September 26, 2011 

A&C-11-471 

Mr. Victor Wicklund, ANM-112 

Transport Standards Staff 

Transport Airplane Directorate 

Federal Aviation Administration 

1601 Lind Ave, SW 

Renton, WA 98057-3356 

Subject: Additional Information to Support Petition for Partial Exemption to 14 CFR 

25.981(a)(3) for the Gulfstream GVI

FAA Project No.: TC8700AT-T 

Reference: 1) A&C-10-444 – Petition for Partial Exemption to 25.981 

2) FAA / Gulfstream Telecon and WebEx Meeting on September 7, 2011

3) ANM 112-08-002 Policy for Issuance of Special Condition and 

Exemptions Related to Lightning Protection of Fuel Tank Structure, 5-26-

2009

4) FAA letter dated March 24, 2011, to Mr. Anthony Beck, Requesting 

Additional Information to Support Petition for Partial Exemption to 14 CFR 

25.981(a)(3)

 

Dear Mr. Wicklund: 

Per FAA letter dated March 24, 2011 (reference 4), the FAA has requested additional 

information in order to substantiate Gulfstream’s petition for exemption to 14CFR 25.981(a)(3) 

(reference 1). Gulfstream is requesting this exemption from 14 CFR 25.981 for Structural 

Lighting Protection per FAA policy (reference 3). 

FAA Comment 1: 

“The analysis should show that the design, manufacturing processes, and airworthiness 

limitations section of the instructions for continued airworthiness include all practical measures 

to prevent, and detect and correct, failures of structural lightning protection features because of 

manufacturing variability, aging, wear, corrosion, and likely damage.” 

GAC Response to Comment 1:

As a means of preventing failures, Gulfstream has defined locations and features in the 

Zone 1A/2A section of the wing that require protection against potential single faults. 
A&C-11-471

September 26, 2011 

Page 2 of 6 

Gulfstream has implemented protective measures and inspection requirements, during and 

post-manufacturing, to ensure proper application is achieved. The type design data 

incorporates notes corresponding to these requirements that must be adhered to for all 

current wing assemblies constructed and all future assemblies. 

Detection and correction of potential failures is accomplished through mandatory inspection 

criteria. Critical Design Configuration Control Limitations (CDCCL) specify tests or 

inspections at a system/component level. Gulfstream aircraft undergo a mandatory 96-

month inspection interval, specified by the Airplane Maintenance Manual (AMM), in which 

fuel tank fasteners, components and sealant are thoroughly inspected. The GVI AMM will 

also require a 96-month inspection, for which the governing engineering document that 

defines entries to the AMM, is reviewed by a qualified Engineering Authorized 

Representative. This review provides assurance that inspection requirements for these 

critical lightning protection provisions satisfy 14 CFR 25.981(a)(3) protection requirements 

as amended by the requested partial exemption. 

FAA Comment 2: 

“The report addresses the basic fasteners used in the wing structure and briefly addresses the fuel 

access doors. However, the report does not address other detailed structural design features that 

could also require specific lightning protection. For example, those features could include fuel 

vents, fuel filler caps, pump housings, wing-to-body structural attachments, and flap/slat/aileron 

hinge fittings that could have fasteners that extend into the fuel tank structural supports and 

brackets for fuel and hydraulic tubes.” 

GAC Response to Comment 2:

The examples cited in FAA Comment 2 reflect components that are part of the zone 3 

section of the GVI wing. Since making our initial request for partial exemption, Gulfstream 

has conducted analyses and tests of these configurations. Testing includes such features 

as: 

 Flight control attachments; 

 Structural design features such as the rear spar splice, mating point of the Sponson 

with the rear spar and the wing-to-fuselage attachments; 

 Multiple common structural attachment features; 

 Fuel hopper access panels; 

 Hydraulic and fuel system mounting brackets; and, 

 Gravity filler port. 

Cases exist where a component is installed using identical design features to other 

components that were already subjected to direct-strike lightning tests, and therefore are not 

being tested again in the less severe, conducted current format. These include the fuel vent 

system NACA scoop and the Fuel Quantity Measuring System brackets through the forward 

spar. These two features are installed with mounting hardware and features like those on 
A&C-11-471

September 26, 2011 

Page 3 of 6 

the fuel access panels; therefore, the results for analyses and tests of that configuration are 

considered applicable for these two components as well. 

FAA Comment 3: 

“The report does not describe Gulfstream’s testing plans in order to demonstrate that the design 

provides redundant and effective lightning protection.” 

GAC Response to Comment 3:

At the time of the petition for exemption submittal (reference 1), Gulfstream was still 

developing the test plan and requirements to show suitability for the requested partial 

exemption. Since requesting the partial exemption, Gulfstream has produced and released 

the test plan, and coordinated with the FAA. 

FAA Comment 4: 

“The assumed failure rate for fasteners in the analysis is in the range of 10-9. Industry 

experience shows fastener failures do occur; therefore, this rate appears to be overly optimistic. 

Additional substantiation for the assumption should be submitted.” 

GAC Response to Comment 4:

The failure rates that were used for the Fault Tree analyses were taken from NPRD-95. 

These rates were expressed in terms of failures per millions of miles driven, and so 

Gulfstream used a rational conversion to translate those values to the airborne environment, 

giving a corresponding failure rate in terms of hours. Since conducting that analysis, NPRD2011 has been published, giving fastener failure rates in hours, suitable for airborne 

environment analyses. 

Because each airplane uses such a large quantity of fasteners in its construction, the 

exposure rate is high. Consequently, even though a fastener may have a failure rate that 

qualifies for “extremely improbable,” it is still possible for failures to occur over the life of a 

fleet of airplanes in which they are used. 

Gulfstream is updating the wing zonal assessment document to reflect the latest accepted 

values of fastener failure rates, in accordance with NRPD-2011. 

FAA Comment 5: 

“The discussion in section 5.6.1 states that: “Sealing and corrosion protection aspects of the 

assembly process ensures no aging effects due to corrosion which could jeopardize fastener 

bonding.” If this statement is intended to indicate that the analysis is based upon no corrosion 

for the life of the airplane, additional substantiation for this assumption should be submitted.” 
A&C-11-471

September 26, 2011 

Page 4 of 6 

GAC Response to Comment 5:

Gulfstream’s GVI Wing uses similar design processes and materials to all previous 

Gulfstream models. These include proven materials, processes, coatings and finishes, used 

independently and in combination, and proven effective through test to provide corrosion 

resistance. These processes and design philosophies are consistent throughout the 

majority of the aircraft industry and have proven to support Gulfstream’s design criteria for 

life cycle. 

Like previous airplane models (e.g., GV & GV-SP), the GVI will have a 96-month inspection 

for corrosion, defined in the Instructions for Continued Airworthiness of the Airplane 

Maintenance Manual. This inspection has proven effective in the existing Gulfstream fleet at 

detecting and correcting corrosion and degradation of sealant in the wing. The GVI 

airplane’s 96-month corrosion inspection will be enhanced to include specific instructions 

that will ensure presence of all defined critical lightning protection features. 

FAA Comment 6: 

“The analysis indicates quality assurance is dependent upon detecting fuel leakage. Additional 

information should be submitted on any limitation for dispatch of the airplane with fuel leakage 

present.” 

GAC Response to Comment 6:

Gulfstream limits dispatch capability in accordance with the severity of the leakage noted. 

For the least severe leak, typically comprised of visible evidence of moisture over a small 

area surrounding the leak source, a maintenance record is noted, and the space is cleaned 

and monitored regularly by additional inspection. This case does not limit dispatch. A leak 

comprising four drops per hour from any location is sufficient to prevent dispatch of the 

airplane until a repair is completed. 

The Fault Tree Analysis accounts for all of the above cases, and still shows the airplane to 

meet the requirements for “extremely improbable” 

Summary:

In summary, Gulfstream has performed analyses of each zone of the GVI wing and determined: 

a) Single point faults; and 

b) Critical design features requiring additional lightning protection 

The fuel tank zonal assessment report is being updated to reflect this additional information and 

to provide final fault tree analyses and failure modes and effects analyses as required to 

substantiate the efforts that have been taken. 
A&C-11-471

September 26, 2011 

Page 5 of 6 

Gulfstream has been conducting extensive lightning effects testing. The intent of testing is to 

demonstrate that special treatments applied to any identified single point faults in critical fuel 

tank zones is sufficient to inhibit the emission of an arc that could ignite fuel vapor. Additionally, 

testing is being used to demonstrate that components in the less critical zone 3 section of the 

wing, when constructed in accordance with the intended design and without the use of 

additional treatments, are not susceptible to emitting arcs that could ignite fuel vapor in the 

event of lightning strike. Test results as well as corrective actions required to meet this intent, if 

any, will be supplied by a formal report upon completion of all testing and analysis of the results 

obtained. Test witnessing has been performed by qualified Gulfstream Engineering 

Airworthiness Representatives as delegated/requested by the FAA. 

Additionally, Gulfstream will ensure that the inspection requirements for the Instructions for 

Continued Airworthiness and for the Aircraft Maintenance Manual are established and submitted 

to the FAA for approval as required. 

Gulfstream believes that the GVI fuel tank system has been designed with all practical 

measures taken to assure protection against potential ignition due to lightning strike. Additional 

measures were considered and the following rationalization is given against such measures: 

 Fuel tank bladders: 

o Reduce accessibility for structural inspections; 

o Introduce new enclosed fuel vapor area possibly increasing threat of ignition due 

to increased fuel vapor concentration, and thus producing a net reduction in 

safety; 

o Increase weight; 

o Increase emissions; 

o Increase per-unit cost and operating costs; and, 

o Reduce range and therefore marketability of the airplane 

 Double-walled construction: 

o Adds complexity for manufacturing; 

o Does not isolate fuel system from lightning strike effects; 

o Introduces more opportunity for manufacturing defects; 

o Introduces new enclosed fuel vapor area possibly increasing threat of ignition 

due to increased fuel vapor concentration, and thus producing a net reduction in 

safety; 

o Increases weight; 

o Increases emissions; 

o Increases per-unit cost and operating costs; and 

o Reduces range and therefore marketability of the airplane 

 Fuel vapor inerting systems: 

o Add weight; 

o Add complexity; 

o Add per-unit and operating cost; and, 
A&C-11-471 

September 26, 2011 

Page 6 of 6 

o 	 Introduce additional reliability concerns 

• 	 Alternate fasteners: 

o 	 Must not compromise strength, therefore would add weight (i.e., conductive steel 

fasteners compared to aluminum and titanium), resulting in: 

• 	 Increased emissions; 

• 	 Reduced range; and 

• 	 Increased chance of corrosion 

o 	 Are not inherently fault tolerant 

• 	 Monolithic construction (i.e., reduced or no-fasteners): 

o 	 Fasteners still required for access panels and other features as discussed in 

FAA Comment 2 above; 

o 	 Does not necessarily produce a fault tolerant design; and 

o 	 Adhesives as a means of mounting various features to the wing structure is a 

significant departure from established structural practices. 

o 	 Still in significant Research and Development phase and not considered mature 

enough for application on GVI platform. 

Please note that the point of contact at Gulfstream for this action is now Mr. Robert Glasscock. 


If there are any additional questions regarding this petition, please contact the undersigned at 


(912) 965-6817 or Keith Candline at (912) 965-4817. 

Sincerely, 

~~~ Robert Glasscock 


GVI Certification 


P.O . Box 2206 • Sav a nna h , GA 31402·2206 USA· Telephone (912) 965·3000 • gulfstream .c om