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Gulfstream requested that the NPRM be withdrawn.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
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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 thesubject 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 removingall 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 mostinstances, 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 notcertificate 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 isnot 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,
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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 ----------------------------------------------------------------------------------------------------------------
Cost per Cost on U.S.
Action Labor cost Parts cost product operators ---------------------------------------------------------------------------------------------------------------- AFM revision....................... 1 work-hour x $85 per hour $0 $85 $103,700
= $85. ----------------------------------------------------------------------------------------------------------------
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 thevarious 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.