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

Tel: +972-3-935-6624 ¤ Fax: +972-3-935-6750 

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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 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 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 

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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 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 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 this 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 strike. 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 interference 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 the 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 

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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.” 

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 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. 

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 tank 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 governmental 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.