444 Salomon Circle  Sparks, Nevada 89434 -9651  (775) 331-0222  Admin. FAX: (775) 331-0370  Eng. FAX: (775)

331-0369 May 6, 2011

U.S. Department of Transportation, Docket Operations West Building Ground Floor, Room W12-140 1200 New Jersey Avenue, SE. Washington, DC 20590

Subject: Petition for Exemption from 14 CFR Part 2 5.843(a) for Pressure Vessel Test

To Whom It May Concern :

Sierra Nevada Corporation (SNC) is undertaking an a mendment to a previously approved STC to expand the maximum operating altitu de, currently limited to 45000 feet by the STC, to the type certificated limit, 47 000 feet. SNC proposes an exemption to the extent necessary to permit certification of the design change by analysis validated by testing instead of a complete pressure vessel te st as required by 14 CFR 25.843(a).

Section of the Federal Aviation Regulations (FAR) a ffected:

Section 25.843(a), Amendment 25-0 – specifies that the complete pressurized cabin must be tested as a pressure vessel for the p ressure differential specified in Sec. 25.365(d).

The Petitioner’s Supportive Information:

Sierra Nevada Corporation (SNC) has certified a cam era portal window via STC on

Cessna model 680 aircraft with an altitude limitati on of 45000 feet. The Cessna model 680 is type certified per TCDS T00012WI for a maxim um operating altitude of 47000 feet. The aircraft operator requires the full type certified altitude envelope to make full use of the mission capabilities of this airplane wi th the camera portal STC and other approved modifications installed; however, certific ation for operation above 45000 feet requires a test of the pressure vessel to a pressur e differential of 1.67 times the maximum relief valve setting.

While Cessna did test a pressure vessel to the 1.67 factor during the original type certification project, the test was never intended to be conducted on a production

aircraft that would be returned to service due to t he risk of potential undetectable damage to the airframe and/or installed equipment, and the potential detrimental effect

on continued airworthiness.

Therefore, as part of the STC, the aircraft with th e camera portal installed was only tested to a maximum pressure differential factor of 1.33 times the maximum relief valve setting as required for operation up to 45000 feet. The STC then limits operation to

45000 feet maximum altitude.444 Salomon Circle  Sparks, Nevada 89434 -9651  (775) 331-0222  Admin. FAX: (775) 331-0370  Eng. FAX: (775)

331-0369 SNC recognizes that the risks associated with emerg ency descent due to a decompression event are higher at altitudes above 4 5000 feet; however, the aircraft pressurization system and emergency procedures in t he event of a decompression event are type certified for operation up to 47000 feet per TCDS T00012WI and are unchanged by the STC. The maximum pressure differe ntial between cabin and ambient conditions, and therefore the loads due to pressure differential, is identical at all altitudes up to 47000 feet because the aircraft pre ssurization system was designed and certified to ensure that the maximum pressure diffe rential will not be exceeded if the aircraft is operated within the AFM limits.

SNC hereby petitions for an exemption from 14 CFR 2 5.843(a) to the extent necessary to permit certification to operate up to a maximum altitude of 47000 feet (in accordance with TCDS T00012WI) by analysis validated by testin g instead of a complete pressure vessel test in accordance with the requirements of 14 CFR 25.843(a). Compliance would be shown by analysis validated by test data, combined with component level tests, to show the installation to meet the require ments of 14 CFR 25.365(d) for operation up to 47000 feet.

Given that there is no difference in the actual ope rational pressure differential and therefore loads applied to the airframe by pressure differential at 47000 feet versus 45000 feet, SNC proposes the following:

• The airplane is type certified per TCDS T00012WI t o operate at altitudes up to

47000 feet.

• The STC camera portal components and fuselage rein forcements (skin and frame doublers) were designed and analyzed to meet the load factors required for operations above 45000 feet. Analysis of the m aterial and joint allowables show no yielding at limit load (1.67 times maximum relief valve setting) and no failure at ultimate load corresponding to 2.5 times the maximum relief valve setting. The analysis identified critically stress ed areas of the components, installation, and affected aircraft structure.

• During the STC project the components associated w ith the camera portal (consisting of machined fittings, fasteners, gasket s, and acrylic window) were

tested and shown to meet the 1.67 limit load factor s and 2.5 ultimate load factors required for operation above 45000 feet. Strain ga ges were used to gather stress and strain data at the critically stressed a reas as identified by analysis.

• During the STC project, SNC tested the aircraft pr essure vessel with camera portal installed per 14 CFR 25.843(a) to 1.33 times the maximum relief valve setting as specified by 14 CFR 25.365(d) for operat ing altitudes up to 45000 feet. Strain gages were used to gather stress and strain data at the critically stressed areas as identified by analysis.

• The strain gage data from both the pressure vessel test and component tests validated the stresses and loads determined by anal ysis.

• The strain gage data and corresponding validated s tresses and loads determined from the aircraft pressure vessel test were extrapo lated and compared to the material and joint allowables to show no yielding a t limit load (1.67 times444 Salomon Circle  Sparks, Nevada 89434 -9651  (775) 331-0222  Admin. FAX: (775) 331-0370  Eng. FAX: (775)

331-0369 maximum relief valve setting) and no failure at ult imate load corresponding to 2.5 times the maximum relief valve setting for operatio n above 45000 feet.

SNC notes that the risk of damage, the difficulty o f detecting damage, the potential cost of repairing damage, and uncertainty in assuring co ntinued airworthiness as a result of testing the operational aircraft pressure vessel to the pressure differential as required by 14 CFR 25.843(a) for operation above 45000 feet is quite high. The cost of a separate full pressure vessel test would be prohibitive for a modification involving a limited

number of units.

SNC feels that granting the petition would be in th e public interest for the following

reasons:

1. Compliance to the required 1.67 factor has been shown by analysis supported and validated by testing.

2. The increase in safety from testing a full press ure vessel would be minimal.

3. Testing an aircraft to limit load to meet the pr essure vessel test requirements

when the aircraft is intended to be returned to ser vice could potentially have a negative effect on safety.

4. The cost of a separate full pressure vessel test would be prohibitively high for a modification involving a limited number of airplane s for the specialized application. Due to the lack of availability of a Cessna model 680 fuselage to use as a test article short of purchasing another a irplane that could not safely be returned to service after test, SNC estimates th e cost to test a full scale representative pressure vessel to be greater than $ 10 million.

5. Limiting the aircraft to a maximum operating alt itude of 45000 feet reduces the utility of the aircraft and increases operating cos ts. The decrease in fuel efficiency as a result of limiting altitude to 4500 0 feet is estimated at between 2.2 and 2.7% for a 2500 nautical mile flight.

6. Reduced fuel efficiency of the modified aircraft as a result of the 45000 foot altitude limitation makes the STC less appealing to other potential operators, thus reducing the marketability of the STC and pote ntially affecting employment at SNC and related suppliers.

In summary, SNC is requesting exemption from the re quirement to test the pressure vessel in accordance with § 25.843(a) to the extent necessary to permit certification by using analysis validated by testing to expand the m aximum operating altitude to 47000

feet in accordance with TCDS T00012WI for Cessna mo del 680 aircraft with the camera portal STC installed.

Please contact the undersigned if you have any ques tions or require further information.