[Federal Register Volume 64, Number 224 (Monday, November 22, 1999)]
[Rules and Regulations]
[Pages 63653-63661]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-30143]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 99-NM-152-AD; Amendment 39-11307; AD 99-19-18]
RIN 2120-AA64


Airworthiness Directives; Bombardier Model DHC-7 and DHC-8 Series 
Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD), 
applicable to certain Bombardier Model DHC-7 and DHC-8 series 
airplanes, that requires revising the Airplane Flight Manual (AFM) to 
include requirements for activation of the airframe pneumatic deicing 
boots. This amendment is prompted by reports of inflight incidents and 
an accident that occurred in icing conditions where the airframe 
pneumatic deicing boots were not activated. The actions specified by 
this AD are intended to ensure that flightcrews activate the pneumatic 
wing and tail deicing boots at the first signs of ice accumulation. 
This action will prevent reduced controllability of the aircraft due to 
adverse aerodynamic effects of ice adhering to the airplane prior to 
the first deicing cycle.

EFFECTIVE DATE: December 27, 1999.

ADDRESSES: Information pertaining to this rulemaking action may be 
examined at the Federal Aviation Administration (FAA), Transport 
Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, 
Washington.

FOR FURTHER INFORMATION CONTACT: Norman Martenson, Aerospace Engineer, 
Manager, International Branch, ANM-116, FAA, Transport Airplane 
Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; 
telephone (425) 227-2110; fax (425) 227-1149.

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 certain Bombardier Model DHC-7 and 
DHC-8 series airplanes was published in the Federal Register on July 
16, 1999 (64 FR 38362). That action proposed to require revising the 
Airplane Flight Manual (AFM) to include requirements for activation of 
the airframe pneumatic deicing boots.

Related Proposals

    In addition to the proposed rule described previously, in June 
1999, the FAA issued 18 other similar proposals that address the 
subject unsafe condition on various airplane models (see below for a 
listing of all 19 proposed rules). These 18 proposals also were 
published in the Federal Register on July 16, 1999. (Docket 99-NM-153-
AD, for Fokker Model F27 Mark 100, 200, 300, 400, 500, 600, and 700 
series airplanes, was also issued as a supplemental notice of proposed 
rulemaking, and published in the Federal Register on August 6, 1999.) 
This final rule contains the FAA's responses to all relevant public 
comments received for each of these proposed rules.

[[Page 63654]]



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      Manufacturer airplane model                           Number                     Federal Register citation
----------------------------------------------------------------------------------------------------------------
Cessna Aircraft Company, Models 500,     99-NM-136-AD                                 64 FR 38374
 550, and 560 Series Airplanes.
Sabreliner Corporation, Models 40, 60,   99-NM-137-AD                                 64 FR 38358
 70, and 80 Series Airplanes.
Gulfstream Aerospace, Model G-159        99-NM-138-AD                                 64 FR 38341
 Series Airplanes.
McDonnell Douglas, Models DC-3 and DC-4  99-NM-139-AD                                 64 FR 38325
 Series Airplanes.
Mitsubishi Heavy Industries, Model YS-   99-NM-140-AD                                 64 FR 38371
 11 and YS-11A Series Airplanes.
Gulfstream American, (Frakes Aviation)   99-NM-141-AD                                 64 FR 38355
 Model G-73 (Mallard) and G-73T Series
 Airplanes.
Lockheed, Models L-14 and L-18 Series    99-NM-142-AD                                 64 FR 38338
 Airplanes.
Fairchild Models F-27 and FH-227 Series  99-NM-143-AD                                 64 FR 38322
 Airplanes.
Aerospatiale Models ATR-42/ATR-72        99-NM-144-AD                                 64 FR 38368
 Series.
Jetstream Model BAe ATP Airplanes......  99-NM-145-AD                                 64 FR 38351
Jetstream Model 4101 Airplanes.........  99-NM-146-AD                                 64 FR 38335
British Aerospace Model HS 748 Series    99-NM-147-AD                                 64 FR 38319
 Airplanes.
Saab Model SF340A/SAAB 340B/SAAB 2000    99-NM-148-AD                                 64 FR 38365
 Series Airplanes.
CASA Model C-212/CN-235 Series           99-NM-149-AD                                 64 FR 38348
 Airplanes.
Dornier Model 328-100 Series Airplanes.  99-NM-150-AD                                 64 FR 38332
Lockheed Model 1329-23 and 1329-25       99-NM-151-AD                                 64 FR 38316
 (Lockheed Jetstar) Series Airplanes.
de Havilland Model DHC-7/DHC-8 Series    99-NM-152-AD                                 64 FR 38362
 Airplanes.
Fokker Model F27 Mark 100/200/300/400/   99-NM-153-AD                                 64 FR 42870
 500/600/700/050 Series Airplanes.
Short Brothers Model SD3-30/SD3-60/SD3-  99-NM-154-AD                                 64 FR 38329
 SHERPA Airplanes.
----------------------------------------------------------------------------------------------------------------

Comments

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

1. Support for the Rule

    One commenter supports the proposed rule.

2. Request To Withdraw the Proposal: No Unsafe Condition

    Several commenters request that the proposal be withdrawn because 
no unsafe condition exists on certain airplanes. One of these 
commenters states that the FAA is merely speculating that the proposed 
Airplane Flight Manual (AFM) revision will improve safety. Further, the 
commenter contends that the FAA cannot substantiate that the proposed 
AFM revision will prevent ice bridging. This same commenter also asks 
if the FAA met its own standards by testing the proposed procedure on 
each of the affected airplanes.
    The FAA does not concur that no unsafe condition exists. As 
discussed in the preamble of the proposed rule, the FAA has reviewed 
the icing-related incident history of certain airplanes, and has 
determined that icing incidents may have occurred because pneumatic 
deicing boots were not activated at the first evidence of ice 
accretion. As a result, the handling qualities or the controllability 
of the airplane may have been reduced due to the accumulated ice. The 
FAA also discussed an accident that occurred as a result of the failure 
of the flightcrew to activate the wing and tail pneumatic deicing 
boots.
    Although there may have been no reported cases of incidents or 
accidents on a specific airplane model, the potential still exists for 
reduced controllability of all airplanes equipped with pneumatic 
deicing boots due to adverse aerodynamic effects of ice adhering to the 
airplane. This AD addresses this unsafe condition.
    Further, ice bridging of deicing boots was considered during 
development of the proposed rule. A broad representation of the 
aviation community was consulted, including airframe manufacturers, air 
carriers, airline pilot associations, airplane owner associations, 
deicing boot manufacturers, and National Aeronautics and Space 
Administration (NASA). Also, articles readily accessible by the general 
piloting community solicited operational information concerning ice 
bridging of deicing boots. The FAA considers that the general consensus 
of the aviation community is that little or no evidence exists of ice 
bridging of deicing boots with current deicing boot designs, and ice 
that is not shed after the initial boot cycle continues to increase in 
thickness and sheds during subsequent cycles.
    In addition, many airplanes equipped with pneumatic deicing boots 
to protect the engine are operated when icing conditions are present, 
i.e., visible moisture and a specific temperature are observed. As 
discussed in Comment #3 (following this response), at least two 
airplane manufacturers have issued AFM's that contain procedures to 
activate the deicing boots at the first sign of ice accumulation. The 
FAA is unaware of any ice bridging problems associated with early 
operations of either the airfoil or engine pneumatic deicing boots.
    In response to the commenter's question regarding the FAA meeting 
its own standards, the FAA infers that the commenter is requesting the 
basis for the FAA's determination that the proposed procedures are 
safe. Most aircraft certification programs have not considered the 
reduced controllability of the aircraft due to adverse aerodynamic 
effects of ice adhering to the pneumatic boots. The requirements of 
this AD (activation of the deicing boot system at the first sign of ice 
formation anywhere on the aircraft, or upon annunciation from an ice 
detector system, whichever occurs first, along with the periodic 
cycling of the boots) will minimize the ice accretions and thereby 
reduce the adverse aerodynamic effects.

3. Request To Withdraw the Proposal: Possible Adverse Effects of 
Residual Ice

    Several commenters state that deicing boots do the best job of 
shedding ice on a single cycle, if ice is permitted to accrete to \1/4\ 
or \1/2\ inch before activation of the boots. One of these commenters 
further contends that the effect of continuous cycling in auto mode may 
not produce a clean shed of ice on each activation, and that residual 
ice must be taken into consideration before any revision to the AFM is 
required. Another commenter states that, although operation in the 
continuous mode upon first indication of ice accretion would eliminate 
the problem of identification of accretion, the commenter is concerned 
that there would then be a potential for degraded performance due to 
residual ice.
    The FAA does not concur that the proposal should be withdrawn 
because of concerns over residual ice. Operation of pneumatic deicing 
boots typically results in persistent ice accretions on the boot 
surfaces, even when \1/4\ to \1/2\

[[Page 63655]]

inch of ice is allowed to accrete prior to activation of the boots. The 
persistent residual and inter-cycle ice accretions typically result in 
adverse aerodynamic effects and degraded airplane flying qualities. 
Activation of the wing and tail pneumatic deicing boots at the first 
sign of ice accretion, or at the annunciation of an ice detector system 
and periodic operation of the deicing boots will also result in 
persistent ice accretions. However, the proposed procedure will 
minimize the residual and intercycle ice accretions because the ice 
will be shed when the minimum thickness or mass required for shedding 
is reached. The residual and intercycle ice accretion thickness 
resulting from this procedure is less than the ice accretion thickness 
typically recommended prior to operation of the pneumatic deicing boot. 
Adverse airplane flying qualities resulting from ice accretions 
typically are affected by the thickness, shape, texture, and location 
of the ice accretion.
    At least two airplane manufacturers have issued AFM's that contain 
procedures to activate the deicing boots at the first sign of ice 
accumulation. Those two airplane models have different wing and 
stabilizer design characteristics and different deicing boot 
configurations. Further, those two airplane models represent a large 
proportion of the airplane fleet equipped with pneumatic deicing boots. 
The FAA has received no reports indicating any adverse effects of 
residual ice resulting from early activation of the deicing boots for 
these airplane designs.
    In addition, a number of airplane models are equipped with deicing 
boot systems that include automatic operating modes, wherein the boots 
automatically cycle at specific time intervals after being activated. 
This automatic cycling has surely resulted in operation of the boots 
with less than the recommended thickness of ice accreted. The FAA has 
received no reports indicating any adverse effects resulting from the 
use of the automatic mode.

4. Request To Withdraw or Delay: Develop More Data

    Several commenters request that the FAA delay issuance of the rule 
until more data are developed and reviewed. Certain of these commenters 
also state that at the public meeting on icing (February 2-4, 1999), 
the consensus was that a uniform procedure cannot be adopted for all 
airplanes. That is, a ``blanket'' proposal for numerous airplanes 
(regardless of design) is inappropriate without specific consideration 
for the individual designs. Another one of these commenters points out 
that each airplane model is unique and that the operating instructions 
for the ice protection system for one airplane model may not be 
appropriate for another airplane model. That commenter further adds 
that the airframe manufacturer is in the best position to determine 
appropriate limitations.
    Another one of the commenters requests that, if the proposal is not 
withdrawn, the issuance of any rulemaking be delayed since certain 
language of the requirements of the AD is confusing.
    The FAA does not concur that a delay in issuing this action is 
appropriate. The FAA concurs that the airframe manufacturers present at 
the February public meeting did not support a common procedure for the 
operation of deicing boots. However, as mentioned previously, there 
have been no adverse reports on the airplane fleet equipped with 
pneumatic deicing boots that operate the boots at the first sign of ice 
accretion. With the exception of ``older'' pneumatic boots (reference 
comment #7, below), the FAA finds that a common procedure for boot 
operation is appropriate. The FAA has determined that the common 
procedures for operation of deicing boots as required by this AD 
(activation of the deicing boot system at the first sign of ice 
formation anywhere on the aircraft, or upon annunciation of an ice 
detector system, whichever occurs first, and periodic cycling of the 
boots) will minimize the ice accretions and thereby reduce the adverse 
aerodynamic effects.
    To withdraw or delay this AD would be inappropriate since the FAA 
has determined that an unsafe condition exists, and that the required 
AFM revision must be accomplished to ensure continued safety of the 
fleet. The fact that other data may be developed at a later time does 
not negate the FAA's responsibility to address the existing identified 
unsafe condition in a timely manner. No change is necessary to the 
final rule in this regard.
    The FAA is unable to respond to one commenter's statement that 
certain language of the proposal was confusing since no example was 
specified.

5. Request To Withdraw Proposals for Certain Airplanes

    Three commenters, all airframe manufacturers, request that the 
proposal be withdrawn for several airplane models [British Aerospace 
Model ATP airplanes, British Aerospace Model HS 748 airplanes, Dornier 
Model 328-100 series airplanes, and deHavilland Model DHC-7/DHC-8 
series airplanes] since they have been certified to be in compliance 
with part 25 of the Federal Aviation Regulations (14 CFR 25.1419). 
Additionally, the commenters point out that those airplanes have been 
certificated in accordance with the appropriate foreign civil 
airworthiness authorities. The commenters further explain that service 
experience of those airplanes does not indicate any deficiencies with 
regard to handling and performance due to airframe accreted ice. In 
conclusion, the commenters state that, in the absence of any evidence 
to suggest deficiencies regarding this subject, they cannot support the 
intent of the rule.
    The FAA acknowledges that an airplane model may have design 
characteristics that mitigate the adverse airplane flying qualities 
resulting from ice accretion on deicing boot surfaces. As discussed in 
the proposal for this AD, the FAA has previously requested that 
interested persons provide information on icing system design and 
operations procedures concerning flight during icing conditions. The 
request also asked manufacturers, who are in the best position to 
determine those operating procedures, to provide data showing that 
their aircraft have safe operating characteristics with ice accreted on 
the protected surfaces (boots). That information was requested 
specifically by letter on October 1, 1998, to certain manufacturers of 
airplanes certified in accordance with part 25 of the Federal Aviation 
Regulations (14 CFR part 25). Except as discussed in Item 6 of the 
comment section of this final rule, no other information received 
caused the FAA to reconsider that an unsafe condition may exist, or 
that a revision of the AFM, such as required by this AD, was unsafe for 
those airplanes.
    Additionally, similar information was specifically requested in the 
discussion section of the proposed rule. Of the comments to the 
proposal that were received by the FAA, no additional data was included 
for Dornier Model 328-100 series airplanes, or deHavilland Model DHC-7/
DHC-8 series airplanes that caused the FAA to reconsider the previous 
conclusion that an unsafe condition exists. Further, no data was 
provided to indicate that the proposal to require activation of wing 
and tail pneumatic deicing boots at the first sign of ice accretion or 
annunciation of an ice detector system was unsafe for any particular 
airplane model.
    United Kingdom Accident Investigation Board Preliminary Report EWC 
91/18 indicated that, while on climb to 16,000 feet in the vicinity of 
Oxford, England, on August 11, 1991, a

[[Page 63656]]

British Aerospace Model ATP airplane suffered a significant degradation 
of flying qualities and propeller icing. According to that report, the 
deicing boots of the airplane were not activated, and the airplane 
stalled, experienced severe uncontrolled roll oscillations, severe 
vibration that rendered the electronic flight instruments partially 
unreadable, and developed a high rate of descent. The deicing boots 
were finally activated and control of the airplane was regained after a 
loss of 3,500 feet in altitude. The report identified causal factors of 
the incident which included rapid accumulation of glaze ice that was 
not evident to the flightcrew, difficulty of the flightcrew to visually 
gauge the ice accretion thickness on the wing's leading edge, and 
propeller vibrations that disguised the onset of wing stall. Even 
though this incident occurred outside of the United States, and 
although this airplane model demonstrated acceptable in-flight icing 
airworthiness relative to FAA and Joint Airworthiness Authorities (JAA) 
requirements, the incident illustrates the vulnerability of this 
airplane model to the safety condition addressed by this AD.
    One commenter, British Aerospace, has requested until October 20, 
1999, to provide additional data to substantiate that the Model ATP 
airplanes and Model HS 748 airplanes can safely operate with ice 
accumulations on the protected surfaces. As discussed previously and in 
the NPRM, the FAA considers that this same vulnerability exists on all 
airplanes equipped with pneumatic deicing boots.
    In the interest of safety, the FAA finds that it is not prudent to 
delay issuance of the final rules on those airplane models. However, 
British Aerospace and any other manufacturer is encouraged to request 
approval of an alternative method of compliance with the airworthiness 
directive based on substantiating data indicating that a particular 
aircraft can safely be operated with the ice that would accumulate on 
the protected surfaces prior to activation of the ice protection 
system.

6. Request To Withdraw the Proposal for Certain Other Airplanes

    Two manufacturers request that the proposals regarding Cessna Model 
500, 501, 550, 551, and 560 series airplanes, and British Aerospace 
(Jetstream) Model 4101 airplanes be withdrawn. The manufacturers advise 
that the testing summarized in their comments provides evidence that 
the current procedures provide a safe method to operate those airplane 
models. The manufacturers conclude that, based on the service history 
and data provided to the FAA, the proposed AFM revision for those 
models is unnecessary.
    The FAA concurs that the notice of proposed rulemaking for Cessna 
Model 500, 501, 550, 551, and 560 series airplanes should be withdrawn 
based on the following information. The manufacturer performed a 
complete evaluation of the stall and handling characteristics with 
simulated ice shapes on the Model 550 (Bravo) series airplanes. Stall 
speeds and warning margins were evaluated with a \1/2\-inch glaze ice 
shape and with an ice shape associated with the system failure. This 
\1/2\-inch ice shape simulated the ice shape prior to deicing boot 
activation. Maneuver margin testing consisted of left and right 40-
degree bank turns. Stall characteristics were evaluated with a \1/2\-
inch rime ice shape configuration. Stall characteristic testing 
consisted of wings level and 30-degree bank turns. At the conclusion of 
the testing it was determined that the airplane had an acceptable stall 
warning margin with ice shapes present. The manufacturer maintains that 
Model 500/501, Model 550/551, and Model 550 (Bravo) series airplanes 
all use a common wing airfoil with some minor differences in span and 
wing loading. These aircraft also use a common tail configuration 
(airfoil, span, and leading edge sweep).
    Additionally, the FAA reviewed the Type Inspection Report (TIR) for 
Model 550 (Bravo) series airplane testing and found that ice shapes 
were placed on both the protected and unprotected surfaces.
    The Model 560 (Ultra) series airplanes underwent an extensive ice 
shape stall investigation. This investigation consisted of stall 
testing of the baseline airplane and the airplane with the most adverse 
simulated intercycle ice shapes. The ice shapes consisted of \1/2\-inch 
shapes on the surfaces protected by boots and 3-inch shapes on 
unprotected flight surfaces. The stall speeds determined by this 
testing were incorporated into the Safeflight Angle of Attack computer 
to increase the stall warning margin during flight in icing conditions. 
The Model 560 series airplanes angle of attack computer was also 
updated to incorporate a normal mode and an ice mode stall warning 
system. The changes to the angle of attack computer on Model 560 and 
560 (Ultra) series airplanes were proposed by Rules Docket No. 98-NM-
312-AD.
    The FAA notes that extensive testing of Model 550 and 560 series 
airplanes (in which acceptable stall protection and maneuver margins at 
operational speeds were demonstrated with expected ice accretion on the 
deicing boot surfaces) indicates that these airplanes can safely 
operate with ice accretions associated with the AFM normal operations 
procedures of the deicing boots. These attributes demonstrate that 
Model 550 and 560 series airplanes satisfactorily address the unsafe 
condition addressed by this AD.
    Since Model 500 series airplanes are similar to Model 550 series 
airplanes, the Model 500 series airplanes also satisfactorily addresses 
the unsafe condition addressed by this AD. The FAA also notes that 
testing of Model 560 series airplanes revealed problems in the stall 
warning margin for flight in icing conditions that were addressed by 
previously issued airworthiness directives.
    The FAA also concurs that the notice of proposed rulemaking for 
British Aerospace Jetstream Model 4101 airplanes should be withdrawn 
based on the following information. In response to the FAA's October 1, 
1998, letter (discussed previously), British Aerospace submitted a 
summary of the handling and performance flight test results that were 
produced during the original flight in icing certification. This 
summary was referenced in their response to the proposed rulemaking. 
The commenter volunteered to provide any reports referenced in the 
summary. The FAA requested and subsequently received copies of the full 
handling and performance flight test results for certification in the 
icing conditions specified in Appendix C of part 25 of the Federal 
Aviation Regulations (14 CFR 25), and the JAA draft issue of 
AMJ25.1419, which was used as guidance for compliance with JAR/FAR 
25.1419. The FAA reviewed these reports and guidance material and finds 
that the Jetstream 4101 airplane was adequately tested with a variety 
of natural ice accretions on both the protected and unprotected 
surfaces. Handling and performance flight test was accomplished for the 
following: Normal Operation of the Deicing Boots, \1/2\ to \3/4\ inch 
of ice on the protected wing leading edges and up to 3 inches of ice on 
unprotected leading edges; Simulated Failure of the Deicing Boots, 
approximately 1 to 1\1/2\ inches of ice on all leading edges; Ice 
Accreted During the Take-off Phase, a thin rough layer of ice accreted 
during the initial take-off phase to 400 feet, prior to operation of 
deicing boots.
    These ice accretion depths were established to address the 
following: Ice accreted during the rest-time of a deicing cycle, 
delayed operation or

[[Page 63657]]

failure of the system, and residual ice accumulations. The flight 
testing examined stall speeds, stall warning margins, stall 
characteristics, maneuver margins, longitudinal controllability, flap 
configuration changes, ability to trim, susceptibility to tailplane 
stall, and longitudinal, lateral, and directional stability. The angles 
of attack for activation of the stall warning system and stall 
identification system (i.e., stick shaker or stick pusher) are reset to 
lower values (i.e., higher speeds) for flight in icing and safe flight 
speeds (minimum operating speeds) established accordingly. Affected AFM 
performance information was derived for icing conditions based on the 
higher operating speeds, in accordance with JAA draft AMJ25.1419.
    The Cessna and British Aerospace aircraft models discussed in this 
comment have been tested and, where appropriate, changes have been made 
to ensure the airplanes are safe for operations with ice accretions on 
the protected surfaces. Without this type of testing and 
substantiation, the FAA must conclude the aircraft affected by this 
final rule may be subject to adverse aerodynamic effects due to ice 
accretions on the protected surfaces prior to deicing boot operation. 
Other manufacturers may also develop the necessary data to substantiate 
that their airplanes are safe with these accretions and request 
approval of an alternative method of compliance.

7. Request To Differentiate Between ``Modern'' Boot Systems and 
``Older'' Boot Systems

    Several commenters request that the difference between the 
``older'' boot systems and the ``modern'' boot systems be explained. 
These commenters express concern that although both systems are 
addressed in the proposal, there may not be a sound technical reason to 
apply the requirements of the proposal to both types of boot systems.
    The FAA acknowledges that definitions of ``older'' and ``modern'' 
pneumatic boot systems should be provided. Therefore, for the purposes 
of this AD, ``modern'' pneumatic boot systems may be characterized by 
short segmented, small diameter tubes, which are operated at relatively 
high pressures [18-23 pounds per square inch (psi)] by excess bleed air 
that is provided by turbine engines. ``Older'' pneumatic boot systems 
may be characterized by long, uninterrupted, large diameter tubes, 
which were operated at low pressures by engine driven pneumatic pumps 
whose pressure varied with engine revolutions per minute (rpm). This 
low pressure coupled with long and large diameter tubes caused early 
de-ice systems to have very lengthy inflation and deflation cycles and 
dwell times. (Dwell time is the period of time that the boot remains 
fully expanded following the completion of the inflation cycle until 
the beginning of the deflation cycle.)

8. Request To Withdraw the Proposal For Airplanes with ``Older'' Boots

    Two commenters request that the proposed rules applying to 
Gulfstream Model G-159 series airplanes and McDonnell Douglas Model DC-
3 and DC-4 series airplanes be withdrawn. Both commenters state that 
those airplane models do not meet the common definition of the word 
``modern.'' (See Comment #7 of this final rule for a definition of 
``modern'' as used in this AD.) One commenter states that the current 
AFM specifically directs the flightcrew to wait for \1/4\ inch of ice 
before activating the boots. Further, the commenter asserts that the 
current procedure was developed during certification and is the basis 
for the airplane's approval for flight into known icing. Additionally, 
the commenters assert that the in-service safety records for more than 
40 years indicates that the existing procedures are appropriate for 
these airplanes. The commenter concludes that the proposed AFM revision 
is in direct opposition to the certification findings.
    The FAA acknowledges that early activation of the ``older'' 
pneumatic deicing boots may create the hazard of ice bridging on the 
``older'' systems. As discussed in Comment #2 previously, ``older'' 
boots may be susceptible to ice bridging, and the FAA concurs that 
requiring the activation of the boots at the first sign of icing may 
actually introduce an unsafe condition on those airplanes. In order to 
address this issue, the FAA is taking the following steps. First, to 
accommodate certain airplane models of the fleet (i.e., Gulfstream 
Model G-159 series airplanes and McDonnell Douglas Model DC-3 and DC-4 
series airplanes) that may be equipped with the ``older'' pneumatic 
deicing boot system, the FAA is considering the issuance of 
supplemental NPRM's for those airplane models. The purpose of the 
supplemental NPRM's would be to require an inspection to determine 
which type of pneumatic deicing boots are installed on the airplanes, 
and to require operation of the boots at the first sign of ice 
accretion if the airplanes have been retrofitted with ``modern'' boots. 
Second, for aircraft with ``older'' pneumatic boots installed, the FAA 
will continue to investigate other solutions to the unsafe condition of 
reduced handling qualities or controllability of the airplane due to 
ice accumulations on the protected surfaces. If other solutions are 
identified, the FAA may consider further rulemaking.

9. Request To Revise AFM Change

    One commenter requests that the proposal to operate the boots at 
the first sign of ice accretion be limited to the holding and approach 
phases of flight. The commenter states that the Aviation Rulemaking 
Advisory Committee (ARAC) Ice Protection Harmonization Working Group 
(IPHWG) completed a comprehensive review of past icing accidents/
incidents. The IPHWG concluded that the only phases of flight that 
demonstrate a safety concern are holding patterns and various approach 
segments; since these operations are conducted at lower airplane speed, 
instability could occur as a result of ice accumulations on the wing 
and tail surfaces.
    The FAA does not concur that the AFM revision should be limited to 
the holding and approach phases of flight. The FAA acknowledges that 
the IPHWG is working on a proposed operations rule that may only be 
applicable during holding and approach phases of flight. However, the 
IPHWG continues to work on the proposed rule and has not reached 
technical agreement. Since discussions are ongoing, it would not be 
appropriate to assume that the IPHWG positions as presented by the 
commenter will necessarily be reflected in the actual published 
proposal.
    Another commenter, an airplane manufacturer, stated that the AFM 
for Model SF340A/SAAB340B/SAAB 2000 series airplanes currently does not 
limit the operation of the deicing boots during specific phases of 
flight. The commenter requests that the AFM change required by 
paragraph (a) of the proposal be revised to limit the applicable phases 
of flight where the AFM specifies that deicing boots should not be 
used. Specifically, the commenter requests that the language be revised 
to read ``Deicing boots must not be used during take-off and landing.''
    The FAA partially concurs, and acknowledges that clarification is 
necessary. It was the FAA's intent that the boots do not have to be 
operated at the first sign of ice accretion during those phases of 
flight if there are existing procedures in the AFM that prohibit the 
operation of the boots during specific phases of flight. However, the 
boots must always be operated at the first sign of ice accretion if, in 
accordance with the AFM, it is acceptable to operate the boots during

[[Page 63658]]

all phases of flight. Therefore, the FAA has revised paragraph (a) of 
the final rule to state, ``Except if the AFM otherwise specifies that 
deicing boots should not be used for certain phases of flight (e.g., 
take-off, final approach, and landing), compliance with the following 
is required.''
    With respect to the request to specify that the deicing boots must 
not be used during take-off and landing, it would be desirable to 
customize the AFM limitation for specific models of airplanes. This 
would allow the AFM to clearly indicate to the flightcrew when the 
deicing boots should be deactivated, rather than necessitating that the 
flightcrew first determine if there are other portions of the AFM that 
indicate that the deicing boots should not be used during specific 
phases of flight. Therefore, the FAA encourages requests for approval 
of alternative methods of compliance to customize the AFM limitation to 
the specific airplane model.
    However, the FAA does not concur with the request to revise the 
final rule that applies to Saab Model SAAB SF340A/SAAB340B/SAAB 2000 
series airplanes since the existing Saab AFM does not indicate that the 
deicing boots should not be used during take-off and landing. If the 
commenter has data to indicate that the deicing boots should not be 
used during those phases of flight, the commenter should take action to 
revise the AFM and request approval of an alternative method of 
compliance.

10. Request To Revise Instructions on When To Deactivate the Boot 
System

    One commenter requests that two changes be made to paragraph (a) of 
the proposal. The first change would be to specify that the wing and 
tail leading edge pneumatic deicing boot system may be deactivated only 
after completion of an entire deicing cycle after leaving icing 
conditions. The commenter also requests that the proposal be revised to 
add related procedures for operating speeds, and that related 
procedures for operation of the autopilot (if any) be discontinued only 
after the airplane is determined to be clear of ice. The commenter 
states that natural ice shedding, melting, or sublimation from the 
protected areas will mostly eliminate residual ice.
    Regarding the commenter's first request, the FAA concurs. For the 
reasons the commenter stated, the FAA has revised paragraph (a) of the 
final rule from: ``The wing and tail leading edge pneumatic deicing 
boot system may be deactivated only after leaving icing conditions and 
after the airplane is determined to be clear of ice;'' to ``The wing 
and tail leading edge pneumatic deicing boot system may be deactivated 
only after completion of an entire deicing cycle after leaving icing 
conditions.''
    Regarding the commenter's second request, the FAA considers that, 
since the suggested change would alter the actions currently required 
by this AD, additional rulemaking would be required. The FAA finds that 
to delay this action would be inappropriate in light of the identified 
unsafe condition. However, the FAA is considering additional rulemaking 
concerning operating speeds during icing conditions.

11. Requests the FAA Consider the Pilot Workload

    One commenter states that the proposal would require the pilot to 
monitor ice formation and to activate the deicing system almost 
constantly. Another commenter suggests that such increase of the 
pilot's workload could, of itself, cause an indirect adverse impact on 
operational safety. The commenters request that the FAA consider the 
additional pilot workload if the proposal is adopted.
    The FAA has previously considered the effects on the pilot of 
requiring that the deicing boots be activated at the first sign of ice 
formation anywhere on the airplane, or upon annunciation from an ice 
detector system. The FAA acknowledges that current procedures 
recommending activation of the deicing boots at a specific ice 
accretion thickness require the flightcrew to closely monitor the ice 
accretion. However, since a number of airplanes affected by this AD are 
equipped with deicing boot systems with automatic operating modes, 
operating the deicing boots at the first sign of ice accretion in an 
appropriate automatic mode will favorably influence flightcrew 
workload. For airplanes not equipped with automatic deicing boot 
operating modes, periodic operation of the boots can be accomplished 
based on time intervals consistent with existing icing conditions. The 
FAA considers that periodic operation of the boots is not a greater 
workload burden than closely monitoring the ice accretion thickness.
    For the reasons stated, the FAA has determined that it is 
unnecessary to revise the final rule.

12. Request To Withdraw the Proposal: Provide Training Instead

    Several commenters request that the FAA withdraw the proposal and 
ensure that appropriate information and training regarding the use of 
the boots is provided to pilots. The commenters also suggest that a 
testing program be accomplished by industry. The commenters assert that 
such training, along with an analysis of the testing program, would 
eliminate the need for requiring that the deicing boots be activated in 
accordance with the proposal. One commenter also adds that the AFM 
should only be changed to add a warning that delayed activation of the 
pneumatic boot system may be unsafe. Another commenter adds that the 
language of the proposed AFM revision may conflict with current AFM 
procedures and could confuse operators.
    The FAA does not concur that substituting mandatory training for 
issuance of an AD is appropriate in this case. The FAA acknowledges 
that, in addition to the issuance of an AD, information specified in 
the revision to the AFM should be integrated into the pilot training 
syllabus. However, the development and use of advisory materials and 
training alone are not adequate to address the unsafe condition. The 
only method of ensuring that certain information is available to, and 
mandatory for, the pilot is through incorporation of the information 
into the Limitations Section of the AFM. The appropriate vehicle for 
requiring such revision of the AFM is issuance of an AD. No change is 
necessary to the final rule in this regard.

13. Request To Consider Procedures Already in Normal Procedures Section

    One commenter requests concurrence that procedures existing in the 
Normal Procedures section of the AFM be considered as compliant with 
the requirements of the proposed AD.
    The FAA does not concur that procedures specified in the Normal 
Procedures section of the AFM are an equivalent method of compliance 
with the AD. The FAA considers that, since the Limitations section of 
the AFM is the only section of the AFM that is mandatory [Sec. 91.9 of 
the Federal Aviation Regulations (14 CFR 91.9)], the subject required 
revision to the AFM must be included in the Limitations section. No 
change is necessary to the final rule in this regard.

14. Request To Limit the AD to Only Those Operations Conducive to Icing

    Two commenters request that the AFM limitation specified in 
paragraph (a) of the proposal be limited to those conditions where 
operations conducive to icing exist. The commenters provide examples of 
conditions where operations not conducive to icing may exist such as 
Hawaii; the Caribbean; short, low altitude flights in the summer; etc. 
One of these commenters

[[Page 63659]]

states that, ``under the proposal, dispatch with an inoperative boot 
would be considered prohibited even though the deicing would never be 
needed.''
    The FAA does not concur that revision of the AD is necessary in 
this regard. Paragraph (a) of the AD specifically states that wing and 
tail leading edge pneumatic deicing boot systems must be activated at 
the first sign of ice formation anywhere on the aircraft, or upon 
annunciation from an ice detector system, whichever occurs first. The 
FAA considers that, regardless of what geographic area an airplane may 
be flying in or what season of the year it may be, the boot system must 
be activated if those specified conditions occur.
    Regarding dispatch with an inoperative boot, current Master Minimum 
Equipment List (MMEL) procedures prohibit dispatch of the airplane into 
known or forecast icing conditions if the deicing boots are 
inoperative. In the event that icing conditions are inadvertently 
encountered during operation in accordance with MMEL provisions, 
procedures exist to instruct the flightcrew to exit the icing 
conditions immediately. The FAA considers that those existing 
procedures will prevent conflict between the requirements of this AD 
and perceived problems regarding dispatch with inoperative boots. No 
change is necessary to the final rule in this regard.

15. Request To Consider Differences in Airplanes Systems

    One commenter requests that the AFM revision specified in paragraph 
(a) of the proposed rule be revised for those airplanes that are 
equipped with icing detection systems. Such a revision should read 
``activate the wing and tail leading edge pneumatic deicing boot system 
upon annunciation from an ice detector,'' rather than ``at the first 
sign of ice anywhere on the aircraft, or upon annunciation from an ice 
detector system, whichever occurs first.'' The commenter states that, 
since the sensor for the ice detection system detects ice buildup at 
the boot, it would make sense for airplanes that have an ice detection 
system to activate the boot only when ice is detected at the boot by 
the ice detection system. The commenter further points out that 
activating the boot when ice is not forming on the boot will not remove 
the ice formations elsewhere on the airplane, but will simply 
deteriorate the condition of the boot and provide no safety benefit. 
Additionally, the commenter adds that if the ice detection system were 
inoperative for dispatch, it would be appropriate as a Master Minimum 
Equipment List (MMEL) condition to activate the boot at the first sign 
of icing.
    The FAA does not concur that the final rule should be revised to 
address procedures specifically for airplanes equipped with icing 
detection systems. Visual detection of icing by the flightcrew has been 
certificated as the primary means of ice detection. Therefore, the FAA 
has determined that, although ice detection systems may alert the 
flightcrew to the presence of icing, the flightcrew is still 
responsible to monitor the airframe for ice accretion. No change is 
necessary to the final rule in this regard. However, in the event a 
turbopropeller airplane equipped with pneumatic deicing boots was also 
equipped with an ice detection system that was approved as the primary 
ice detection system, the operator could request an alternative method 
of compliance in accordance with paragraph (b) of the final rule.

16. Request To Require Additional Operational Procedures

    Several commenters propose that the FAA consider that minimum speed 
restrictions be used in conjunction with the early activation of the 
deicing boots. Some of the commenters specify that these speed 
additions be applied during landing approach. One of the commenters 
expresses concern that various reports and research indicate that 
increasing the angle-of attack with even a small ice formation on the 
airfoil can cause large increases in drag and loss of lift. The 
commenter contends that control of the angle-of-attack is critical in 
maintaining airfoil performance, and concludes that additional 
operational procedures must be added.
    The FAA concurs that certain operational procedures may be 
beneficial when used with early activation of the deicing boots. As a 
complement to this AD, the FAA is considering rulemaking regarding 
minimum speeds in icing conditions. As mentioned previously, the FAA 
encourages manufacturers to present data via a request for approval of 
an alternative method of compliance to substantiate that their 
airplanes are either capable of flying safely with ice that accumulates 
prior to boot activation, or that they are not capable of flying safely 
but there are other means to address the unsafe condition. For example, 
in the case of Cessna Model 560 series airplanes, the stall warning 
margins were modified to ensure the airplane could safely operate with 
ice accretions on the protected surfaces. No change is necessary to the 
final rule in this regard.

17. Request To Mandate Installation of an Ice Detection System

    One commenter suggests that a required installation of a reliable 
ice detection system might alleviate the difficulties associated with 
flightcrew recognition of airfoil ice accretions. The commenter notes 
that, historically, the problem of ice detection has been the ability 
of the flightcrew to either identify that the airfoil has ice adhering 
to it or accurately determine that a certain thickness of ice exists on 
the airfoil prior to activation of the boot system.
    The FAA concurs that installation of a reliable ice detection 
system would alleviate the difficulties associated with flightcrew 
recognition of airfoil ice accretions. This issue is being addressed by 
an ARAC working group. Upon receipt of a recommendation from ARAC, the 
FAA may consider further rulemaking. In the interim, the FAA is issuing 
these airworthiness directives to impose a relatively simple deicing 
boot operational change to address the reduced handling qualities or 
controllability of the airplane due to ice accumulations on the 
protected surfaces. No change is necessary to the final rule in this 
regard.

18. Request To Require Action To Reduce Adhesion Characteristics

    One commenter requests that action be taken to minimize or reduce 
the ice adhesion characteristics of boot material. The commenter 
asserts that one reason flightcrews may be seeing large amounts of 
residual ice may be that, as the boot ages, the tendency for residual 
ice to stick to the boot surface may increase if the adhesion qualities 
of the boot materials are not properly maintained. In addition, the 
commenter suggests that the use of certain compounds (e.g., ICEX, an 
ice-phobic chemical spray) can reduce ice adhesion by substantial 
margins.
    The FAA does not concur with the commenter's request to require 
rulemaking to reduce adhesion characteristics of boot material. The FAA 
considers that normal wear and tear on the deicing boot materials is to 
be expected, and the adhesion characteristics of the boot increases as 
the boot surface degrades over time. Operators have the responsibility 
to monitor the performance of the deicing boots installed on their 
airplanes, and to perform maintenance as required.
    The FAA acknowledges that use of certain ice-phobic chemicals may

[[Page 63660]]

provide an additional safety benefit. However, a variety of factors 
(e.g., normal wear and tear, ``patching,'' and oxidation of boot 
material) exist in varying degrees on individual airplanes. As a 
result, the optimum frequency of application will vary during the life 
of the boot. The FAA has received no quantitative data to demonstrate 
the adequacy of particular amounts of ice phobic chemical sprays or to 
provide adequate intervals of application. Therefore, the FAA cannot 
establish an appropriate application interval at this time. However, if 
additional data becomes available, the FAA may consider further 
rulemaking.

19. Request To Consider the Associated Maintenance Procedures and 
Increased Costs

    Several commenters point out that certain maintenance requirements 
should be considered if the proposed AFM revision is required. One 
commenter notes that a detailed review of maintenance procedures should 
be conducted regarding the deicing boots to ensure that, as the boot 
ages, the boot system continues to effectively shed ice.
    Several commenters request that the FAA also consider the 
additional costs that the proposed AFM revision would require. One 
commenter states that the added cycling of the boots will require 
additional maintenance. The commenters express concern that the boots 
will wear out faster, need to be replaced at an accelerated rate, and 
thereby add additional costs.
    The FAA acknowledges the concerns of these commenters. The FAA 
considered the deicing boot fatigue issues surrounding the proposed AD, 
such as the reliability of the deicing boots. Reliability of the 
deicing boots is affected by several factors, including: maintenance 
practices; abrasion during dry air, rain, hail, snow, and icing 
operations; oxidation; and, fatigue resulting from boot cycling.
    However, none of the commenters provided cost estimates for any of 
the maintenance costs or replacement costs. The FAA did receive certain 
other information from a large operator of two airplane models that 
will be affected by this final rule. (One of the airplane models in 
that fleet currently observes the early-activation procedures required 
by this final rule and the other airplane model does not.) The operator 
stated that the largest contributor to periodic replacement of deicing 
boots on the fleet was erosion of the boot surface, rather than fatigue 
that would be caused by activation of deicing boots at the first sign 
of ice accretion.
    The FAA recognizes that, in accomplishing the requirements of any 
AD, operators may incur ``incidental'' costs in addition to the 
``direct'' costs that are reflected in the cost analysis presented in 
the AD preamble. However, the cost analysis in AD rulemaking actions 
typically does not include incidental costs. In the case of this AD, 
for example, the requirements are to revise the AFM to include certain 
information. How operators actually ``implement'' that information 
thereafter (once it is placed in the AFM) may vary greatly among them: 
for some operators, implementation may necessitate extensive retraining 
among their flightcrews; for others, implementation may merely be 
considered a typical part of the routine, continuous training of their 
flightcrews. In light of this, it would be nearly impossible for the 
FAA to calculate accurately or to reflect all costs associated with the 
AFM revision required by this AD. The FAA has determined that direct 
and incidental costs are still outweighed by the safety benefits of the 
AD.

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 described 
previously. 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

    The FAA estimates that 183 airplanes of U.S. registry will be 
affected by this AD.
    It will take approximately 1 work hour per airplane to accomplish 
the required AFM revisions, at the average labor rate of $60 per work 
hour. Based on these figures, the cost impact of the AD on U.S. 
operators is estimated to be $10,980, or $60 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.

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 12612, 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:

99-19-18  Bombardier, Inc. (Formerly de Havilland, Inc.): Amendment 
39-11307. Docket 99-NM-152-AD.

    Applicability: Model DHC-7 and DHC-8 series airplanes equipped 
with pneumatic deicing boots, certificated in any category.
    Compliance: Required as indicated, unless accomplished 
previously.
    To ensure that flightcrews activate the wing and tail pneumatic 
deicing boots at the first signs of ice accumulation on the 
airplane, accomplish the following:
    (a) Within 10 days after the effective date of this AD: Revise 
the Limitations Section of the FAA-approved Airplane Flight Manual 
(AFM) to include the following requirements for activation of the 
ice protection systems. This may be accomplished by inserting a copy 
of this AD in the AFM.
    `` Except if the AFM otherwise specifies that deicing 
boots should not be used for certain phases of flight (e.g., take-
off, final approach, and landing), compliance with the following is 
required.
     Wing and Tail Leading Edge Pneumatic Deicing Boot 
System, if installed, must be activated:


[[Page 63661]]


--At the first sign of ice formation anywhere on the aircraft, or 
upon annunciation from an ice detector system, whichever occurs 
first; and
--The system must either be continued to be operated in the 
automatic cycling mode, if available; or the system must be manually 
cycled as needed to minimize the ice accretions on the airframe.

     ``The wing and tail leading edge pneumatic deicing boot 
system may be deactivated only after completion of an entire deicing 
cycle after leaving icing conditions.''
    (b) 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, International Branch, ANM-116, FAA, 
Transport Airplane Directorate. The request shall be forwarded 
through an appropriate FAA Operations Inspector, who may add 
comments and then send it to the Manager, International Branch, ANM-
116 ACO.

    Note 1: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the International Branch, ANM-116 ACO.
    (c) Special flight permits may be issued in accordance with 
Secs. 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.
    (d) This amendment becomes effective on December 27, 1999.

    Issued in Renton, Washington, on November 10, 1999.
John J. Hickey,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 99-30143 Filed 11-19-99; 8:45 am]
BILLING CODE 4910-13-U