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 25.843(a) for Pressure Vessel Test 

To Whom It May Concern: 

Sierra Nevada Corporation (SNC) is undertaking an amendment to a previously 

approved STC to expand the maximum operating altitude, currently limited to 45000 

feet by the STC, to the type certificated limit, 47000 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 test as required by 14 CFR 25.843(a). 

Section of the Federal Aviation Regulations (FAR) affected: 

Section 25.843(a), Amendment 25-0 – specifies that the complete pressurized 

cabin must be tested as a pressure vessel for the pressure differential specified 

in Sec. 25.365(d).

The Petitioner’s Supportive Information: 

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

Cessna model 680 aircraft with an altitude limitation of 45000 feet. The Cessna model 

680 is type certified per TCDS T00012WI for a maximum 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 with the camera portal STC and other 

approved modifications installed; however, certification for operation above 45000 feet 

requires a test of the pressure vessel to a pressure 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 the 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 the 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 emergency descent due to a 

decompression event are higher at altitudes above 45000 feet; however, the aircraft 

pressurization system and emergency procedures in the 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 differential 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 pressurization system was designed and 

certified to ensure that the maximum pressure differential will not be exceeded if the 

aircraft is operated within the AFM limits. 

SNC hereby petitions for an exemption from 14 CFR 25.843(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 testing 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 requirements of 14 CFR 25.365(d) for 

operation up to 47000 feet. 

Given that there is no difference in the actual operational 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 to operate at altitudes up to 

47000 feet. 

• The STC camera portal components and fuselage reinforcements (skin and 

frame doublers) were designed and analyzed to meet the load factors required 

for operations above 45000 feet. Analysis of the material 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 stressed areas of the components, 

installation, and affected aircraft structure. 

• During the STC project the components associated with the camera portal 

(consisting of machined fittings, fasteners, gaskets, and acrylic window) were 

tested and shown to meet the 1.67 limit load factors and 2.5 ultimate load factors 

required for operation above 45000 feet. Strain gages were used to gather 

stress and strain data at the critically stressed areas as identified by analysis. 

• During the STC project, SNC tested the aircraft pressure 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 operating 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 analysis. 

• The strain gage data and corresponding validated stresses and loads determined 

from the aircraft pressure vessel test were extrapolated and compared to the 

material and joint allowables to show no yielding at limit load (1.67 times 
444 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 ultimate load corresponding to 2.5 

times the maximum relief valve setting for operation above 45000 feet. 

SNC notes that the risk of damage, the difficulty of detecting damage, the potential cost 

of repairing damage, and uncertainty in assuring continued 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 the 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 pressure vessel would be minimal. 

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

when the aircraft is intended to be returned to service 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 airplanes 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 airplane that could not safely 

be returned to service after test, SNC estimates the cost to test a full scale 

representative pressure vessel to be greater than $10 million. 

5. Limiting the aircraft to a maximum operating altitude of 45000 feet reduces the 

utility of the aircraft and increases operating costs. The decrease in fuel 

efficiency as a result of limiting altitude to 45000 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 potentially affecting 

employment at SNC and related suppliers. 

In summary, SNC is requesting exemption from the requirement 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 maximum operating altitude to 47000 

feet in accordance with TCDS T00012WI for Cessna model 680 aircraft with the camera 

portal STC installed. 

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