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

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 1 GALP Ref :FAA / 11154/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 partial relief from the following regulation:

Section 25.981(a)(3) as amended by Amendment 25-102

(a) No ignition source may be present at each point in the fuel tank 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 ignition 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 variability, 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 sectio n §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 s ource. Demonstration of compliance with this requirement would typically require a structured, quantitative safety analysis; design areas that have latent failure conditions typically would be driven 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 Regulation is Impractical

Since the issuance of §25.981(a)(3), as amended by Amendment 25-102, the FAA has conducted certification projects in which applicants found it impractical to meet the requirements of that regulation for some areas of lightning protection for fuel tank structure. Partial exemptions were issued for these projects. These same difficulties exist for the G280 project.

The difficulty of designing multiple-fault-tolerance structure, and the difficulty of detecting failures of hidden structural design features 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 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 inspected or tested after assembly without damaging the structure, joint, or fastener. Examples of such features include a required interference fit between the shank of a fastener and the hole in which the fastener is installed, metal foil or mesh imbedded in composite material, a required clamping force provided by fastener to pull two structural parts together, and required faying surface bond between the flush surfaces of adjacent pieces of structural material such as in a wing skin joint or mounting bracket installation. In addition, other features that can be physically inspected or tested may be located within the fuel tanks; therefore, it is not practical to inspect for failures of those features at short intervals. Examples of such failure include separation or loosening of cap seals over fastener 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 remaining life of the airplane.

Accounting for such long failur e 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 examined 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 th is examination that providing multiple fault tolerance for some structural elements is not practical.

As a result of certification projects, previous to the G280, the FAA has determined that compliance with §25.981(a)(3) is impractical for some areas of lightning 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 certification projects progressing before the revision is complete. This is discussed in FAA Memorandum ANM-112-08-002, Policy on Issuance of Special Conditions and exemptions Related to Lightning Protection of Fuel 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 Zone 2A. The G280 fuel tanks that are located in this area 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 st rike. The Fwd fuel tank skin is attached to the supporting structure by mainly by welding with a relatively small amount of fasteners. The types of fasteners used have fastener/hole in terference fit and are installed using a predefined tension/torque. 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. 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. Lightning protective sealant is installed over t he fasteners ends located inside the fuel tanks to provide fault tolerance. GALP has considered installing lightning protective caps on top of the sealant as an additional protective feature; however this was ruled out as it was not considered as an “independent feature” for providing fault tolerance.Gulfstream Aerospace LP ¤ PO Box 1036 ¤ Airport City 70100 ¤ ISRAEL

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 3 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 fo r a new type certification program.” Since the G280 is designed to carry less that 30 passengers and a payload less than 7500 pounds, 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 practi cable 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. The G280 design incorporates the following features:

 The ECS pack is located in the aft fuselage far from the fuel tanks and no other heat sources are in or adjacent fuel tanks, except jet pumps that which convey a negligible amount of heat.

 For the Fwd and Aft tanks the ratio between the skin surface exposed external air flow and the volume of is greater than 1. This makes them similar to an unheated conventional aluminum wing tank.

 Special louvers are installed in the bottom fairing to increase the cooling air flow around the

Fwd Center and Aft tanks. Temperature measurements in the fuel tanks during flight tests indicate similar behavior of the temperatures in the fuselage tanks and the wing tank. A Monte Carlo analysis performed for the Center t ank based on temperatures measured in flight yielded a flammability exposure of 3.04%.



### Issue of Public Interest: Granting this request for partial exemption 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 workers at Gulfstream Aerospace and its suppliers around the US and the world.

 Several of major corporations and govern mental 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 complying with the requirements of §25.981(a)(3) for the fasteners in some areas of the G280 Fwd Center and Aft fuel tanks.