DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2016-9522; Product Identifier 2016-NM-144-AD; Amendment
39-19065; AD 2017-20-08]
RIN 2120-AA64

Airworthiness Directives; Gulfstream Aerospace Corporation
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: We are superseding Airworthiness Directive (AD) 2009-17-01,
which applied to certain Gulfstream Model G-IV, GIV-X, GV-SP airplanes
and Model GV airplanes. AD 2009-17-01 required an inspection for
sealant applied to the exterior of the auxiliary power unit (APU)
enclosure (firewall), and a revision of the airplane flight manual
(AFM), as applicable. This AD requires revising the AFM and revising
the applicability to include additional airplanes. This AD was prompted
by a report indicating that the type design sealant applied to the APU
enclosure failed certain tests. We are issuing this AD to address the
unsafe condition on these products.

DATES: This AD is effective November 6, 2017.

The Director of the Federal Register approved the incorporation by
reference of certain publications listed in this AD as of November 6,
2017.

ADDRESSES: For service information identified in this final rule,
contact Gulfstream Aerospace Corporation, Technical Publications Dept.,
P.O. Box 2206, Savannah, GA 31402-2206; telephone 800-810-4853; fax
912-965-3520; email pubs@gulfstream.com; Internet http://
www.gulfstream.com/product_support/technical_pubs/pubs/index.htm. You may
view this referenced service information at the FAA, Transport Standards Branch,
1601 Lind Avenue SW., Renton, WA. For information on the availability
of this material at the FAA, call 425-227-1221. It is also available on
the Internet at http://www.regulations.gov by searching for and
locating Docket No. FAA-2016-9522.

Examining the AD Docket

You may examine the AD docket on the Internet at http://www.regulations.gov
by searching for and locating Docket No. FAA-2016-
9522; or in person at the Docket Management Facility between 9 a.m. and
5 p.m., Monday through Friday, except Federal holidays. The AD docket
contains this AD, the regulatory evaluation, any comments received, and
other information. The address for the Docket Office (phone: 800-647-
5527) is Docket Management Facility, U.S. Department of Transportation,
Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200
New Jersey Avenue SE., Washington, DC 20590.

FOR FURTHER INFORMATION CONTACT: Ky Phan, Aerospace Engineer,
Propulsion and Services Section, FAA, Atlanta ACO Branch, 1701 Columbia
Avenue, College Park, GA 30337; phone: 404-474-5536; fax: 404-474-5606;
email: ky.phan@faa.gov.

SUPPLEMENTARY INFORMATION:

Discussion

We issued a notice of proposed rulemaking (NPRM) to amend 14 CFR
part 39 to supersede AD 2009-17-01, Amendment 39-15991 (74 FR 40061,
August 11, 2009) ("AD 2009-17-01"). AD 2009-17-01 applied to certain
Gulfstream Model G-IV, GIV-X, GV-SP airplanes, and Model GV airplanes.
The NPRM published in the Federal Register on January 4, 2017 (82 FR
737) ("the NPRM"). The NPRM was prompted by a report indicating that
the type design sealant (Aerospace Material Specification (AMS) 3374),
applied to the APU enclosure, does not meet the requirement in 14 CFR
25.1191(b)(1) for a firewall to be fireproof, and failed a
certification test and a company test. The NPRM proposed to require
revising the AFM and revising the applicability to include additional
airplanes. We are issuing this AD to provide the flight crew with
operating procedures for airplanes that have AMS 3374 or Gulfstream
Material Specification (GMS) 4107 sealant applied to the APU enclosure
(firewall). Under certain anomalous conditions such as an APU failure/
APU compartment fire, AMS 3374 or GMS 4107 sealant could ignite the
exterior surfaces of the APU enclosure, and result in propagation of an
uncontained fire to other critical areas of the airplane.

Comments

We gave the public the opportunity to participate in developing
this AD. The following presents the comments received on the NPRM and
the FAA's response to each comment.

Request To Withdraw the NPRM

Gulfstream requested that the NPRM be withdrawn. The commenter
stated that the FAA's findings and decisions in the proposed AD are not
based on analysis of the commenter's supporting data and accepted risk
and safety assessment methodologies. The commenter asserted that its
risk assessments, performed using the FAA's Transport Airplane Risk
Assessment Methodology (TARAM) Handbook, are within the allowable
guidelines of the FAA's TARAM Handbook.

We do not agree with the commenter's request because this final
rule is consistent with FAA policy and orders. The FAA's TARAM is used
to assess risk associated with a wide variety of potential safety
issues. The FAA typically follows the defined risk guidelines contained
in the TARAM Handbook for transport category airplanes. However,
occasionally, other factors affect the decision on whether to issue an
AD. FAA Order 8110.107A, "Monitor Safety/Analyze Data," paragraph 2-
10.e. states:

In rare situations the Aviation Safety Engineer or FAA
management may, based on factors unrelated to the risk analysis,
make recommendations not consistent with risk guidelines for ADs or
other mandatory corrective actions. The decision to accept or reject
these recommendations is made during the CARB [Corrective Action
Review Board].

One such factor, unrelated to risk analysis, is whether the
affected system provides an emergency or safety function. Examples of
emergency/safety systems include seatbelts, life rafts, oxygen systems,
and firewalls. Failure of emergency systems typically do not cause an
accident, but can greatly increase the probability of fatalities in the
event of an additional unrelated failure. TARAM analyses of emergency/
safety systems typically indicate a low TARAM risk. This is due to the
fact that the precipitating event is very rare, for example, high-g
decelerations due to an accident, ditching, explosive decompression,
and engine fire. Ultimately, the decision regarding whether to mandate
airworthiness action for a condition is the responsibility of the FAA's
CARB, which for this AD was comprised of representatives from the
Atlanta ACO Branch, Transport Airplane Directorate, Atlanta MIDO
Section, and the Aircraft Evaluation Group. The CARB unanimously
concluded that factors other than the TARAM risk indicated the need to
mandate corrective action for Gulfstream APU firewalls assembled using
AMS 3374 sealant, in addition to the previously mandated requirements
for Gulfstream APU firewalls assembled using GMS 4107 sealant.

Several 14 CFR part 25 regulations are intended to prevent the
spread of a fire to other critical areas of an airplane in the event of
an in-flight or ground fire; one of these regulations is 14 CFR 25.1191
("Firewalls"). The 14 CFR part 25 regulations include requirements
for (1) fire detection, (2) fire suppression, and (3) fire containment
by a firewall. The AMS 3374 sealant, as applied to the Gulfstream APU
firewall type designs that are the subject of this AD, has been shown
by fire testing to result in backside (cold side) ignition of the
firewall when exposed to a 2,000 degree Fahrenheit flame for 15
minutes, thus violating the 14 CFR part 25 requirement for the firewall
to be fireproof (refer to FAA Advisory Circular (AC) 20-135,
"Powerplant Installation and Propulsion System Component Fire
Protection Test Methods, Standards and Criteria," dated February 6,
1990 ("AC 20-135"), for firewall fire testing guidance.) Previous
fire testing also confirmed that Gulfstream APU firewalls assembled
with GMS 4107 exhibited backside ignition during those tests. The
backside ignition of the Gulfstream APU firewalls occurred in an area
of the airplane that does not have fire detection or fire suppression.
This is a non-compliance with the requirements of 14 CFR 25.1191(b)(1)
for firewalls to be fireproof. If an APU fire occurred in flight or on
the ground on such a non-compliant airplane, it could result in
backside ignition of the firewall, potentially resulting in propagation
of an uncontained fire to other critical areas of the airplane. The
area outside and adjacent to the Gulfstream APU firewall contains many
airplane critical systems such as empennage structure, flight control
components, fuel lines, and oil lines. The FAA finds that APU
operations on the affected Gulfstream models without a firewall that is
fireproof, as required by 14 CFR 25.1191, constitutes an unsafe
condition. The FAA performed an additional TARAM analysis, which indicated
a higher risk than the results of the original Gulfstream TARAM analysis. However,
we want to point out that neither TARAM analysis was the sole consideration for
mandating corrective action. We have made no changes to this AD in this regard.

Request for Separate AD Action for AMS 3374 Sealant

Gulfstream requested that the FAA issue a separate rulemaking
action to address the use of AMS 3374 sealant. The commenter deems it
inaccurate to associate the GMS 4107 sealant unsafe condition with the
application of the AMS 3374 sealant. Gulfstream also considers the
corrective actions to be significantly different for the two types of sealants.

We disagree with the commenter's request. Many of the Gulfstream
airplanes affected by this AD have both GMS 4107 and AMS 3374 sealants
used in the fabrication of APU firewalls. The use of GMS 4107 and/or
AMS 3374 sealants, per the Gulfstream type design for the APU firewalls
that are the subject of this AD action, has resulted in backside
ignition of the APU firewall in fire tests that were intended to
demonstrate that the firewalls are fireproof. The corrective action for
both types of sealants is identical, applying restrictions on APU
operations. The corrective actions specified in the AD being
superseded, AD 2009-17-01, did not address APU firewalls fabricated
using AMS 3374 sealant. Subsequent fire testing has shown that AMS 3374
sealant, used as specified in the Gulfstream type design, does not
comply with the regulations that require a firewall to be fireproof;
therefore, AD 2009-17-01 must be superseded to include APU firewalls
fabricated using AMS 3374 sealant. Future rulemaking to incorporate a
solution proposed by Gulfstream might be considered when and if a
proposed solution is made available to the FAA. We have made no changes
to this AD in this regard.

Request To Clarify Terminology

Gulfstream requested that the FAA revise the NPRM by removing all
of the statements that AMS 3374 sealant is flammable. The commenter
stated that it is not accurate to make a general statement that AMS
3374 sealant is flammable because there are many applications where AMS
3374 sealants are compliant with applicable fireproof certification
requirements.

We partially agree with the commenter's request. The FAA's
certification requirement is that firewalls be fireproof, not that the
sealant be fireproof. The FAA does not have specific requirements for
sealant, apart from the requirement that its use in the assembly of
firewalls must result in a fireproof firewall assembly.

Also, the commenter's statement that there are many applications
where AMS 3374 sealants are compliant with applicable fireproof
certification requirements may be partially correct. There could be
firewalls assembled using AMS 3374 sealants that do meet the applicable
fireproof certification requirements. The issue addressed by this final
rule is that Gulfstream's application of AMS 3374 sealant to the APU
firewall assemblies affected by this rulemaking action is not compliant
with the airworthiness requirement for the firewall to be fireproof.
The AMS 3374 sealant does meet the requirements of an industry
specification, the Society of Automotive Engineers (SAE) Standard AMS
3374. Compliance with an SAE standard is not equivalent to, and does
not satisfy, compliance with the FAA certification requirement that
firewalls be fireproof. AC 20-135 is used throughout the aviation
industry as guidance material for how to show compliance with the FAA's
requirement that firewalls be fireproof. Regarding the use of sealants,
AC 20-135 provides the following guidance:

Outgassing. A characteristic of bonded construction firewall
materials and seal materials is the outgassing of the volatile
constituents of the bonding resins or seal materials. This can occur
from either the hot or cool side surface of the specimens during the
test. These gasses are, in most instances, highly flammable.
Ignition occurring on the cool side is unacceptable in passing the
fire test. . . . For these types of construction, no "cool side"
ignition is allowed and verification is required.

There are many variables that determine if a given firewall
configuration meets the airworthiness requirement to be fireproof.
Sealants are known to outgas volatile constituents. In the case of the
Gulfstream APU firewall type design, outgassed constituents of AMS 3374
sealant ignited on the backside during fire testing, and therefore, the
firewall does not meet the definition of fireproof per AC 20-135. We
have changed the wording in this final rule to specify that the type
design sealant (AMS 3374), as applied in the Gulfstream APU firewall,
does not meet the airworthiness requirement in 14 CFR 25.1191(b)(1),
for a firewall to be fireproof.

We do not agree that AMS 3374 sealant is compliant with applicable
fireproof certification requirements because the FAA does not
certificate sealants; the FAA certificates that firewalls are
fireproof. Therefore, we have made no changes to this AD in this
regard.

Request To Revise the Estimated Costs of Compliance

Gulfstream requested that the estimated costs of compliance in the
NPRM be revised to include costs associated with an operator's
inability to use the APU during normal operations, and the cost
associated with a terminating action. The commenter noted that the
estimated costs in the NPRM are associated with physically revising the
AFM by inserting the applicable AFM supplement (AFMS). The commenter
stated that the costs associated with a terminating action that would
allow an operator to use its APU in flight is much more expensive, and
depending on the number of airplanes that need to be retrofitted, the
costs are likely to be tens of millions of dollars.

The FAA did not include any costs associated with an operator's
inability to use the APU during normal operations because APU usage is
not required by the FAA for the operation of any of the affected
aircraft. This final rule does allow APU usage during certain
emergencies.

We acknowledge that the costs associated with a terminating action,
which would allow an operator to have use of its APU in flight, may be
higher because the costs associated with retrofit of the airplane are
likely to be higher than for implementing the change to the AFMS. This
final rule only provides the costs associated with implementing the
AFMS that restricts APU operations. There are no hardware or
modification costs associated with this final rule.

We do not agree with the commenter's request to revise the
estimated costs of compliance. The FAA uses a standard labor rate of
$85 per hour for evaluation of all airworthiness actions, regardless of
who performs the corrective action. The only cost associated with this
final rule is for revising the AFM by inserting the applicable AFMS.
Therefore, we have made no changes to this final rule regarding this
issue.

Additional Change Made to This Final Rule

The FAA no longer considers this final rule to be an "interim
action" and reference to "interim action," which was included in the
NPRM, has been omitted from this final rule. The FAA will accept the
AFMS restrictions on APU operation as terminating action. If Gulfstream
proposes design changes that would eliminate the APU firewall unsafe
condition addressed by this AD, the FAA might consider further rulemaking.

Conclusion

We reviewed the relevant data, considered the comments received,
and determined that air safety and the public interest require adopting
this AD with the changes described previously, and minor editorial
changes. We have determined that these minor changes:

Are consistent with the intent that was proposed in the
NPRM for correcting the unsafe condition; and

Do not add any additional burden upon the public than was
already proposed in the NPRM.

We also determined that these changes will not increase the
economic burden on any operator or increase the scope of this AD.

Related Service Information Under 1 CFR Part 51

We reviewed the following Gulfstream AFMSs. The AFMSs provide
operating limitations on the use of the APU during certain ground and
flight operations. These documents are distinct since they apply to
different airplane models.

Gulfstream GIV/G300/G400 AFM Supplement GIV-2016-01, dated
July 27, 2016, to the Gulfstream GIV AFM, dated April 22, 1987; the
Gulfstream G300 AFM, dated January 15, 2003; and the Gulfstream G400
AFM, dated November 18, 2002.

Gulfstream G450/G350 AFM Supplement G450-2016-01, dated
July 27, 2016, to the Gulfstream G450 AFM, dated August 12, 2004; and
the Gulfstream G350 AFM, dated October 28, 2004.

Gulfstream GV AFM Supplement GV-2016-01, dated July 27,
2016, to the Gulfstream GV AFM, dated April 11, 1997.

Gulfstream G550/G500 AFM Supplement G550-2016-01, dated
July 27, 2016, to the Gulfstream G550 AFM, dated August 14, 2003; and
the Gulfstream G500 AFM, dated December 5, 2003.

Gulfstream GVI (G650) AFM Supplement G650-2016-01, dated
July 27, 2016, to the Gulfstream GVI (G650) AFM, dated September 7,
2012.

Gulfstream GVI (G650ER) AFM Supplement G650ER-2016-03,
dated July 27, 2016, to the Gulfstream GVI (G650ER) AFM, dated October
2, 2014.

This service information is reasonably available because the
interested parties have access to it through their normal course of
business or by the means identified in the ADDRESSES section.

Costs of Compliance

We estimate that this AD affects 1,220 airplanes of U.S. registry.
We estimate the following costs to comply with this AD:

Estimated Costs

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to
issue rules on aviation safety. Subtitle I, Section 106, describes the
authority of the FAA Administrator. Subtitle VII, Aviation Programs,
describes in more detail the scope of the Agency's authority.

We are issuing this rulemaking under the authority described in
Subtitle VII, Part A, Subpart III, Section 44701, "General
requirements." Under that section, Congress charges the FAA with
promoting safe flight of civil aircraft in air commerce by prescribing
regulations for practices, methods, and procedures the Administrator
finds necessary for safety in air commerce. This regulation is within
the scope of that authority because it addresses an unsafe condition
that is likely to exist or develop on products identified in this
rulemaking action.

This AD is issued in accordance with authority delegated by the
Executive Director, Aircraft Certification Service, as authorized by
FAA Order 8000.51C. In accordance with that order, issuance of ADs is
normally a function of the Compliance and Airworthiness Division, but
during this transition period, the Executive Director has delegated the
authority to issue ADs applicable to transport category airplanes to
the Director of the System Oversight Division.

Regulatory Findings

We have determined that this AD will not have federalism
implications under Executive Order 13132. This AD will not have a
substantial direct effect on the States, on the relationship between
the national government and the States, or on the distribution of power
and responsibilities among the various levels of government.

For the reasons discussed above, I certify that this AD:

(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),

(3) Will not affect intrastate aviation in Alaska, and

(4) 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.

List of Subjects in 14 CFR Part 39

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

Adoption of the Amendment

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

PART 39--AIRWORTHINESS DIRECTIVES

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

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

Sec. 39.13 [Amended]

2. The FAA amends Sec. 39.13 by removing Airworthiness Directive (AD)
2009-17-01, Amendment 39-15991 (74 FR 40061, August 11, 2009), and
adding the following new AD: