[Federal Register Volume 66, Number 208 (Friday, October 26, 2001)]
[Rules and Regulations]
[Pages 54111-54119]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-26951]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 95-NM-15-AD; Amendment 39-12485; AD 2001-22-06]
RIN 2120-AA64


Airworthiness Directives; Boeing Model B-17E, F, and G Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD), 
applicable to all Boeing Model B-17E, F, and G airplanes, that requires 
inspections to detect cracking and corrosion of the wing spar chords, 
bolts and bolt holes of the spar chords, and wing terminals; and 
correction of any discrepancy found during these inspections. This 
amendment is prompted by reports of cracking and corrosion of the wing 
spar. The actions specified by this AD are intended to prevent reduced 
structural integrity of the wing of the airplane due to the problems 
associated with corrosion and cracking of the wing spar.

EFFECTIVE DATE: November 30, 2001.

ADDRESSES: Information concerning this amendment may be obtained from 
or examined at the Federal Aviation Administration (FAA), Transport 
Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, 
Washington 98055-4056.

FOR FURTHER INFORMATION CONTACT: James G. Rehrl, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 
1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 
227-2783; fax (425) 227-1181.

SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal 
Aviation Regulations (14 CFR part 39) to include an airworthiness 
directive (AD) that is applicable to all Boeing Model B-17E, F, and G 
airplanes was published in the Federal Register on March 16, 1995 (60 
FR 14233). That action proposed to require inspections to detect 
cracking and corrosion of the wing spar chords, bolts and bolt holes of 
the spar chords, and wing terminals; and correction of any discrepancy 
found during these inspections.
    Of the approximately 12,600 Boeing Model B-17E, B-17F, and B-17G 
bombers produced during World War II, only about a dozen remain in 
operation. Since the last B-17 was completed in April 1945, each is now 
at least 56 years old. Those remaining are flown primarily in various 
forms of airshow displays.

Comments

    Interested persons have been afforded an opportunity to participate 
in the making of this amendment. Due consideration has been given to 
the comments received.

Requests To Withdraw Proposed Rule

    Many commenters contend that the proposed AD is unjustified and 
that it should be withdrawn accordingly. The commenters present various 
reasons for this request.
    Several commenters assert that cracking in the spar chord is not a 
safety issue because no wing or structural failures, incidents, or 
accidents have resulted from the cracking addressed by the proposed AD. 
One commenter states that the documented support for the necessity of 
the proposed AD (as described in the proposal) is flawed and without 
technical or event-based merit. Another states that no proper basis or 
need for the issuance of an AD has been established.
    Several commenters also refer to B-17s flying with known cracks 
without incident, some of which are subject to an unspecified type of 
inspection. One commenter notes that cracks were present in some B-17s 
during World War II, and limits on the degree of cracking that was 
acceptable were described in the Structural Repair Manual. The same 
commenter notes that battle damage was corrected with strap or angle 
reinforcements. Another commenter reports finding corroded or cracked 
spars on several airplanes under major restoration, and on one that ran 
off a runway into a ravine, consequently requiring major repairs. The 
commenter indicates that, despite the extreme conditions that this 
latter airplane encountered, and the implied severity of the spar 
cracks, no components failed. One commenter reports inspecting the 
cracks on a particular B-17 and noticing surface corrosion in the 
cracked area of one B-17. The commenter concludes that since corrosion 
takes a period of time (sometimes years) to form, the cracks must have 
been there for several years. Another commenter reports that a hairline 
crack was observed in the left wing of an airplane in 1979, and that 
there has been no change or increase in the size of the crack during 
years of subsequent flying. (The commenter did not specify which 
structural member contained the crack.) The commenter indicates that a 
B-17 engineer indicates that there is no safety problem with hairline 
cracks.
    The FAA acknowledges that no accidents are known to have occurred 
as a result of the conditions addressed by the proposed AD. 
Nevertheless, the FAA, as well as the operators, are aware of cracks in 
the wing spar chords of certain B-17 airplanes. To date five of the B-
17s either flying or capable of being restored to flight status are 
known to have cracks in their wing spar chords. The FAA has determined 
that there is no design feature to prevent the crack propagation from 
becoming transverse and severing the spar chord. The integrity of this 
structure is, therefore, essential for continued safe flight and 
landing.
    Several commenters point to the service history of the B-17 as 
evidence that the proposed actions are not necessary. A few commenters 
state that, in proposing this rule, the FAA failed to take into account 
the ruggedness of the B-17, and they reference occurrences during World 
War II in which some B-17s returned with all four spars broken

[[Page 54112]]

as a result of combat damage. One commenter states that the reason for 
the airplanes being able to return safely in spite of the degree of 
damage is that 90 percent of the wing strength is in the skin and ribs 
of the airplane.
    Several commenters justify their requests to withdraw the proposal 
based on the fact that the current usage of the airplane is far less 
demanding--in terms of weights, altitudes, and environments--than the 
conditions encountered during wartime operations. Several commenters 
note that none of the subject airplanes fly at gross weight, with most 
of them, according to one commenter, flying at 10,000 to 15,000 pounds 
under gross. One commenter states that the airplanes subject to the 
proposed AD are flown only 50 to 250 hours per year. Additionally, the 
commenters assert that the current pilots of these airplanes are more 
schooled and proficient than those flying them 50 years ago.
    Several commenters also cite the excellent maintenance record on 
the B-17s as a reason that the proposal should be withdrawn. They point 
out that the subject airplanes are under ``constant surveillance,'' and 
are well maintained. The commenters also suggest that the remaining B-
17s are better maintained now than when they were new, with many of 
them having been completely restored and many of them being hangared 
during the airshow off-season.
    The FAA concurs with the commenters' assertions that the subject 
airplanes are operating in environments much more favorable than those 
encountered during World War II. In addition, the FAA recognizes that, 
for the most part, these airplanes are meticulously maintained. 
However, the FAA does not concur with the request to withdraw the 
proposal based on the conditions in which B-17s operate today, because 
such conditions are only partially relevant. Of much greater 
significance are the conditions to which any particular airplane has 
been exposed over its life-span. While most B-17s may be hangared and 
well-maintained now, most, if not all, of the affected airplanes have 
been exposed to years of grueling operations such as fire-fighting, 
aerial application, etc. Furthermore, even if the airplanes had been 
hangared continuously since World War II, moisture could accumulate 
from condensation. In fact, most of the subject airplanes have spent 
much of their life-span in open storage with no particular protection 
from the elements.
    One commenter indicates that applicable military technical orders 
(the basis to which these aircraft are maintained) allow flights with 
known cracks in the spar chord if the cracks meet specified criteria. 
The commenter reports that this allowance has been validated by combat 
operations, current usage of the airplanes, and the type certificate.
    Contrary to the commenters' assertions, continued flight with known 
structural defects, such as those addressed by the proposed AD, is 
considered a violation of section 91.7 of the Federal Aviation 
Regulations (14 CFR 91.7), which requires the pilot in command to 
discontinue a flight when an unairworthy structural condition occurs. 
The FAA finds that a military technical order written almost 60 years 
ago during wartime conditions (when emphasis was placed on short-term 
airworthiness risks as opposed to long-term risks such as fatigue and 
corrosion) is not an appropriate basis for allowing continued flight 
with cracks of this nature. The FAA also is not aware of any specific 
FAA approval, either directly or by reference, of a military technical 
order that allowed continued flight operations for B-17s with 
unrepaired cracked spar chords. In any event, this AD would supersede 
such an approval.
    One commenter justifies its objection to the proposed rule on the 
fact that B-17s are not operated for hire. (The Limited Category type 
certification basis prohibits using B-17 airplanes for carriage of 
passengers or cargo for compensation or hire.) The FAA infers that the 
commenter is implying that a lesser safety standard is therefore 
acceptable. The FAA does not concur with the commenter's justification. 
The corrective action specified in this AD is needed to ensure the 
safety of not only the crew members and any other persons on board, but 
also of the many spectators that are in proximity to the affected 
airplanes as they participate in airshows.
    Several commenters report that removal of the wings requires 
significant disassembly and express concern that such removal could 
reduce the structural integrity of the spar chord-to-terminal fitting 
joints. Two commenters state that it has not been determined that these 
cracks reduce the structural integrity of the wing assembly. One 
commenter states that replacement of used aircraft hardware with new 
hardware will affect the aircraft's ``preset'' and ``harmonics,'' and 
may establish a stress concentration, which would reduce the integrity 
of the aircraft.
    The FAA does not concur. The wings have already been removed and 
repaired on at least three B-17 airplanes. The FAA has received no 
comments indicating the removal and subsequent reinstallation of the 
wings reduced the structural integrity of those airplanes. 
Nevertheless, wing removal is not required in all instances, as 
discussed below. No change to the final rule in this regard is 
necessary.

Clarification of Discussion Section of Proposed Rule

    Certain commenters request clarification and correction of language 
that appears in the Discussion section of the preamble to the notice of 
proposed rulemaking (NPRM). One commenter presents an analysis of the 
Discussion section, which includes a number of questions and 
suggestions for editorial changes. The commenters specifically request 
that the FAA correct certain language related to the description of the 
wing spar chord to wing terminal fitting joint. One commenter asks for 
clarification regarding the description of the wing spar chord to wing 
terminal fitting through bolts being ``seized'' in the joint. 
Additionally, the commenters request correction of the discussion of 
spar loading that appeared in the NPRM. Additionally, the commenters 
pose various questions, such as:

--When was the cracking problem discovered by the FAA?
--On how many airplanes was the cracking discovered?
--How many cracked spars have been found?
--How was the cause of the bolt corrosion and spar chord cracking 
attributed to moisture entrapment? Was the moisture accumulation 
observed or ``is this a guess?''

    The FAA finds that clarification of these issues is necessary. The 
commenters note correctly that spar chords mate with the cylindrical, 
tapering inner wing attach fitting inserts. Each of the eight joints is 
held together by eight close-tolerance bolts. The FAA was informed of 
the cracking of the wing spar chord and corrosion of these bolts on 
April 26, 1994. One B-17 had been inspected at that time, and 
approximately one-third of the 64 bolts in the eight joints were 
replaced due to corrosion. At least two bolts had lost almost half of 
the cross-sectional area. Some of the eight spar chords were cracked, 
and one chord end was broken into pieces. Since receiving that report, 
the FAA has learned that cracks have been discovered in the wing spar 
chord-to-wing terminal fittings of five of the 12 airplanes either 
flying or capable of being restored to flight status.
    The FAA notes that cracks have propagated to observed lengths 
greater

[[Page 54113]]

than seven inches. As the cracks propagate outboard into the region of 
increasing longitudinal tensile and compressive stresses, there is no 
design feature to prevent the crack propagation from becoming 
transverse and severing the spar chord. Because this area is subject to 
high axial loads and this structure is necessary for the continued safe 
flight of the airplane, cracking in this area is critical.
    Evidence that the bolt corrosion and spar chord cracking were due 
to moisture entrapment came from several sources. The first operator to 
report this condition found corrosion of the joint bolts and the spar 
chords. By design, the spar chord tubes are open at the outboard end, 
and the presence of the wing terminal fittings inside the spar chords 
traps water at the inboard ends. Cracks known to date run 
longitudinally along the spars, which indicates that circumferential 
loads are cracking the spars. Pressure from corrosion products between 
the spar chord-to-terminal joints would create such circumferential 
loads.
    Commenters correctly note that the bolts in these fittings are not 
seized. Rather, moisture trapped in the inner wing spars has caused 
some of the bolts to corrode, which makes removal difficult.
    Since the Discussion section of the preamble of an NPRM is not 
restated in a final rule, no change to this final rule is necessary in 
this regard.

Questions Concerning Applicability of Proposed Rule

    One commenter asserts that all B-17 aircraft with large, visible 
cracks were built by Douglas, and all had history of damage or severe 
use. The commenter states that Vega- and Boeing-built B-17s do not have 
a problem with cracking.
    The FAA infers from these remarks that the commenter requests that 
Vega- and Boeing-built airplanes be excluded from the applicability of 
this AD. The FAA does not concur. The FAA notes that, of the 
approximately 12,600 Model B-17E, B-17F, and B-17G airplanes produced, 
nearly 3,000 were manufactured under license by Douglas, and 
approximately 2,750 were manufactured under license by Vega, a 
subsidiary of Lockheed.
    The dozen or so airplanes still in operation--only about one of 
every 1,000 produced--comprise a statistically insignificant sample; 
therefore, no conclusions can be drawn statistically from the origin of 
the particular airplanes in which cracks have been discovered. 
Additionally, the commenter fails to present any evidence, such as 
differences in design or production methods, that would suggest 
airplanes manufactured by Boeing or Vega are less likely to experience 
the unsafe condition addressed by this AD. Further, the FAA is not 
aware of any such differences. No change to the applicability of this 
final rule is necessary.
    Another commenter requests that the applicability of the proposed 
AD exclude certain airplanes that have already undergone wing removal, 
removal of terminals, replacement of close tolerance bolts, and repair 
of spar tubes.
    The FAA does not concur that a general exclusion should be made for 
those airplanes since the previous actions accomplished on those 
airplanes may not provide the necessary level of safety. Operators have 
not submitted formal documentation to the FAA describing such previous 
actions, and so cannot establish that any actions accomplished 
previously on these airplanes definitively meet the criteria of this 
AD. In addition, it appears likely that there may be repairs 
accomplished previously, such as stop-drilling of cracks found in the 
spar chords, that do not adequately address the unsafe condition.
    However, paragraph (d) of this final rule provides operators with 
the opportunity to present the FAA with data to justify approval of an 
inspection or repair accomplished previously as an alternative method 
of compliance. This provision enables the FAA to review such 
inspections and repairs and determine whether further action is 
necessary. Also, NOTE 2 of this AD states specifically that operators 
of airplanes on which the terminal fitting-to-spar chord joint was 
separated prior to the effective date of this AD, and on which 
inspection(s) of and/or repair(s) to the wing terminals-to-spar chords 
were accomplished prior to the effective date of this AD, should submit 
requests for approval of alternative methods of compliance to the FAA.

Question Concerning Cause of Cracking

    Several commenters question whether the cracks have been caused by 
corrosion. The commenters state there is no documented proof that 
corrosion between the steel wing terminal fitting and the aluminum spar 
chord is the cause of the cracking. Several commenters state that the 
cracks are due to operational abuses (e.g., heavy landings, operating 
above gross weights). Another commenter states that the cracks known to 
be present on B-17s have not been attributed to any single cause. That 
commenter states that environmental stresses (i.e., temperature changes 
between the aluminum spar and the steel trunnion) contributed to the 
cracking. One commenter states that moisture accumulation and 
consequent corrosion cannot be the cause of the cracking addressed by 
this AD because most B-17 owners store their airplanes indoors where 
moisture cannot accumulate on the spars. Other commenters suggest that 
observed cracking is due to a reported manufacturing procedure in which 
the terminal fittings, as well as the spar chord-to-terminal fitting 
bolts, were driven into place with hammers.
    The FAA clarifies that cracking that has been discovered is not 
consistent with the damage that would result from overstresses such as 
those suggested by the commenters. However, on the other hand, the 
cracking is consistent with the pressure that would result from 
products of corrosion in the joints. The FAA finds that the 
longitudinal nature of the cracks discovered so far is indicative of 
expansion due to corrosion products in the spar chord to terminal 
fitting joints. It should be noted that the wing terminal fittings are 
steel, while the spar chords are constructed of aluminum. Because steel 
and aluminum are dissimilar metals, aluminum will tend to galvanically 
corrode if in direct contact with steel, as it is in the B-17 design. 
The faying surfaces of these joints have not been the subject of 
routine maintenance inspections because of the age of the subject 
airplanes.

Requests Concerning Separation of Wing Spar Chord-to-Wing Terminal 
Joint

    Several commenters indicate that separation of the wing spar chord-
to-wing terminal joint is unnecessary, and that the proposed 
requirement to remove all 64 bolts in the eight wing spar chord-to-wing 
terminal joints is likewise unnecessary. These commenters offer various 
proposals with regard to alternative inspection and repair procedures 
and compliance times, which are discussed in the following paragraphs.
    Several commenters request that the FAA change the requirements to 
remove the most inboard bolt in each wing spar chord joint and to 
remove all 64 bolts, as specified in proposed paragraphs (a)(2) and 
(b)(2)(i), respectively, so that the three most inboard fasteners in 
each joint would not have to be removed. One commenter states that the 
most inboard bolt in each of the eight wing spar chord-to-wing terminal 
joints should not be removed due to interference with other wing 
structure and the fact that the bolt is only \5/8\ inch

[[Page 54114]]

from the end of the spar. Some commenters state that the three most 
inboard bolts should not be removed for the reason mentioned previously 
(for the most inboard bolt), and because removal of the next two most 
inboard bolts would necessitate disassembly of a wing rib to access 
those bolts.
    The FAA finds that some commenters were apparently misled by the 
preamble of the proposed AD as to whether the inspections specified in 
paragraph (b) of the AD could be accomplished without actually 
separating the wing spar-to-wing terminal joint.
    The FAA acknowledges that significant disassembly would be required 
to remove the three most inboard bolts on the front and rear spars. The 
FAA clarifies that the intent of paragraph (b)(2) of the proposed rule 
(designated as paragraph (b)(2)(i) of this final rule) is that the use 
of equivalent inspections that do not involve separating the terminal 
fitting from the spar chord to detect cracking and corrosion may be 
acceptable. The FAA has determined that an acceptable level of safety 
can be achieved without removing the three most inboard bolts of a 
joint provided: (1) The dye penetrant inspection of the spar-chord 
tube-end reveals no cracks; (2) the other five bolts are removed and an 
eddy current inspection verifies that the holes are free of cracks; and 
(3) a borescope inspection using 10-power magnification reveals that 
the first, second, and third most inboard bolts are free of corrosion. 
These inspections must be performed on a repetitive basis at 36-month 
intervals. Paragraph (b) of this final rule has been reformatted, and 
this new alternative procedure is specified in paragraph (b)(2)(ii) of 
this AD.
    Further, the FAA has made editorial changes to paragraphs 
(b)(2)(ii), (b)(2)(ii)(B), and (b)(2)(ii)(C) of the final rule to more 
clearly specify which bolts are being referred to in those paragraphs.
    In addition, the FAA has determined that the requirement to perform 
the high frequency eddy current inspection in accordance with paragraph 
(a)(2) of the proposed AD (which included removing the most inboard 
bolt during the initial inspection) can be omitted from this AD without 
unduly affecting aviation safety, since this inspection is adequately 
addressed by paragraph (b) of this AD. Therefore, paragraph (a) of this 
AD has been re-structured and re-numbered accordingly.
    One commenter that has accomplished extensive repairs on a Model B-
17 airplane in the area that is the subject of this AD states that 
separating the terminal fitting from the spar chord is the only method 
that will adequately address the unsafe condition (corrosion and 
cracking of the wing spar, which could result in reduced structural 
integrity of the wing of the airplane). The FAA infers that this 
commenter is requesting that the FAA revise paragraph (b)(2) of the 
proposed AD to eliminate reference to alternative inspection procedures 
that may not include separating the terminal fitting from the spar 
chord.
    The FAA partially concurs with the commenter's request. The FAA 
concurs that inspections that involve separating the terminal fitting 
from the spar chord are required for all airplanes with cracks that are 
unacceptable for repair. The FAA points out that it also has not 
approved any alternative inspection procedures for airplanes that have 
no cracks or repairable cracks. The FAA also points out that this AD 
does not grant blanket approval for alternative inspection procedures. 
All inspections in accordance with this AD are required to be 
accomplished in accordance with a method approved by the FAA.
    However, the FAA does not concur that inspections must include 
separation of the terminal fitting from the spar chord. The FAA finds 
that it may be possible, depending on the degree of cracking detected, 
for alternative inspection procedures to provide an acceptable level of 
safety, even if such procedures do not involve separating the terminal 
fitting from the spar chord. Paragraph (b)(2)(i) of this AD specifies 
that alternative inspection procedures must meet certain minimum 
requirements, which are specified in paragraphs (b)(2)(i)(A), 
(b)(2)(i)(B), and (b)(2)(i)(C) of this AD. However, the FAA does not 
have the resources to develop these procedures for operators. No change 
to the final rule is necessary in this regard.

Requests Concerning Proposed Compliance Time

    One commenter requests that the proposed 18-month compliance time 
specified in paragraph (b) of the AD be changed to allow the 
inspections and any needed repairs to be performed during the winter 
months away from the airshow season.
    The FAA does not concur that the compliance time should be revised. 
An 18-month compliance time, as proposed, does allow compliance during 
the winter months; therefore, no change to paragraph (b) is necessary 
in that regard.
    However, in light of the concern raised by the commenter, the FAA 
has determined that the compliance time for the initial inspections 
specified in paragraph (a) of this AD can be changed to 180 days 
without a significant adverse effect on aviation safety. Paragraph (a) 
of this AD has been revised accordingly.

Discussion of Repairs

    One commenter suggests that each spar chord should be treated with 
corrosion inhibitor after bolt removal, replacement, or remedial 
action. The FAA infers that the commenter is requesting that a 
requirement for application of corrosion inhibitor be added to 
applicable paragraphs in the final rule.
    The FAA concurs with the commenter's suggestion that each spar 
chord should be treated with corrosion inhibitor. Therefore, paragraph 
(a)(2) of the final rule (formerly paragraph (a)(3) in the NPRM) has 
been revised to include a requirement for application of a corrosion 
inhibitor as suggested. The FAA has determined that such a requirement 
will increase the long-term corrosion resistance characteristics of the 
affected structure without imposing a significant burden on the 
operators of the affected airplanes.
    One commenter requests that the FAA require that repairs be 
performed in accordance with published repair manuals for the B-17. For 
those repairs not covered by a published repair manual, the commenter 
believes that repairs should be accomplished with the aid of FAA 
Designated Engineering Representatives (DER) or other recognized 
experts.
    The FAA does not concur. All repairs required by this AD must be 
approved by the Manager of the Seattle Aircraft Certification Office, 
FAA, regardless of whether those repairs are addressed in a published 
B-17 repair manual. Although a World War II-era repair manual may be of 
some assistance in that regard, it must be recognized that the value of 
such a manual is very limited. The primary concern was short-term 
airworthiness; that is, that an airplane was to be repaired 
sufficiently to safely complete further combat missions. Long-term 
considerations, such as fatigue and corrosion, were secondary.
    The FAA also recognizes that there have been considerable advances 
in repair and corrosion-prevention practices over the last half-
century. As suggested by the commenter, the FAA encourages review of 
any needed repair by an appropriately qualified DER since that would 
undoubtedly hasten FAA approval of the repair. (Because the repair 
would be related to compliance with an airworthiness directive, a DER 
would be authorized only to

[[Page 54115]]

recommend its approval.) However, no change to this final rule is 
necessary in this regard.
    One commenter requests that the final rule be revised to require 
replacement of bolts only ``as needed,'' rather than requiring 
replacement of any corroded bolt. The commenter states that it has 
accomplished a repair that involved removal of the wings and the 
terminal attach fittings. In the course of the repair, approximately 
one-third of the wing terminal-to-spar bolts were found to be corroded 
to the point where replacement was required. However, the commenter 
points out that there was no corrosion of the shear plane of any bolt. 
Based on the commenter's statements, the FAA infers that the commenter 
is requesting that the final rule be revised to require replacement of 
bolts only if corrosion is found at the shear plane area.
    The FAA does not concur with the commenter's request. The FAA finds 
that it would be inappropriate to allow a bolt found to be corroded to 
remain installed on an airplane. The FAA has determined that bolts in 
wing spar chord-to-wing terminal joints are critical to the safety of 
flight; therefore, those bolts must be free of discrepancies, including 
corrosion. In addition to the criticality of the bolts to flight 
safety, the bolts must be removed to be inspected fully, and the FAA 
has determined that it is more cost effective for operators to replace 
corroded bolts with new bolts, rather than to perform frequent 
repetitive inspections of corroded, or corrosion-reworked, bolts. No 
change to the final rule is necessary in this regard.
    In lieu of repairing any cracks found, one commenter requests that 
the FAA allow operators to attach 4130 steel straps to the outside of 
the wing using existing rivet holes. The FAA does not concur that this 
would be an acceptable alternative because steel straps fastened to the 
outside of the wings would not provide adequate load paths for the 
spar-chord loads. No change to the final rule is necessary in this 
regard.

Economic Considerations

    Some commenters question the cost impact information presented in 
the preamble of the NPRM. These commenters take offense to assumptions 
made in that section that ``no operator has yet accomplished any of the 
proposed requirements'' and that ``no operator would accomplish those 
actions if this AD were not adopted.'' One commenter states that all 
owners/operators have already voluntarily undertaken inspections and 
repairs as a community. The commenter adds that results of those 
inspections revealed that virtually all cracks have been discovered 
using detailed visual inspection and non-destructive test (NDT) 
inspection methods that did not involve de-mating of the spar/wing 
terminal. Other commenters also submit information concerning 
previously accomplished inspections and corrective actions.
    The FAA finds that clarification of language presented in the cost 
impact information of this AD is necessary. The FAA and other federal 
agencies are required to propose or adopt a regulation only upon 
reasoned determination that the benefits of the intended regulation 
justify its costs. The two assumptions mentioned above merely represent 
a degree of conservatism taken by the FAA in determining that this AD 
will, in fact, be cost effective. They are in no way intended to be 
judgmental of what a particular operator would or would not do in the 
absence of this AD.
    Nevertheless, the FAA has not been provided with specific data 
indicating that any of the previously accomplished repairs and 
inspections provide the level of safety intended by this AD to 
adequately address the identified unsafe condition. It also must be 
recognized that, in the absence of an AD, operators of B-17s would not 
be required to perform the needed inspections and repairs. If there is 
no binding requirement to do so, the statutory responsibility of the 
FAA to ensure the safety of the occupants of those airplanes and 
persons on the ground watching the airplanes during airshows would not 
be fulfilled.
    Another commenter states that the statement in the proposal that 
indicates the proposed AD ``would not have a significant economic 
impact on a substantial number of small entities'' is inconsistent with 
the estimated cost of $90,000 per airplane.
    The FAA notes that the phrase referenced by the commenter refers to 
a statutory requirement imposed by the Regulatory Flexibility Act. That 
act is intended to protect small businesses and organizations from 
federal rulemaking by requiring agencies to develop and analyze 
information concerning the effect of rules on small entities. When the 
effects of a rule are likely to be ``significant'' on a ``substantial 
number of small entities,'' the agency is expected to take steps that 
will reduce the burden. Regarding regulatory flexibility findings in 
conjunction with the requirements of ADs, very few ADs will ever reach 
the level of having a ``significant economic impact, positive or 
negative, on a substantial number of small entities,'' since either 
most aircraft operators do not meet the agency's criteria for small 
entities, or because the cost of an individual AD usually does not 
exceed the agency limit for significant impact (which is $100 million 
per year). A statement concerning the impact, or lack of it (as in the 
case of this AD), is required to be included in the certification 
statement of each AD.
    Some commenters state that they cannot afford to separate the wing 
spar chord-to-terminal joints to perform the inspection. The commenters 
state that the AD, as proposed, would create severe economic hardship, 
result in grounding of airplanes, and force the sale of non-flying 
airplanes at a financial loss. One commenter acknowledges that the cost 
estimates fall within federal guidelines for a rule that is ``not a 
significant impact;'' however, the commenter contends that, for the 
most part, these airplanes are owned by non-profit organizations that 
do not have $90,000 in discretionary funds. Another commenter states 
that, under the circumstances, issuance of a precautionary 
manufacturer's service bulletin or an FAA Advisory Circular would be 
more than adequate.
    The FAA recognizes the economic impact of the proposed rule. 
However, the FAA notes that an unsafe condition exists in regard to the 
integrity of the affected joints, which are essential for safe flight. 
The FAA also points out that, as explained previously, paragraph (b)(2) 
of the AD provides operators the option of performing an alternative 
inspection without separating the joints. The FAA expects that costs 
for accomplishment of the alternative inspection will likely be lower 
than $90,000 per airplane.
    Some commenters believe that the cost of compliance will be much 
greater than the estimated $90,000 per airplane. One commenter states 
that a consensus of affected owners/operators is that the wing spar/
terminal de-mate would require 2,250 to 2,500 work hours. The commenter 
notes that this requirement entails removal and reinstallation of four 
engines, and complete de-rigging of the control, electrical wiring, 
engine control, and fuel systems. Therefore, the commenter estimates 
that costs would be from $125,000 to $150,000 per airplane and a four-
to six-month cessation in aircraft financial support activities.
    However, one commenter that has actually performed the alternative 
inspections outlined in paragraph (b) of the proposed AD states that 
the cost impact was much lower than the estimated amount.

[[Page 54116]]

    The FAA finds that no change to the cost impact information, below, 
is necessary. The FAA based the cost impact information presented in 
this AD on the best data available to date for airplanes on which the 
wing spar chord-to-wing terminal fitting separation has been 
accomplished. Although the costs may vary somewhat, the actual cost for 
a particular airplane is not expected to differ greatly from the 
estimated cost of $90,000 per airplane.

Issues Related to Inspection Methods and Procedures

    One commenter proposes an alternative to the inspection 
requirements of paragraph (a) of the proposed AD. The commenter 
suggests that within 90 days, and annually thereafter, a dye penetrant 
check be accomplished on the inboard butt of each of the eight spar 
tubes. The commenter adds that within 12 months, and tri-annually 
thereafter, the interior of all eight spar tubes should be treated with 
a moisture and corrosion inhibitor.
    The commenter also proposes that within 12 months, and thereafter 
at 10-year or 1,000-flight-hour intervals, a detailed inspection 
designed to detect cracking in the wing spar tubes or terminal bolt 
holes should be accomplished. This inspection would include removal and 
inspection of the terminal attach bolts at the fifth and seventh most 
inboard locations. The commenter also suggests that operators should 
inspect annually and report on the status, migration (or lack thereof), 
and condition of any cracks determined to be within acceptable 
tolerance criteria.
    The FAA does not concur with this request. Since corrosion is 
believed to be the cause of the cracking, the FAA finds that the 
proposed inspection program at the intervals suggested by the commenter 
would not ensure such timely detection of cracking. In addition, an 
inspection interval based on flight hours is inappropriate because 
damage resulting from corrosion is related to calendar time, not flight 
time.
    Two commenters indicate that no bolts should be removed during the 
inspections required by this proposed AD unless there is obvious damage 
to the bolt.
    One of these commenters states that no bolts should be removed 
``without due cause,'' because the bolts have been in the holes of the 
joint for more than 50 years, and molecular transfer will have taken 
place between the mating surfaces. The commenter asserts that 
replacement of the bolts is likely to cause reduced structural 
integrity of the wing terminal-to-spar joints.
    Another commenter states that replacement of hardware or parts from 
an airplane with new parts or hardware changes the harmonics of the 
airplane's vibration frequency and establishes a stress point at the 
location of the replacement. This commenter states that the engineers 
and master mechanics consulted did not recommend replacement of 
hardware unless major damage is detected during a visual inspection, 
because the stress created by removal could cause significant damage.
    The FAA infers that these commenters are requesting that paragraph 
(b) of the proposed rule be revised to eliminate the requirement to 
remove the bolts that join the wing terminals to the spar chords.
    The FAA does not concur with the request to eliminate the 
requirement to remove the bolts. The FAA has determined that performing 
inspections of the bolts and bolt holes without removing the bolts does 
not ensure that corrosion or cracking would be detected. The FAA finds 
that, to ensure the continued safety of the fleet of airplanes, it is 
necessary to require at least a one-time removal of five of the bolts 
in each joint to inspect the shear planes of the bolts for corrosion 
and to inspect the bolt holes for cracks. However, as discussed 
previously, the FAA has revised this final rule to allow for the three 
inboard fasteners in each joint to remain in place, provided that 
certain conditions are met.
    One commenter inquires as to what eddy current inspection methods 
are approved. The FAA is unaware of any military or industry standards 
for eddy current inspections. As stated in paragraph (b)(1) of this AD, 
eddy current inspections must be approved by the Manager of the Seattle 
Aircraft Certification Office, FAA. The manufacturer, The Boeing 
Company, has agreed to allow its specifications to be used for the eddy 
current inspections. The FAA has added a new NOTE 9 to this final rule 
to indicate that this information is available to operators as needed.
    One commenter requests that the FAA develop criteria containing 
acceptance/rejection standards of cracks characterized (by the 
commenter) as insignificant, monitorable, and unacceptable. The 
commenter believes that ``blanket condemnation of any cracking is 
unwarranted.''
    The FAA does not concur that continued flight with any cracking is 
acceptable. As specified in paragraph (c) of this final rule, any 
cracking discovered as a result of the required inspections must be 
repaired prior to further flight. Such repairs may or may not require 
separating the wing spar chord-to-wing terminal joint, depending upon 
the severity of the cracking. However, if cracking is found and 
repaired without separating the wing spar chord-to-wing terminal joint, 
repetitive inspections would be required. The FAA expects that the 
operator would propose its inspection program as part of the 
documentation needed to secure approval of the proposed repair, in 
accordance with paragraph (c) of this AD. Continued crack growth 
following repair requires separation of the wing spar chord-to-wing 
terminal joint in order to positively address continued cracking 
problems. No change to the final rule is required in this regard.
    Two commenters question the reference to ``acceptance/rejection 
criteria contained in sensitivity level Group IV, MIL-I-25135'' which 
is contained in paragraph (a)(1) of the proposed rule. One commenter 
notes that the referenced military specification does not contain 
acceptance/rejection criteria pertaining to cracks, nor was the 
specification intended to do so.
    The FAA finds that clarification is necessary. The commenter 
correctly notes that the military specification cited in the proposed 
rule does not contain acceptance/rejection criteria on cracking. The 
FAA clarifies that the intent of paragraph (a)(1) is that the dye 
penetrant inspection be performed in accordance with MIL-STD-6866, 
using a fluorescent Type 1 penetrant, Method C, Sensitivity Level 3, 
inspection. Any cracking that is detected must be repaired in 
accordance with a method approved by the Manager of the Seattle 
Aircraft Certification Office, FAA, prior to further flight. To 
eliminate any confusion in this regard, the wording of paragraph (a)(1) 
of this final rule has been revised accordingly.
    In addition, the FAA recognizes that a variety of dye penetrant 
inspection procedures may be acceptable. Therefore, the FAA has added 
Note 4 to the final rule to clarify that operators wanting to use an 
alternative procedure may request approval from the Seattle Aircraft 
Certification Office per the provision of paragraph (d) of this AD.

Clarification of Visual Inspections

    The FAA has revised the final rule to clarify that the type of 
visual inspection required by paragraphs (a)(2), (b)(1), and 
(b)(2)(i)(C) is a ``detailed visual inspection.'' Further, the 
definition of this inspection has been included in a new Note 8 of the 
final rule.

[[Page 54117]]

Addition of Other New Notes

    The FAA has also revised the final rule to include new Notes 1 and 
11:
    As a result of communications with the Air Transport Association 
(ATA) of America, the FAA has learned that, in general, some operators 
may misunderstand the legal effect of ADs on airplanes that are 
identified in the applicability provision of the AD, but that have been 
altered or repaired in the area addressed by the AD. The FAA points out 
that all airplanes identified in the applicability provision of an AD 
are legally subject to the AD. If an airplane has been altered or 
repaired in the affected area in such a way as to affect compliance 
with the AD, the owner or operator is required to obtain FAA approval 
for an alternative method of compliance with the AD, in accordance with 
the paragraph of each AD that provides for such approvals. Therefore, a 
new Note 1 has been added to this final rule to clarify this long-
standing requirement.
    In addition, a new Note 11 has been added to the final rule to 
strongly encourage owners and operators of the affected airplanes to 
coordinate their requests for approvals of alternative methods of 
compliance or adjustment of the compliance times pertaining to this AD. 
Coordination of a single request (in lieu of a separate request from 
each owner/operator) will allow the FAA to more quickly review and 
respond.

Conclusion

    After careful review of the available data, including the comments 
noted above, the FAA has determined that air safety and the public 
interest require the adoption of the rule with the changes previously 
described. The FAA has determined that these changes will neither 
increase the economic burden on any operator nor increase the scope of 
the AD.

Cost Impact

    There are approximately 12 airplanes of the affected design in the 
worldwide fleet. The FAA estimates that 10 airplanes of U.S. registry 
will be affected by this AD, that it will take approximately 1,500 work 
hours per airplane to accomplish the required actions, and that the 
average labor rate is $60 per work hour. Based on these figures, the 
cost impact of the AD on U.S. operators is estimated to be $900,000, or 
$90,000 per airplane.
    The cost impact figure discussed above is based on assumptions that 
no operator has yet accomplished any of the requirements of this AD 
action, and that no operator would accomplish those actions in the 
future if this AD were not adopted. The cost impact figures discussed 
in AD rulemaking actions represent only the time necessary to perform 
the specific actions actually required by the AD. These figures 
typically do not include incidental costs, such as the time required to 
gain access and close up, planning time, or time necessitated by other 
administrative actions.

Regulatory Impact

    The regulations adopted herein will not have substantial direct 
effects 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. Therefore, in 
accordance with Executive Order 13132, it is determined that this final 
rule does not have sufficient federalism implications to warrant the 
preparation of a Federalism Assessment.
    For the reasons discussed above, I certify that this action (1) is 
not a ``significant regulatory action'' under Executive Order 12866; 
(2) is not a ``significant rule'' under DOT Regulatory Policies and 
Procedures (44 FR 11034, February 26, 1979); and (3) will not have a 
significant economic impact, positive or negative, on a substantial 
number of small entities under the criteria of the Regulatory 
Flexibility Act. A final evaluation has been prepared for this action 
and it is contained in the Rules Docket. A copy of it may be obtained 
from the Rules Docket at the location provided under the caption 
ADDRESSES.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Safety.

Adoption of the Amendment

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the Federal Aviation Administration amends part 39 of 
the Federal Aviation Regulations (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. Section 39.13 is amended by adding the following new 
airworthiness directive:

2001-22-06 Boeing: Amendment 39-12485. Docket 95-NM-15-AD.
    Applicability: All Model B-17E, F, and G airplanes, certificated 
in any category.

    Note 1: This AD applies to each airplane identified in the 
preceding applicability provision, regardless of whether it has been 
modified, altered, or repaired in the area subject to the 
requirements of this AD. For airplanes that have been modified, 
altered, or repaired so that the performance of the requirements of 
this AD is affected, the owner/operator must request approval for an 
alternative method of compliance in accordance with paragraph (d) of 
this AD. The request should include an assessment of the effect of 
the modification, alteration, or repair on the unsafe condition 
addressed by this AD; and, if the unsafe condition has not been 
eliminated, the request should include specific proposed actions to 
address it.


    Note 2: For airplanes on which the terminal fitting-to-spar 
chord joints were separated prior to the effective date of this AD, 
and inspections of and/or repairs to the wing terminals-to-spar 
chords were accomplished prior to the effective date of this AD: 
Applications for approval of an alternative method of compliance to 
the requirements of paragraphs (a) and (b) of this AD must be 
submitted to the FAA in accordance with the provisions of paragraph 
(d) of this AD.

    Compliance: Required as indicated, unless accomplished 
previously.
    To prevent reduced structural integrity of the wing of the 
airplane, accomplish the following:

Inspections and Corrective Actions

    (a) Within 180 days after the effective date of this AD, 
accomplish the requirements of paragraphs (a)(1) and (a)(2) of this 
AD.
    (1) Perform a dye penetrant inspection to detect cracking of 
each inboard end of the eight aluminum wing spar chords, in 
accordance with MIL-STD-6866, using a fluorescent Type 1 penetrant, 
Method C, Sensitivity Level 3, inspection. If any cracking is 
detected, prior to further flight, repair in accordance with a 
method approved by the Manager, Seattle Aircraft Certification 
Office (ACO), FAA.

    Note 3: The part number (P/N) for the upper wing spar chords is 
3-14231-0, and the P/N for the lower wing spar chords is 3-14231-1.


    Note 4: Operators desiring to use an alternative dye penetrant 
procedure may request approval from the Seattle ACO in accordance 
with paragraph (d) of this AD.


    Note 5: The following are the P/N's for the terminal fitting-to-
spar chord joint assemblies:


[[Page 54118]]



------------------------------------------------------------------------
               Assemblies                      Assembly part number
------------------------------------------------------------------------
Left Upper Front Spar Joint Assembly...  75-4781-0
Right Upper Front Spar Joint Assembly..  75-4781-1
Left Lower Front Spar Joint Assembly...  65-4782-512
Right Lower Front Spar Joint Assembly..  65-4782-513
Left Upper Rear Spar Joint Assembly....  75-4783-0
Right Upper Rear Spar Joint Assembly...  75-4783-1
Left Lower Rear Spar Joint Assembly....  75-4784-0
Right Lower Rear Spar Joint Assembly...  75-4784-1
------------------------------------------------------------------------


    Note 6: The following are the P/N's for the bolts for the spar 
chords:


------------------------------------------------------------------------
               Bolts for:                        Bolt part number
------------------------------------------------------------------------
Upper and Lower Front Spar Chords......  NAS56A36
Upper Rear Spar Chord..................  NAS56A34
Lower Rear Spar Chord..................  NAS56A40-5
------------------------------------------------------------------------

    (2) Perform a detailed visual inspection to detect corrosion of 
the bolts, as installed, and replace any corroded bolt with a new 
bolt having a P/N in the NAS 6606 series in accordance with Army 
Technical Order Number 01-20EF-2. Prior to further flight, for all 
bolt replacements, accomplish the requirements of paragraphs 
(a)(2)(i), (a)(2)(ii), (a)(2)(iii), and (a)(2)(iv) of this AD in 
accordance with Army Technical Order Number 01-20EF-2.

    Note 7: The following are the P/N's for the replacement bolts 
for the spar chords:


------------------------------------------------------------------------
         Replacement bolts for:            Replacement bolt part number
------------------------------------------------------------------------
Upper and Lower Front Spar.............  NAS 6606-51
Upper Rear Spar........................  NAS 6606-47
Lower Rear Spar........................  NAS 6606-56
------------------------------------------------------------------------


    Note 8: For the purposes of this AD, a detailed visual 
inspection is defined as: ``An intensive visual examination of a 
specific structural area, system, installation, or assembly to 
detect damage, failure, or irregularity. Available lighting is 
normally supplemented with a direct source of good lighting at 
intensity deemed appropriate by the inspector. Inspection aids such 
as mirror, magnifying lenses, etc., may be used. Surface cleaning 
and elaborate access procedures may be required.''

    (i) Install a washer having P/N MS 20002C6 under the head of the 
bolt, a self-locking nut having P/N NAS 1804-6, and a washer having 
P/N MS 200026 under the nut, for each replacement bolt.
    (ii) Torque any replacement bolt to 95-105 inch-pounds.
    (iii) Oversize replacement bolts by \1/16\ inch, as necessary.
    (iv) Apply corrosion inhibiting compound (using BMS 3-23, Type 
II or equivalent compound) to the spar chord after bolt removal, 
replacement, or other remedial action.
    (b) Within 18 months after the effective date of this AD, 
accomplish the requirements of either paragraph (b)(1) or (b)(2) of 
this AD.
    (1) Perform detailed visual and high frequency eddy current 
inspections, that include separating all eight wing terminal-to-spar 
chord joints, to detect cracking and corrosion of the wing terminals 
and spar chords, in accordance with a method approved by the 
Manager, Seattle ACO; or
    (2) Accomplish either paragraph (b)(2)(i) or (b)(2)(ii) of this 
AD.
    (i) Perform an equivalent inspection(s) to that required by 
paragraph (b)(1) of this AD in accordance with a method approved by 
the Manager, Seattle ACO. To be considered acceptable, the 
equivalent inspection(s) must include, at a minimum, the criteria 
specified in paragraphs (b)(2)(i)(A), (b)(2)(i)(B), and (b)(2)(i)(C) 
of this AD.
    (A) The inspection must include removal of all 64 bolts that 
join the eight wing terminals to the eight spar chords; and
    (B) The inspection must adequately detect cracking of the spar 
chord, and corrosion between the terminal fitting and the spar 
chord; and
    (C) The inspection must include a detailed visual inspection to 
detect corrosion of the attachment bolts; and a high frequency eddy 
current, and borescope inspection at 10-power magnification, of the 
bolt holes common to the spar chord-to-wing terminal interface.
    (ii) Perform a dye penetrant inspection to detect cracking of 
the spar chord tube end; remove the most outboard five bolts in the 
joint and perform an eddy current inspection to detect cracking of 
the holes; and perform a 10-power magnification borescope inspection 
to detect corrosion of the most inboard three bolts. If the criteria 
specified in paragraphs (b)(2)(ii)(A), (b)(2)(ii)(B), and 
(b)(2)(ii)(C) of this AD are met, removal of the three most inboard 
bolts of each terminal-to-spar chord joint is not required. Repeat 
the requirements of this paragraph thereafter at intervals not to 
exceed 36 months.
    (A) Results of the dye penetrant inspection of the spar chord 
tube end indicate that there is no cracking; and
    (B) Results of the eddy current inspection indicate that the 
holes of the five most outboard bolts in the joint are free of 
cracks; and
    (C) Results of the 10-power magnification borescope inspection 
indicate that the most inboard three bolts are free of corrosion.

    Note 9: The Boeing Company will make its specifications for eddy 
current inspections available to operators as needed.

    (c) If any cracking and/or corrosion is detected during any of 
the inspections required by paragraphs (a) and (b) of this AD, prior 
to further flight, repair in accordance with a method approved by 
the Manager, Seattle ACO.

Alternative Methods of Compliance

    (d) An alternative method of compliance or adjustment of the 
compliance time that provides an acceptable level of safety may be 
used if approved by the Manager, Seattle ACO. Operators shall submit 
their requests through an appropriate FAA Principal Maintenance 
Inspector, who may add comments and then send it to the Manager, 
Seattle ACO.

    Note 10: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the Seattle ACO.



[[Page 54119]]


    Note 11: The FAA strongly encourages owners and operators of the 
affected airplanes to coordinate their requests for approvals of 
alternative methods of compliance or adjustment of the compliance 
times pertaining to this AD. Coordination of a single request (in 
lieu of a separate request from each owner/operator) will allow the 
FAA to more quickly review and respond.

Special Flight Permits

    (e) Special flight permits may be issued in accordance with 
sections 21.197 and 21.199 of the Federal Aviation Regulations (14 
CFR 21.197 and 21.199) to operate the airplane to a location where 
the requirements of this AD can be accomplished.

Effective Date

    (f) This amendment becomes effective on November 30, 2001.

    Issued in Renton, Washington, on October 19, 2001.
Ali Bahrami,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 01-26951 Filed 10-25-01; 8:45 am]
BILLING CODE 4910-13-U