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

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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 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 GALP Ref :FAA /11154A/CG

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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 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 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 structure 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 

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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 fasteners 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 practicable 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: 
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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-icing 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-icing 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 covered 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 short 
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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 tank 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 for 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 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, Aft and Wing fuel tanks.