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 Gulfstr eam’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 conti nued airworthiness include all practical measures to prevent, and detect and correct , failures of structural lightni ng protection featur es because of manufacturing variabilit y, 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 meas ures 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 fa steners used in the wing structur e and briefly addresses the fuel access doors. However, the report does not addre ss 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-bo dy 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 onA&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 testin g plans in order to demonstrate that the design provides redundant and eff ective 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 ex emption, 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 ra nge of 10-9. Industry experience shows fastener failures do occur; theref ore, 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 prot ection aspects of the assembly process ensures no aging effects due to corrosion which could jeopardize fastener bonding.” If this statement is intended to indi cate that the analysis is based upon no corrosion for the life of the airplane, additional substant iation 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