $6.5 million, and no entity has 
business receipts in excess of $16.5 million. Using the total number of 
airplanes owned as a size criterion, the FAA selected a sample of 41 of 
the largest affected entities, and found median sales shown by http://www.manta.com to be just $250,000 annually. Firms in agricultural 
aviation appear to be inherently of small size. Accordingly, the FAA 
estimates that 545 small entities will be affected by this proposed 
rule.
---------------------------------------------------------------------------

    \1\ FAA Registry, http://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/releasable_aircraft_download. Data downloaded on 4/14/08.
---------------------------------------------------------------------------

Description of the Projected Reporting, Recordkeeping, and Other 
Compliance Requirements of the Proposed AD

    The proposed AD does not impose any additional reporting or 
recordkeeping requirements beyond those required by the 2006 AD. The 
proposed rule would retain the requirements of AD 2006-07-15 and impose 
a life-limit on the wing front lower spar caps, which would require 
operators of affected airplanes to replace the wing front lower spar 
caps when the life-limit is reached.
    The estimated compliance cost varies widely by airplane submodel; 
from a cost of zero for the more than 200 older airplanes that we 
estimate will retire \2\ before the life-limit on their wing front 
lower spar caps is reached, to a cost of $320,000 (5 replacements at 
$64,000 per

[[Page 20436]]

replacement) for two airplanes. Individual airplane compliance costs 
will likely result in costs to the small entities that own these 
airplanes. The exact cost will vary, depending on the number of 
affected Thrush airplanes owned by the entity and the specific 
compliance cost for each airplane. The ownership table below shows the 
variation in the number of owners with particular numbers of airplanes. 
The table shows that almost 75% of the 546 individual owners have only 
one affected airplane, and more than 90% of owners have no more than 
two affected airplanes. The average (mean) number of affected airplanes 
held is 1.48, while the median number held is just 1.00, so the median 
airplane cost is equivalent to the median owner cost.
---------------------------------------------------------------------------

    \2\ As fully analyzed in the ``Cost of Compliance'' section of 
this proposed rule, the FAA estimates that the airplanes affected by 
this proposed rule retire at age 40.

                   Number of Thrush AD Owners Having Particular Numbers of Affected Airplanes
----------------------------------------------------------------------------------------------------------------
                                                 Number of affected
                                                  airplanes held by     Number of owners        Cumulative %
                                                    single owner
----------------------------------------------------------------------------------------------------------------
                                                                   1                   406                  74.4
                                                                   2                    86                  90.1
                                                                   3                    26                  94.9
                                                                   4                    13                  97.3
                                                                   5                     7                  98.5
                                                                   6                     2                  98.9
                                                                   7                     2                  99.3
                                                                   8                     1                  99.5
                                                                   9                     2                  99.8
                                                                  13                     1                 100.0
----------------------------------------------------------------------------------------------------------------
Total.........................................                   808                   546
Mean..........................................                  1.48
Median........................................                  1.00
----------------------------------------------------------------------------------------------------------------
Source: FAA Registry. Data downloaded on 4/18/08.

    In the ``Cost of Compliance'' section of this proposed AD, the FAA 
estimates total cost (undiscounted) to be $37.1 million and the present 
value cost to be $25.2 million. The FAA estimates that 545 of the 546 
airplanes affected by this proposed AD are small firms, and, in fact, 
98.8% of the proposed AD's estimated cost is attributed to small 
entities. The following documents and analyzes the impact of this cost 
on the substantial number of small firms identified in this proposed 
AD.

Economic Impact on Small Entities

    Because the Registry does not collect financial or business data on 
these entities, and such data is not readily available elsewhere, the 
FAA also used Census Bureau size distribution data to assess the 
economic impact on small firms. The FAA used data from the 2002 Census 
since this is the latest Census for which size distribution by business 
receipts is readily available. These data are available in a special 
Census compilation for the SBA.\3\ The FAA used the data for NAICS 
115112--Soil Preparation, Planting, and Cultivating (including Crop 
Dusting, Crop Spraying), but did not use the data for NAICS 115310--
Support Activities for Forestry (including Forest Fire Suppression) 
since, as noted above, a very high percentage of the affected small 
firms, if not all, meet the classification standard of NAICS 115112. 
Moreover, the size distribution of NAICS 115310 appears to be similar 
to that of NAICS 115112. The concentration of the affected airplanes in 
one NAICS industry, noted above, makes the use of Census data feasible 
and appropriate.
---------------------------------------------------------------------------

    \3\ Small Business Administration, Office of Advocacy. http://www.sba.gov/advo/research/us_rec02.txt.
---------------------------------------------------------------------------

    The relevant Census data are provided in the table below:

       2002 Census Data for NAICS 115112--Soil Preparation, Planting, and Cultivating (Including Crop Dusting, Crop Spraying)--Small Size Classes
--------------------------------------------------------------------------------------------------------------------------------------------------------
              Measure                    Total        $0-$99,999    $100,000-$499,999  $500,000-$999,0000  $1,000,000-$4,999,999  $5,000,000-$10,000,000
--------------------------------------------------------------------------------------------------------------------------------------------------------
Firms.............................            2336             509               992                412                   394                     29
Percentage of firms...............  ..............           21.8%             42.5%              17.6%                 16.9%                   1.2%
Upper bound percentile............  ..............           21.8%             64.3%              81.9%                 98.8%                 100.0%
Est. Receipts ($000)..............      $1,531,004         $25,681          $257,447           $286,462              $772,401               $189,013
Receipts/Firm ($).................        $655,396         $50,454          $259,523           $695,296            $1,960,409             $6,517,690
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: ``Firms'' and ``Est. Receipts'' from Small Business Administration, Office of Advocacy. http://www.sba.gov/advo/research/us_rec02.txt.

    The table shows the number of firm and business receipt data for 
the five smallest size classes of NAICS 115112 that encompass the size 
range of the firms affected by this proposed AD. In the ``Percentage of 
firms'' row, for each size class, the FAA calculates that class's 
number of firms as a percentage of the total number of firms in the 
five size classes. Cumulating this percentage from the smallest to 
largest size class establishes the ``Upper bound percentile''--the 
cumulated percentage of firms of business receipt size ranging up to 
the upper bound of the size class.

[[Page 20437]]

The proposed AD's cost for the firms at the upper bound percentiles is 
then estimated as the corresponding percentiles in the estimated firm-
level compliance cost data. In order to assess the economic impact of 
the proposed AD, these costs are calculated as a percentage of the 
Census data upper bounds. For example, the upper bound percentile for 
the 100-500 thousand dollar size class is 64.3%, so the NAICS 115112 
firms at that percentile are estimated to have $500,000 business 
receipts of $500,000. As shown in the table below, the FAA then 
determined the estimated compliance cost of firms at the same 
percentile in the compliance cost data to be $61,754. The FAA assumes 
these firms are the same so the percentage cost impact (Proposed AD 
Cost/Firm Size) is 12.4%. This procedure assumes the size distribution 
of the 808 firms affected by the proposed AD have a distribution 
similar to the overall distribution of the small firms in NAICS 115112. 
It also assumes there is a perfect rank correlation between the size of 
the affected firms and the firms' compliance cost. While the latter 
assumption is certainly not the case, any deviation from such perfect 
correlation can only increase the impact of the proposed AD because 
smaller firms will have larger costs. Accordingly, the FAA's 
determination that the proposed AD will have a significant impact on a 
substantial number of small entities is unaffected.

                                   Economic Impact of Thrush AD on Small Firms
----------------------------------------------------------------------------------------------------------------
                                                             Estimated firm
                                                              size (Census    Proposed AD Cost/    Cumulative
        Proposed AD cost to firm           Firm percentile   Bureau receipts      Firm Size      number of firms
                                                              upper bound)        (percent)
----------------------------------------------------------------------------------------------------------------
$0......................................            21.8th          $100,000               0.0             119.2
$61,754.................................            64.3rd           500,000              12.4             351.5
$91,335.................................            81.9th         1,000,000               9.1             447.9
$273,734................................            98.8th         5,000,000               5.5             540.2
----------------------------------------------------------------------------------------------------------------

    The above table shows a zero-cost impact on a firm at the 21.8th 
percentile. This result reflects the estimate in the ``Cost of 
Compliance'' section of this proposed AD that more than 200 older 
airplanes will retire before their spar cap life-limits are reached. As 
already mentioned, the proposed AD cost for a firm at the 64.3rd 
percentile is $61,754, which as a percentage of estimated firm size 
(size class upper bound) is 12.4% of annual business receipts. This 
impact declines to 9.1% for a firm at the 81.9th percentile and to 5.5% 
for a firm at the 98.8th percentile. As a result, the overall pattern 
is zero impact for the smallest of the small firms, owners of the 
oldest airplanes, but a highly positive impact for the medium-sized 
small firms. In percentage terms, this impact falls for the largest 
small firms, but remains at a substantial level. While the FAA can make 
no definitive inference on the impact of the proposed AD on firms 
between the 21.8th and 64.3rd percentiles, the FAA notes the cost 
varies from 9.1% up to 12.4% of annual business receipts for 96 firms 
between the 81.9th and 64.3rd percentiles and from 5.5% to 9.1% for 92 
firms between the 98.8th percentile and the 81.9th percentile. These 
estimated percentage impacts are substantial and therefore, the FAA 
concludes that this proposed AD will have a significant impact on a 
substantial number of small entities.

Duplicative, Overlapping or Conflicting Federal Rules

    There are no Federal rules that duplicate, overlap, or conflict 
with this proposed AD.

Significant Alternatives to the Proposed AD

    The FAA considered relying on repetitive inspections as the sole 
safety method, but given that the past required repetitive inspections 
have not fully addressed this critical safety issue, the FAA has 
determined that a part life limit is also necessary. A life limit on 
the wing front lower spar caps is the only available sufficient action 
presently known to the FAA. Consequently, there are no significant 
viable alternatives to the proposed AD.

Request for Comments

    The FAA has determined that this proposed rulemaking will have a 
significant economic impact on a substantial number of small entities. 
The FAA requests comments with supporting justification regarding this 
determination.

International Trade Impact Analysis

    The Trade Agreement Act of 1979 prohibits Federal agencies from 
establishing any standards or engaging in related activities that 
create unnecessary obstacles to the foreign commerce of the United 
States. The statute does not consider legitimate domestic objectives, 
such as safety, as unnecessary. The statute also requires consideration 
of international standards and, where appropriate, that they be the 
basis for U.S. standards. The FAA is issuing this proposed AD because 
of a known safety problem and, therefore, the proposed AD is not 
considered an unnecessary obstacle to international trade.

Unfunded Mandates Reform Act Assessment

    Title II of the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-
4) requires each Federal agency to prepare a written statement 
assessing the effects of any Federal mandate in a proposed or final 
agency rule that may result in an expenditure of $100 million or more 
(adjusted annually for inflation with the base year 1995) in any one 
year by State, local, and tribal governments in the aggregate, or by 
the private sector. The Act deems such a mandate to be a ``significant 
regulatory action.'' The FAA currently uses an inflation-adjusted value 
of $136.1 million.
    This proposed AD does not contain such a mandate.

Regulatory Findings

    We have determined that this proposed AD would not have federalism 
implications under Executive Order 13132. This proposed AD would not 
have a substantial direct effect on the States, on the relationship 
between the national Government and the States, or on the distribution 
of power and responsibilities among the various levels of government.
    For the reasons discussed above, I certify that the proposed 
regulation:
    1. Is not a ``significant regulatory action'' under Executive Order 
12866;
    2. Is not a ``significant rule'' under the DOT Regulatory Policies 
and Procedures (44 FR 11034, February 26, 1979); and
    ------3. Could have a significant economic impact on a substantial 
number of small entities under the criteria of the Regulatory 
Flexibility Act.

[[Page 20438]]

    We prepared a regulatory evaluation of the estimated costs to 
comply with this proposed AD and placed it in the AD docket.

Examining the AD Docket

    You may examine the AD docket that contains the proposed AD, the 
regulatory evaluation, any comments received, and other information on 
the Internet at http://www.regulations.gov; or in person at the Docket 
Management Facility between 9 a.m. and 5 p.m., Monday through Friday, 
except Federal holidays. The Docket Office (telephone (800) 647-5527) 
is located at the street address stated in the ADDRESSES section. 
Comments will be available in the AD docket shortly after receipt.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Incorporation by 
reference, Safety.

The Proposed Amendment

    Accordingly, under the authority delegated to me by the 
Administrator, the FAA proposes to amend 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

    1. The authority citation for part 39 continues to read as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701.


Sec.  39.13  [Amended]

    2. The FAA amends Sec.  39.13 by removing Airworthiness Directive 
(AD) 2006-07-15, Amendment 39-14542 (71 FR 16691, April 4, 2006), and 
adding the following new AD:

Thrush Aircraft, Inc. (Type Certificate previously held by Quality 
Aerospace, Inc. and Ayres Corporation): Docket No. FAA-2007-27862; 
Directorate Identifier 2007-CE-036-AD.

Comments Due Date

    (a) We must receive comments on this airworthiness directive 
(AD) action by July 6, 2009.

Affected ADs

    (b) The following lists a history of the ADs affected by this AD 
action:
    (1) This AD supersedes AD 2006-07-15, Amendment 39-14542;
    (2) AD 2006-07-15 superseded AD 2003-07-01, Amendment 39-13097;
    (3) AD 2003-07-01 superseded AD 2000-11-16, Amendment 39-11764;
    (4) AD 2000-11-16 superseded AD 97-17-03, Amendment 39-10195; 
and
    (5) AD 97-17-03 superseded AD 97-13-11, Amendment 39-10071.

Applicability

    (c) This AD affects the following airplane models and serial 
numbers (S/Ns) in Table 1 that are certificated in any category when 
wing front lower spar cap part numbers (P/N) 20207-1, 20207-2, 
20207-11, 20207-12, 20207-13, 20207-14, 20207-15, or 20207-16 are 
installed. This AD applies to the S/Ns in Table 1 with or without a 
``DC'' suffix. This AD does not affect airplanes with wing front 
lower spar cap P/N 22507 (any dash number). The table also 
identifies the group that each airplane belongs in when determining 
inspection compliance times and life limit times for the parts:

               Table 1--Applicability and Airplane Groups
------------------------------------------------------------------------
              Model                     Serial Nos. (S/N)        Group
------------------------------------------------------------------------
(1) S-2R.........................  5000R through 5100R,                1
                                    except 5010R, 5031R,
                                    5038R, 5047R, and 5085R.
(2) S2R-G1.......................  G1-101 through G1-106.....          1
(3) S2R-R1820....................  R1820-001 through R1820-            1
                                    035.
(4) S2R-T15......................  T15-001 through T15-033             1
                                    (also see paragraph (d)
                                    of this AD).
(5) S2R-T34......................  6000R through 6049R, T34-           1
                                    001 through T34-143, T34-
                                    145, T34-171,
                                    T34[dash]180, and T34-181
                                    (also see paragraph (e)
                                    of this AD).
(6) S2R-G10......................  G10-101 through G10-138,            2
                                    G10-140, and G10-141.
(7) S2R-G5.......................  G5-101 through G5-105.....          2
(8) S2R-G6.......................  G6-101 through G6-147.....          2
(9) S2RHG-T65....................  T65-002 through T65-018...          2
(10) S2R-R1820...................  R1820-036.................          2
(11) S2R-T34.....................  T34-144, T34-146 through            2
                                    T34-170, T34-172 through
                                    T34-179, and T34-189
                                    through T34-234 (also see
                                    paragraph (e) of this AD).
(12) S2R-T45.....................  T45-001 through T45-014...          2
(13) S2R-T65.....................  T65-001 through T65-018...          2
(14) 600 S2D.....................  All serial numbers                  3
                                    beginning with 600-1311D.
(15) S-2R........................  1380R, 1416R through                3
                                    2592R, 3000R, and 3002R.
(16) S2R-R1340...................  R1340-001 through R1340-            3
                                    035.
(17) S2R-R3S.....................  R3S-001 through R3S-011...          3
(18) S2R-T11.....................  T11-001 through T11-005...          3
(19) S2R-G1......................  G1-107 through G1-115.....          5
(20) S2R-G10.....................  G10-139, G10-142 through            5
                                    G10-165.
(21) S2R-G6......................  G6-148 through G6-155.....          5
(22) S2RHG-T34...................  T34HG-102.................          5
(23) S2R-T15.....................  T15-034 through T15-040             5
                                    (also see paragraph (d)
                                    of this AD).
(24) S2R-T34.....................  T34-236 through T34-270             5
                                    (also see paragraph (e)
                                    of this AD).
(25) S2R-T45.....................  T45-015...................          5
(26) S-2R........................  5010R, 5031R, 5038R,                6
                                    5047R, and 5085R.
------------------------------------------------------------------------

    (d) The S/Ns of Model S2R-T15 airplanes could incorporate T15-
xxx and T27-xxx (xxx is the variable for any of the S/Ns beginning 
with T15- and T27-). This AD applies to both of these S/N 
designations as they are both Model S2R-T15 airplanes.
    (e) The S/Ns of Model S2R-T34 airplanes could incorporate T34-
xxx, T36-xxx, T41-xxx, or T42-xxx (xxx is the variable for any of 
the S/Ns beginning with T34-, T36-, T41-, and T42-). This AD applies 
to all of these S/N designations as they are all Model S2R-T34 
airplanes.
    (f) Any Group 3 airplane that has been modified with a hopper of 
a capacity more than 410 gallons, a piston engine greater than 600 
horsepower, or a gas turbine engine greater than 600 horsepower, is 
a Group 1 airplane for the purposes of this AD. Inspect the airplane 
at the Group 1 compliance time specified in this AD. Replace the 
wing front lower spar caps in accordance with the formulas given in 
paragraph (j) of this AD.
    (g) Group 6 airplanes were originally manufactured with higher 
horsepower radial engines, but were converted to lower horsepower 
radial engines. They are now configured identically to Group 3 
airplanes.

[[Page 20439]]

Unsafe Condition

    (h) This AD is the result of the analysis of data from 117 wing 
front lower spar cap fatigue cracks found on similar design Model 
600 S2D and S2R (S-2R) series airplanes and the FAA's determination 
that the replacement of high time wing front lower spar caps is 
necessary to address the unsafe condition for certain airplanes. 
Since we issued AD 2006-07-15, analysis reveals that inspections are 
not detecting all existing cracks, and incidences of undetected 
cracks are increasing. This AD retains the actions of AD 2006-07-15 
and imposes a life limit on the wing front lower spar caps that 
requires you to replace the wing front lower spar caps when the life 
limit is reached. This AD also changes the requirements and 
applicability of the groups discussed above and removes the 
ultrasonic inspection method. We are issuing this AD to prevent wing 
front lower spar cap failure caused by undetected fatigue cracks. 
Such failure could result in loss of a wing.

Compliance

    (i) To address the problem, do the following, unless already 
done:
    (1) If you have already done an inspection required by AD 2006-
07-15, within the next 30 days after the effective date of this AD, 
identify the number of hours time-in-service (TIS) since your last 
inspection required by AD 2006-07-15. You will need this to 
establish the inspection interval for the next inspection required 
by this AD.
    (2) Inspect the two outboard bolt hole areas (whether 1/4-inch 
and 5/16-inch diameter bolt holes or both 5/16-inch diameter bolt 
holes) on each wing front lower spar cap for fatigue cracking using 
magnetic particle or eddy current procedures. If Kaplan splice 
blocks, P/N 22515-1/-3 or 88-251, are installed following Quality 
Aerospace, Inc. Custom Kit No. CK-AG-30, dated December 6, 2001, 
inspect the three outboard bolt hole areas on each wing front lower 
spar cap for fatigue cracking using magnetic particle or eddy 
current procedures. Use the compliance times listed in paragraph 
(i)(3) of this AD for the initial inspection and the compliance time 
listed in paragraphs (i)(5), (i)(6), or (i)(7) of this AD for the 
repetitive inspections. The cracks may emanate from the bolt hole on 
the face of the wing front lower spar cap or they may occur in the 
shaft of the hole. Inspect both of those areas.
    (i) If using the magnetic particle method, inspect using the 
``Inspection'' portion of the ``Accomplishment Instructions'' and 
``Lower Splice Fitting Removal and Installation Instructions'' in 
Ayres Corporation Service Bulletin No. SB-AG-39, dated September 17, 
1996. Do the inspection following American Society for Testing and 
Materials E 1444-01, using wet particles meeting the requirements of 
the Society for Automotive Engineers AMS 3046. CAUTION: Firmly 
support the wings during the inspection to prevent movement of the 
wing front lower spar caps when the splice blocks are removed. This 
will allow easier realignment of the splice block holes and the 
holes in the wing front lower spar cap for bolt insertion and 
prevent damage to the bolt hole. Damage to the bolt hole inner 
surface or edge of the bolt hole can cause cracks to begin 
prematurely.
    (ii) The inspection must be done by or supervised by a Level 2 
or Level 3 inspector certified following the guidelines established 
by the American Society for Nondestructive Testing or MIL-STD-410.
    (iii) If using eddy current methods, a procedure must be sent to 
the FAA, Atlanta Aircraft Certification Office (ACO), for approval 
before doing the inspection. Send your proposed procedure to the 
FAA, Atlanta ACO, ATTN: Cindy Lorenzen, One Crown Center, 1895 
Phoenix Boulevard, Suite 450, Atlanta, Georgia 30349. You are not 
required to remove the splice block for the eddy current 
inspections, unless corrosion is visible. Eddy current inspection 
procedures previously approved under AD 2006-07-15, AD 2003-07-01, 
AD 2000-11-16, AD 97-13-11, and/or AD 97-17-03 remain valid for this 
AD.
    (iv) If you change the inspection method used (magnetic particle 
or eddy current), the TIS intervals for repetitive inspections are 
based on the method used for the last inspection.
    (3) If airplanes have not yet reached the threshold for the 
initial inspection required in AD 2006-07-15, initially inspect 
following the wing front lower spar cap hours total TIS schedule 
below or within the next 50 hours TIS after the effective date of 
this AD, whichever occurs later:

                                        Table 2--Initial Inspection Times
----------------------------------------------------------------------------------------------------------------
                                              Initially inspect upon accumulating the following hours total TIS
               Airplane group                                  on the wing front lower spar cap
----------------------------------------------------------------------------------------------------------------
(i) Group 1................................  2,000 hours TIS.
(ii) Group 2...............................  1,400 hours TIS.
(iii) Group 3..............................  6,400 hours TIS.
(iv) Group 5...............................  1,000 hours TIS.
(v) Group 6................................  (A) S/N 5010R: 5,530 hours TIS.
                                             (B) S/N 5038R: 5,900 hours TIS.
                                             (C) S/N 5031R: 6,400 hours TIS.
                                             (D) S/N 5047R: 6,400 hours TIS.
                                             (E) S/N 5085R: 6,290 hours TIS.
(vi) Any airplane with the entire Custom     2,000 hours TIS.
 Kit CK-AG-41 installed.
----------------------------------------------------------------------------------------------------------------

    (4) Airplanes in all groups must meet the following conditions 
before doing the repetitive inspections required in paragraphs 
(i)(5), (i)(6), or (i)(7) of this AD:
    (i) No cracks have been found previously on wing front lower 
spar cap; or
    (ii) Small cracks have been repaired through cold work (or done 
as an option if never cracked) following Ayres Corporation Service 
Bulletin No. SB-AG-39, dated September 17, 1996; or
    (iii) Small cracks have been repaired by reaming the 1/4-inch 
bolt hole to 5/16 inches diameter (or done as an option if never 
cracked) following Ayres Corporation Custom Kit No. CK-AG-29, Part 
I, dated December 23, 1997; or
    (iv) Small cracks have been repaired through previous 
alternative methods of compliance (AMOC); or
    (v) Small cracks have been repaired by installing Kaplan splice 
blocks, P/N 22515-1/-3 or 88-251 (or done as an option if never 
cracked) following Quality Aerospace, Inc. Custom Kit No. CK-AG-30, 
dated December 6, 2001.
    (5) Repetitively inspect Groups 1, 2, 3, and 6 airplanes that do 
not have ``big butterfly'' plates and lower splice plates, P/Ns 
20211-09 and P/N 20211-11, installed following Ayres Corporation 
Custom Kit No. CK-AG-29, Part II, dated December 23, 1997; or that 
do not have ``big butterfly'' plates and lower splice plates, P/Ns 
94418-5 and 94418-7 or P/Ns 94418-13 and 94418-15, installed 
following Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, 
dated March 8, 2007; and meet the conditions in paragraph (i)(4) of 
this AD. Follow the wing front lower spar cap hours TIS compliance 
schedule below:

[[Page 20440]]



  Table 3--Repetitive Inspection Times for Airplane Groups 1, 2, 3, and 6 Without ``Big Butterfly'' Plates and
                                               Lower Splice Plates
----------------------------------------------------------------------------------------------------------------
 When airplanes accumulate the following
  hours TIS on the wing front lower spar   Inspect within the following   Inspect thereafter at intervals not to
cap since the last inspection required in       hours TIS after the                    exceed. . .
              AD 2006-07-15,                effective date of this AD,
----------------------------------------------------------------------------------------------------------------
(i) Magnetic Particle inspection:........  ............................  250 hours TIS.
    (A) 350 or more hours TIS............   (A) 50 hours TIS...........
    (B) 175 through 349 hours TIS........  (B) 75 hours TIS............
    (C) Less than 175 hours TIS..........  (C) upon accumulating 250
                                            hours TIS.
(ii) Eddy Current inspection:............  ............................  350 hours TIS.
    (A) 500 or more hours TIS............  (A) 50 hours TIS............
    (B) 275 through 499 hours TIS........  (B) 75 hours TIS............
    (C) Less than 275 hours TIS..........  (C) upon accumulating 350
                                            hours TIS.
----------------------------------------------------------------------------------------------------------------

    (6) Repetitively inspect Groups 1, 2, 3, 5, and 6 airplanes that 
have ``big butterfly'' plates and lower splice plates, P/Ns 20211-09 
and 20211-11, installed following Ayres Corporation Custom Kit No. 
CK-AG-29, Part II, dated December 23, 1997; or that have ``big 
butterfly'' plates and lower splice plates, P/Ns 94418-5 and 94418-
7, or 94418-13 and 94418-15, installed following Thrush Aircraft, 
Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8, 2007; and 
meet the conditions in paragraph (i)(4) of this AD. Follow the wing 
front lower spar cap hours TIS compliance schedule below:

  Table 4--Repetitive Inspections Times for Airplane Groups 1, 2, 3, 5, and 6 With ``Big Butterfly'' Plates and
                                               Lower Splice Plates
----------------------------------------------------------------------------------------------------------------
 When airplanes accumulate the following
  hours TIS on the wing front lower spar   Inspect within the following   Inspect thereafter at intervals not to
cap since the last inspection required in       hours TIS after the                    exceed. . .
              AD 2006-07-15,                effective date of this AD,
----------------------------------------------------------------------------------------------------------------
(i) Magnetic particle inspection:........  ............................  450 hours TIS.
    (A) 650 or more hours TIS............  (A) 50 hours TIS............
    (B) 375 through 649 hours TIS........  (B) 75 hours TIS............
    (C) Less than 375 hours TIS..........  (C) upon accumulating 450
                                            hours TIS.
(ii) Eddy Current inspection:............  ............................  625 hours TIS.
    (A) 900 or more hours TIS............  (A) 50 hours TIS............
    (B) 550 through 899 hours TIS........  (B) 75 hours TIS............
    (C) Less than 550 hours TIS..........  (C) upon accumulating 625
                                            hours TIS.
----------------------------------------------------------------------------------------------------------------


    Note 1:  Group 5 airplanes had P/Ns 20211-09 and 20211-11 
installed at the factory.

    (7) Repetitively inspect airplanes that incorporate Thrush 
Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8, 
2007, in its entirety that meet the conditions in paragraph (i)(4) 
of this AD. Follow the wing front lower spar cap hours TIS 
compliance schedule below:

Table 5--Repetitive Inspection Times for Airplanes With Thrush Aircraft,
 Inc. Custom Kit No. CK-AG-41, Revision A, Incorporated in its Entirety
------------------------------------------------------------------------
     When using the following      Repetitively inspect at intervals not
       inspection methods,                     to exceed. . .
------------------------------------------------------------------------
(i) Magnetic particle inspection.  900 hours TIS
(ii) Eddy current inspection.....  1,250 hours TIS.
------------------------------------------------------------------------

     (8) Initially replace the wing front lower spar caps, P/Ns 
20207-1, 20207-2, 20207-11, 20207-12, 20207-13, 20207-14, 20207-15, 
or 20207-16, at the times specified in Table 6 of this AD. 
Repetitively replace thereafter at the life limit times specified in 
Table 7 of this AD.

     Table 6--Initial Compliance Time for Wing Front Lower Spar Cap
                               Replacement
------------------------------------------------------------------------
                                   Replace the wing front lower spar cap
Total hours TIS on the wing front  upon accumulating the following hours
          lower spar cap               TIS on the spar cap after the
                                         effective date of this AD.
------------------------------------------------------------------------
(i) Group 1 with a radial engine   500 hours.
 and more than 15,000 hours TIS.
(ii) Group 1 with a radial engine  1,000 hours.
 and 12,000 to 15,000 hours TIS.
(iii) Group 1 with a radial        1,500 hours.
 engine and 9,000 to 11,999 hours
 TIS.
(iv) Group 1 with a radial engine  2,000 hours.
 and 7,400 to 8,999 hours TIS.
(v) Group 1 with a radial engine   Use Table 7(xxii).
 and less than 7,400 hours TIS.
(vi) Group 1 with a turbine        500 hours.
 engine and more than 14,000
 hours TIS.

[[Page 20441]]

 
(vii) Group 1 with a turbine       1,000 hours.
 engine and 11,000 to 14,000
 hours TIS.
(viii) Group 1 with a turbine      1,500 hours.
 engine and 8,000 to 10,999 hours
 TIS.
(ix) Group 1 with a turbine        2,000 hours.
 engine and 4,200 to 7,999 hours
 TIS.
(x) Group 1 with a turbine engine  Use Table 7(xxiii).
 and less than 4,200 hours TIS.
(xi) Group 2 with more than 9,000  500 hours.
 hours TIS.
(xii) Group 2 with 6,000 to 9,000  1,000 hours.
 hours TIS.
(xiii) Group 2 with 3,900 hours    1,500 hours.
 to 5,999 hours TIS.
(xiv) Group 2 with less than       Use Table 7(xxiv).
 3,900 hours TIS.
(xv) Group 3 and 6 with more than  500 hours.
 28,800 hours TIS.
(xvi) Group 3 and 6 with 27,800    1,000 hours.
 to 28,799 hours TIS.
(xvii) Group 3 and 6 with less     Use Table 7(xxv).
 than 27,800 hours TIS.
(xviii) Group 5 with more than     500 hours.
 8,000 hours TIS.
(xix) Group 5 with 5,000 to 7,999  1,000 hours.
 hours TIS.
(xx) Group 5 with 2,400 to 4,999   1,500 hours.
 hours TIS.
(xxi) Group 5 with less than       Use Table 7(xxvi).
 2,400 hours TIS.
------------------------------------------------------------------------


             Table 7--Wing Front Lower Spar Cap Life Limits
------------------------------------------------------------------------
                                          Replace wing front lower spar
                                           cap upon the accumulation of
             Airplane Group               the following hours TIS on the
                                                    spar cap:
------------------------------------------------------------------------
(xxii) Group 1 with a radial engine....  9,400 hours TIS.
(xxiii) Group 1 with a turbine engine..  6,200 hours TIS.
(xxiv) Group 2.........................  5,400 hours TIS.
(xxv) Groups 3 and 6...................  28,800 hours TIS.
(xxvi) Group 5.........................  3,900 hours TIS with original
                                          wing front lower spar cap P/N
                                          20207-11 or 20207-12.
                                         5,400 hours TIS after original
                                          wing front lower spar cap has
                                          been replaced with any P/N
                                          20207-xx wing front lower spar
                                          cap.
------------------------------------------------------------------------


    Note 2: There is evidence of sharp, uneven edges on the spar cap 
bolt holes that resulted from the manufacturing process in Group 5 
airplanes. Once the original spar caps are replaced, the life limit 
increases.

    (j) As previously stated in paragraph (f) of this AD, any Group 
3 airplane that has been modified with a hopper of a capacity more 
than 410 gallons, a piston engine greater than 600 horsepower, or a 
gas turbine engine greater than 600 horsepower, is a Group 1 
airplane for the purposes of this AD. Replace the spar caps using 
the following formulas.
    (1) For airplanes that were originally Group 3 airplanes and 
later modified by installing a piston engine of greater than 600 
horsepower and/or a hopper capacity of greater than 410 gallons, 
calculate the equivalent Group 1 hours TIS on each spar cap as 
follows:
[GRAPHIC] [TIFF OMITTED] TP04MY09.003

    (2) For airplanes that were originally Group 3 airplanes and 
later modified by installing a turbine engine of greater than 600 
horsepower, with or without installing a hopper with greater than 
410 gallon capacity, calculate the equivalent Group 1 hours TIS on 
each spar cap as follows:
[GRAPHIC] [TIFF OMITTED] TP04MY09.004

    (ii) Equivalent Group 1 hours TIS = 6,200 x Usage Factor
    (3) When the equivalent Group 1 hours TIS on the wing front 
lower spar cap equals the life limit of 9,400 hours TIS if a radial 
piston engine is installed or reaches 6,200 hours TIS if a turbine 
engine is installed, the wing front lower spar cap must be replaced. 
Use Table 6 if over the life limit.
    (4) See the appendix to this AD for examples of how to calculate 
the applicable life limit.
    (k) If any cracks are found during any inspection required by 
this AD, you must

[[Page 20442]]

repair the cracks or replace the wing front lower spar cap before 
further flight.
    (1) Use the cold work process to ream out small cracks as 
defined in Ayres Corporation Service Bulletin No. SB-AG-39, dated 
September 17, 1996, and deburr the bolt hole edges with the splice 
blocks removed after cold work is performed; or
    (2) If the crack is found in a 1/4-inch bolt hole, ream the 1/4-
inch bolt hole to 5/16 inches diameter as defined in Part I of Ayres 
Corporation Custom Kit No. CK-AG-29, dated December 23, 1997; or
    (3) Install Kaplan splice blocks, P/N 22515-1/-3 or 88-251, 
following Quality Aerospace, Inc. Custom Kit No. CK-AG-30, dated 
December 6, 2001; or
    (4) Replace the affected wing front lower spar cap following an 
FAA-approved procedure (the applicable maintenance manual contains 
these procedures) or replace both lower spar caps and the 
surrounding structure following Thrush Aircraft, Inc. Custom Kit No. 
CK-AG-41, Revision A, dated March 8, 2007. Although not mandatory, 
the FAA recommends installing Custom Kit No. CK-AG-41, Revision A, 
in its entirety. The additional structure provided in the custom kit 
will provide a greater level of safety than the minimum acceptable 
level of safety provided by replacing just the lower spar cap.
    (l) If a crack is found, the reaming associated with the cold 
work process may remove a crack if it is small enough. Some aircraft 
owners/operators were issued AMOCs with AD 97-17-03 to ream the \1/
4\-inch bolt hole to \5/16\ inches diameter to remove small cracks. 
Ayres Corporation Custom Kit No. CK-AG-29, Part I, dated December 
23, 1997, also provides procedures to ream the \1/4\-inch bolt hole 
to \5/16\ inches diameter, which may remove a small crack. Resizing 
the holes to the required size to install a Kaplan splice block may 
also remove small cracks. If you use any of these methods to remove 
cracks and the airplane is re-inspected before further flight and no 
cracks are found, you may continue to follow the repetitive 
inspection intervals for your airplane listed in paragraphs (i)(5), 
(i)(6), or (i)(7) of this AD.
    (m) For all inspection methods (magnetic particle or eddy 
current), hours TIS for initial and repetitive inspections intervals 
and wing front lower spar cap life limit start over when the wing 
front lower spar cap is replaced with a new P/N 20207-1, 20207-2, 
20207-11, 20207-12, 20207-13, 20207-14, 20207-15, or 20207-16. These 
wing front lower spar caps must be inspected as specified in 
paragraphs (i)(3), (i)(5), (i)(6), and (i)(7) of this AD.
    (1) If the wings or wing front lower spar caps were replaced 
with new or used wings or wing front lower spar caps during the life 
of the airplane and the logbook records positively show the hours 
TIS of the replacement wings or wing front lower spar caps, then 
initially inspect at applicable times specified in paragraph (i)(3) 
of this AD. Repetitively inspect thereafter at intervals specified 
in paragraphs (i)(5), (i)(6), or (i)(7) of this AD. Replace the wing 
front lower spar caps upon reaching the life limit specified in 
Table 7 of this AD.
    (2) If the wings or wing front lower spar caps were replaced 
with new or used wings or wing front lower spar caps during the life 
of the airplane and logbook records do not positively show the hours 
TIS of the replacement wings or wing front lower spar caps, then 
inspect within 50 hours TIS after the effective date of this AD, 
unless already done. Repetitively inspect thereafter at intervals 
specified in paragraphs (i)(5), (i)(6), or (i)(7) of this AD. 
Replace the wing front lower spar caps within 500 hours TIS after 
the effective date of this AD.
    (3) If both wing front lower spar caps are replaced by 
installing the entire Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, 
Revision A, dated March 8, 2007, then initially inspect at 2,000 
hours TIS as shown in paragraph (i)(3) of this AD. Repetitively 
inspect thereafter at intervals specified in paragraph (i)(7) of 
this AD. Replace the wing front lower spar caps at times specified 
in paragraph (i)(8) of this AD.
    (n) Any wing front lower spar cap that is removed and is at or 
beyond the replacement time specified in this AD must be disposed of 
following the procedures in 14 CFR Part 43.10.
    (o) Replacement times start over when the wing front lower spar 
cap is replaced with a new P/N 20207-1, 20207-2, 20207-11, 20207-12, 
20207-13, 20207-14, 20207-15, or 20207-16. These wing front lower 
spar caps are now life-limited parts and must be replaced upon the 
accumulation of the hours TIS specified in Table 7 of this AD.
    (p) Report any cracks you find within 10 days after the cracks 
are found or within 10 days after the effective date of this AD, 
whichever occurs later. Send your report to Cindy Lorenzen, 
Aerospace Engineer, ACE-115A, Atlanta ACO, One Crown Center, 1895 
Phoenix Blvd., Suite 450, Atlanta, GA 30349; telephone: (770) 703-
6078; facsimile: (770) 703-6097; e-mail: [email protected]. The 
Office of Management and Budget (OMB) approved the information 
collection requirements contained in this regulation under the 
provisions of the Paperwork Reduction Act and assigned OMB Control 
Number 2120-0056. Include in your report the following information:
    (1) Aircraft model and serial number;
    (2) Engine model;
    (3) Aircraft hours TIS;
    (4) Left and right wing front lower spar cap hours TIS;
    (5) Hours TIS on the spar cap since last inspection;
    (6) Crack location and size;
    (7) Procedure (magnetic particle, ultrasonic, or eddy current) 
used for the last inspection;
    (8) Description of any previous modifications and hours TIS when 
the modification was done, such as engine model change, installation 
of winglets, hopper capacity increase, cold working procedure done 
on bolt holes, or installation of butterfly plates; and
    (9) Information on corrective action taken or installation of 
Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated 
March 8, 2007, and when this corrective action was taken.

Special Flight Permits

    (q) Under 14 CFR part 39.23, we are limiting the special flight 
permits for this AD by the following conditions:
    (1) The hopper is empty;
    (2) Vne is reduced to 126 miles per hour (109 knots) indicated 
airspeed (IAS); and
    (3) Flight into known turbulence is prohibited.

Alternative Methods of Compliance (AMOCs)

    (r) The Manager, Atlanta Aircraft Certification Office, FAA, 
ATTN: Cindy Lorenzen, Aerospace Engineer, ACE-115A, Atlanta Aircraft 
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite 
450, Atlanta, GA 30349; telephone: (770) 703-6078; facsimile: (770) 
703-6097; e-mail: [email protected]; or Keith Noles, Aerospace 
Engineer, ACE-117A, Atlanta Aircraft Certification Office, One Crown 
Center, 1895 Phoenix Blvd., Suite 450, Atlanta, Georgia 30349; 
telephone: (770) 703-6085; facsimile: (770) 703-6097; e-mail: 
[email protected], has the authority to approve AMOCs for this 
AD, if requested using the procedures found in 14 CFR 39.19. Before 
using any approved AMOC on any airplane to which the AMOC applies, 
notify your appropriate principal inspector (PI) in the FAA Flight 
Standards District Office (FSDO), or lacking a PI, your local FSDO.
    (s) AMOCs approved for AD 2006-07-15, AD 2003-07-01, AD 2000-11-
16, AD 97-13-11, and/or AD 97-17-03 are approved as AMOCs for this 
AD except for those pertaining to ultrasonic inspection methods.

Related Information

    (t) To get copies of the service information referenced in this 
AD, contact Thrush Aircraft, Inc. at 300 Old Pretoria Road, P.O. Box 
3149, Albany, Georgia 31706-3149 or go to http://www.thrushaircraft.com. To view the AD docket, go to the U.S. 
Department of Transportation, Docket Operations, M-30 West Building 
Ground Floor, Room W12-140, New Jersey Avenue, SE., Washington, DC, 
or on the Internet at http://www.regulations.gov. The docket number 
is Docket No. FAA-2007-27862; Directorate Identifier 2007-CE-036-AD.

Appendix to Docket No. FAA-2007-27862

    The following are examples of calculating Equivalent Group 1 
hours.
    Example 1: S/N xxx was originally a Group 3 airplane; later it 
was modified with a Wright R-1820-71, 1200 horsepower, radial engine 
when the wing front lower spar caps had 15,700 hours TIS on them. 
The wing front lower spar caps have accumulated an additional 8,200 
hours since the engine conversion for a total of 23,900 hours TIS on 
the wing front lower spar caps.

Usage Factor = 15,700 hours/28,800 + 8,200 hours/9,400= 1.417 
Equivalent Group 1 hours = 9,400 x 1.417 = 13,320 hours.

    The spar caps will need to be replaced within the next 1,000 
hours TIS after the effective date of this AD as determined by Table 
6 for a Group 1 airplane with a radial engine with between 12,000 
and 15,000 hours TIS.

[[Page 20443]]

    ------Example 2: S/N yyy was originally a Group 3 airplane; 
later it was modified with a PT6A-34, 750 horsepower, turbine engine 
when the wing front lower spar caps had 5,300 hours TIS on them. The 
wing front lower spar caps now have 7,700 hours TIS.

Usage Factor = 5,300 hours/28,800 + (7,700 - 5,300)/6,200 = 0.571 
Equivalent Group 1 hours = 6,200 x 0.571 = 3,540 hours.

    The spar caps will need to be replaced at 6,200 Equivalent Group 
1 total hours TIS, which is within the next 2,660 hours TIS (6,200 - 
3,540=2,660).

    Issued in Kansas City, Missouri, on April 27, 2009.
Kim Smith,
Manager, Small Airplane Directorate, Aircraft Certification Service.
[FR Doc. E9-10162 Filed 5-1-09; 8:45 am]
BILLING CODE 4910-13-P