Gulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 GALP Ref :FAA / 11154A/CG 1 GALP Ref :FAA / 11154A/CG
Petition for partial Exemption Gulfstream Aerospace LP

Gulfstream Model G280 

### Partial relief from section 14 CFR Part 25.981(a)(3) as amended by Amendment 25-102 of Title 14, Code of Federal Regulations (14 CFR) as it relates to the structural lightning protection of fuel tank fasteners.


### Existing Regulations Gulfstream Aerospace LP (GALP) requests pa rtial relief from the following regulation: Section 25.981(a)(3) as amended by Amendment 25-102 (a) No ignition source may be pres ent at each point in the fuel t ank or fuel tank system where catastrophic failure could occur due to ignition of fuel or vapors. This must be shown by: (1)

... (2)

... (3) Demonstrating that an igniti on source could not result from each single failure, from each single failure in combination with each latent failure condition not shown to be extremely remote, and from all combinations of failures not shown to be extremely improbable. The effects of manufacturing vari ability, aging, wear, corrosion, and likely damage must be considered. This requirement was originally adopted in Amendment 25-102 and requires the assumption that the fuel tanks are always flammable when showing the probability of an ignition source being present is extremely remote. (Amendment 25-102 included section §25.981(c) that required minimizing fuel tank flammability and this was defined in the preamble as being equivalent to unheated aluminum fuel tanks located in the wing.) This requirement defines three types of scenarios that must be addressed in order to show compliance with §25.981(a)(3). The first scenario is that any single failure, regardless of the probability of occurrence of the failure, must not cause an ignition source. The second scenario is that any single failure, regardless of the probability of occurrence, in combination with any latent failure condition not shown to be at least extremely remote, most not cause an ignition source. The third scenario is that any combination of failures not shown to be extremely improbable must not cause an ignition source. Demonstration of compliance with this requirement would typically require a structured, quantitative safety analysis; design areas that have latent failure conditions typically would be dr iven by these requirements to have multiple fault tolerance, or “triple redundancy”. This means that ignition sources are still prevented even after two independent failures.


### Compliance with Existing Re gulation is Impractical Since the issuance of §25.981(a)(3), as ame nded by Amendment 25-102, the FAA has conducted certification projects in which applicants found it impractical to meet the require ments of that regulation for some areas of lightning protection for fuel tank structure. Pa rtial exemptions were issued for these projects. These same difficulties exist for the G280 project. The difficulty of designing multiple-fault-tolerance st ructure, and the difficulty of detecting failures of hidden structural design featur es in general, makes compliance with §25.981(a)(3) uniquely challenging and impractical for certain aspects of the electrical bonding of structural elements.Gulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 GALP Ref :FAA / 11154A/CG 2 

### Such bonding is needed to prevent occurrence of fuel tank ignition sources from lightning strikes. The effectiveness and fault tolerance of electrical bonding features for structural joints and fasteners is partially dependent on design features that cannot be effectively insp ected or tested after assembly without damaging the st ructure, joint, or fastener. Examples of such features include metal foil or mesh imbedded in composite material, a required clamping force provided by fastener to pull two structural parts together, and required faying su rface bond between the flus h surfaces of adjacent pieces of structural material such as in a wing ski n joint or mounting bracket installation. In addition, other features that can be physi cally inspected or tested may be located within the fuel tanks; therefore, it is not practical to inspect for failures of those features at shor t intervals. Examples of such failure include separation or loosening of cap seals over fast ener ends and actual structural failures of internal fasteners. This inability to practically detect manufacturing errors and failures critical to lightning protection results in degraded conditions that occur and remain in place for a very long time, possibly for the rema ining life of the airplane. Accounting for such long failure latency periods in the system safety analysis required by §25.981(a)(3) would require multiple fault tolerance in the structural lighting protection design. As part of the design development activity for the G280, GALP has exami ned possible design provisions to provide multiple fault tolerance in the structural design to prevent ignition sources from occurring in the event of lightning attachment to the airplane critical locations. GALP has concluded from this examination that providi ng multiple fault tolerance for some st ructural elements is not practical. As a result of certification proj ects, previous to the G280, the FAA has determined that compliance with §25.981(a)(3) is impractical for some areas of li ghtning protection for fuel tank structure, and that application of §25.981(a)(3) to those design areas is therefore inappropriate. The FAA plans further rulemaking to revise §25.981(a)(3 ). As appropriate, the FAA plans to issue special conditions or exemptions, for certificat ion projects progressing before the revisi on is complete. This is discussed in FAA Memorandum ANM-112-08-002, Policy on Issuanc e of Special Conditions and exemptions Related to Lightning Protection of F uel Tank Structure, dated May 26, 2009.

Supportive Information: The G280 design Per the guidance of SAE ARP 5414, the area susceptible to lightning attachment lightning strike is Zones 1A and 2A. The design of the G280 is such that there is no fuel tank in Zone 1A. The G280 fuel tanks that are located in Zone 2A are the inner part of the wing tanks (up to 500 mm from the fuselage) and the fwd and aft fuel tanks. Fairings covers most of the upper part of the inner wing and the bottom side of wing tank and the sides of the Fwd and Aft tanks, therefore only a small area is exposed to a direct lightning strike. The Fwd fuel tank skin is attached to the supporting stru cture by mainly by welding with a relatively small amount of fasteners. The types of fasteners used in the Fwd and center are installed using a predefined tension/torque which ensures contact between the countersunk head and the skin panel and also between the nut and the inner structure. The Aft fuel tank skin is attached by Solid rivets where good contact is ensured between the rivet shank and the side of the skin panel hole. Solid rivets also provide good contact between the rivet head and the fuel tank skin.

Access panels are provided to enable periodical inspections of the inside of the fuel tanks in order to detect loose fasteners or sealant. The fuel tank access panels are installed using Dome Nuts that prevent any contact between fuel vapors and a spark due to a failed fastener.Gulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 GALP Ref :FAA / 11154A/CG 3 The installation of the fasteners is performed through strict control of fastener/hole fit, fastener and hole quality, and installation practices as defined in relevant IAI process specifications. After drilling,
the quality of the hole is inspected by a quality assurance inspector who authorizes continuation of the process and installation of the fasteners. A second inspection is performed after completion of the process. Lightning protective sealant is installed over the fastener s ends located inside the fuel tanks to provide fault tolerance. Some additional design features were evaluated by GALP but ruled out. These include: 1) GALP has considered installing lightning protective caps on top of the sealant as an additional protective feature; this was ruled out as it was not considered as an “independent feature” for providing fault tolerance. 2) Use of welding in the Aft and Center tanks; this was ruled out for the Center tank due to level of loads carried by the skin and the attachments of the skin to its supporting structure and for the Aft tank due to its size and hence the ability to ensure the quality of the welding. 3) Extension of the upper and lower wing-fuselage attachment fairings to cover all of zone 2A part that is adjacent to the fuselage; this was ruled out due to increase in weight and aerodynamic drag and adverse affect on the wing cooling due to decrease in the part of the wing that is exposed to the external air flow. The Fwd and Aft fuel tanks are located in areas where the aircraft loads are low. The tanks were tested for vibrations and sloshing and analyzed to insure no fatigue cracking will occur. The Center tank is fatigue tested to two and a half lifetimes and a teardown inspection will be performed to ensure that there will be no fatigue cracking. The G280 Maintenance Manual will include instructions for inspection of the fuel tank fasteners within the scope of the Zonal inspections. In addition, internal inspection of the wing tanks will be performed every 4 years for the internal wing , Fwd, Center and Aft fuel tanks. The internal inspection will include fasteners sealant and wiring of the fuel gauging system.
Flammability prevention In FAA Memorandum ANM-112-08-002 it is stated that: “... it is not envisioned that the FAA would grant exemptions to both §§25.981(a)(3) and the flammability requirements of §25.981 for a new type certification program.” The applicable flammability requirements of §25.981 are as amended by Amendment 25-102. These require that: “(c) The fuel tank installation must include either – (1) Means to minimize the development of flammable vapors in the fuel tanks (in the context of this rule,

“minimize” means to incorporate practicable design methods to reduce the likelihood of flammable vapor);
or (2) Means to mitigate the effect of an ignition of fuel vapor within fuel tanks such that no damage caused by an

ignition will prevent continued safe flight and landing." Out of the two proposed methods in § 25.981(c), GALP has elected to show compliance with §§ 25.981(c) (1), i.e. "to include means to minimize the development of flammable vapors in the tank". In the context of this rule, the term "to minimize" is "means to incorporate practi cable design methods to reduce the likelihood of flame vapors". The means that have been incorporated in the design of the G280 to minimize the development of fuel vapors and are recommended by the AC 25.981-2 are:Gulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 GALP Ref :FAA / 11154A/CG 4 1. Installation of sufficient baffling in the tanks to reduce sloshing
2. Introducing fuel during refueling and transfer at the bottom of the fuel tank and at low velocity (per section 6 of the SAE AIR 1662) by using a tube outlet with "bell mouth" shape and directing it to flow on a grounded surface.
3. Refueling left and right wings in parallel to reduce refuel velocity.
4. Limiting the approved fuels to high flash points fuel types (JET A/A1 and JP-8 and equivalent).

5. Controlling the fuel tanks temperature by:

a. Using aluminum fuel tanks
b. Avoid locating the fuel tanks near heat sources. The only tank which is near a heat source is the wing fuel tank; the aft chamber behind the wing leading edge, where the temperature is regulated to a 130+/-10 Deg. F by the anti-icin g system and protected by over temperature sensor adjusted to 180 Deg. F. . However, as specified in AC 25.981-2A paragraph 10a (1) (b), the "Thermal anti-icing system and thermal anti-ic ing blankets typically do not significantly increase flammability of fuel tanks". In addition, motive flows of the transfer jet pumps reach the wing tanks. The temperature of the motive fuel flow is higher than the temperature of the fuel in the wing tanks. However, its influence on the temperature in the wing is negligible due to the fact that most of the motive flow is used to motivate the feed pumps (more than 60%) and then burned by the engines.

c. The Fwd, Aft and Wing tanks are located outside of the fuselage contour. They are exposed to large aerodynamic surface area with an aerodynamic surface are to volume ratio grater than one (Aft=3.47, Fwd=2.73 and wings =2.97). Therefore, as per AC25.981-2A paragraph 10. (1) (b), they are considered as "conventional unheated aluminum wing tank".
d. As for the Center tank, since it is cover ed by aerodynamic fairing, dedicated ventilation means, such as air scoops and air gap between the tank and surroundings, are used, to force air to flow around the tank and cool the fuel during flight.

In order to derive the thermal data for each fuel tank, several flight tests for measuring the various fuel tanks temperature data were conducted. Among these tests, one represented a shortGulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 GALP Ref :FAA / 11154A/CG 5 mission flight and one represented a long mission, performed per the guidance of Appendix 4 of AC
25.981-2A and AC 25-7A. The temperature data recorded in the flight tests were used for conducting a Thermal Flight test data analysis in which the delta temperature and time constants for each fuel tank were estimated. Based on this analysis, a "fuel tank flammability assessment" (Monte-Carlo) to calculate the Flammability Exposure Evaluation Time (FEET) for each t ank with 10000 iterations was performed.
The results are as follows:

 Fwd tank – 0.29%

 Aft tank - 0.66%

 Wing tank – 1.26%

 Center tank – 3.06% The results of this assessments show clearly that fo r average normal day the Total FEET in the G280 Fwd, Aft and Wing fuel tanks are much less than 3% limit set by amendment 25-125 (which is not applicable to the G280). The FEET for the Center tank is slightly above 3%, but the 0.06% deviation is considered within the calculation accuracy tolerance

. 

### Issue of Public Interest: Granting this request for partial exem ption is in the public interest. GALP has concluded there is no adverse affect on safety based on the current G280 design features noted in the above sections of this petition. Without the partial exemption, a substantial delay will result in completing type certification and delivery of the Gulfstream Model G280 aircraft from the current plan. This delay would have significantly negative impact in several areas:

 Continued resource demand of the FAA.

 Reduction in employment of production worker s at Gulfstream Aero space and its suppliers around the US and the world.

 Several of major corporations and governm ental entities around the US and the world anticipating the delivery of the G280 aircraft to meet their business and critical mission needs would need to find alternatives.

 The balance of trade between the US and other nations would be negatively impacted due to delayed deliveries and lost sales.

 Taxation at the local, state, and federal levels would be adversely impacted from loss of wage taxes and sales taxes.

Conclusion: GALP respectfully requests a partial exemption from co mplying with the requirements of §25.981(a)(3) for the fasteners in some areas of the G280 Fwd Center, Aft and Wing fuel tanks.