# Fokker50Freighter STC Corp

13394 E. Control Tower Road Englewood, Colorado 80112

Page 1 of 4 February 20, 2013

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

Subject: Docket number FAA-2012-0396 – Re-submittal of Pet ition for Exemption from 14 CFR 25.841(a) and 14 CFR 25.841(b)(6) - Increase in max imum operating altitude for Fokker50Freighter aircraft equipped with Large Cargo Door installatio ns per STC ST01916LA.

To Whom It May Concern:

Proposed Action:

Fokker50Freighter STC Corporation (FFF STC) located at 13394 E. Control Tower Road in Englewood Colorado requested an exemption from 14 CFR 25.841( a) and 25.841(b)(6) for Fokker Model F27 Mark 050 aircraft modified by Supplemental Type Certific ate ST01916LA by letter dated March 26, 2012. The FAA requested by letter dated June 5, 2012, and aga in by letter dated January 29, 2013, additional information. The intent of this letter is to provid e the required information and re-submit the petiti on for exemption.

The proposed exemption would allow the modified air craft, with appropriate supplemental oxygen systems installed, to operate with cabin pressure a ltitudes up to 11,300 feet when operated without passengers, by a maximum of three crew members, who are trained in the dangers of hypoxia and knowledgeable of supplemental oxygen systems which would be used by the required pilots anytime the aircraft is operated with a cabin pressure altitude of more than 10,000 feet.

Sections of the Code of Federal Regulations Affecte d:

14CFR25.841(a) at Amendment 25-87 requires that “Pressurized cabi ns and compartments to be occupied must be equipped to provide a cabin pre ssure altitude of not more than 8,000 feet at the maximum operating altitude of the airplane u nder normal operating conditions.”

14CFR25.841(b)(6) at Amendment 25-87 requires “Warning indication at the pilot or flight engineer station to indicate when the safe or prese t pressure differential and cabin pressure altitude limits are exceeded. Appropriate warning m arkings on the cabin pressure differential indicator meet the warning requirement for pressure differential limits and an aural or visual signal (in addition to cabin altitude indicating me ans) meets the warning requirement for cabin pressure altitude limits if it warns the flight cre w when the cabin pressure altitude exceeds 10,000 feet.”

Petitioner’s Supportive Information:

The Fokker 50 aircraft (Type Certificate Data Sheet A-817 Model F27 Mark 050) is a pressurized passenger aircraft certified for a maximum operatin g altitude of 25,000 feet. A Large Cargo Door (LCD) can be installed in these aircraft per Supplemental Type Certificate ST01916LA granted to the Fokker50Freighter STC Corporation in November of 20 06. For structural reasons, the LCD installation uses a reduced nominal pressure differential of 4.1 6 psi versus 5.45 psi for the standard aircraft. Th is is accomplished by modifying the maximum relief value setting on the outflow valves as part of the STC

## Fokker50Freighter STC Corp

Page 2 of 4 installation. With the reduced pressure differentia l, the aircraft is currently limited to 20,000 feet maximum operating altitude in order to have a cabin pressure altitude of not more than 8,000 feet, for compliance with 14 CFR 25.841(a).

The request for exemption from 14 CFR 25.841(a) is to allow for the removal of the current STC maximum operating altitude limitation, such that th e modified aircraft can then be operated up to the standard aircraft certified operating altitude of 2 5,000 feet. The change in operating procedures to t he higher flight altitude would result in a maximum ca bin pressure altitude of 11,300 feet when the aircr aft is operated at its maximum altitude of 25,000 feet wit h the reduced pressurization levels required by the

LCD installation.

FFF STC recognizes that the risks of hypoxia for ex posures above 8,000 feet are higher, especially for the general population; however, we believe that curren t FAA operational regulations show that safety will not be adversely affected with the limited cabin al titudes and operational procedures being proposed. The current FAA general aviation operating rules permit flight with a cabin pressure altitude of up to 12, 500 feet for an unlimited amount of time and up to 14,0 00 feet for a maximum of 30 minutes without the use of supplemental oxygen (14 CFR 91.211). Whereas the commuter/on-demand and air carrier operating rules permit flight with a cabin pressure altitude of up to 10,000 feet for an unlimited amount of tim e and up to 12,000 feet for a maximum of 30 minutes witho ut the use of supplemental oxygen by the flight dec k crew (14 CFR 135.89 and 14 CFR 121.329). With the 2 0,000 foot operational limitation removed the Fokker 50 with LCD could potentially have a cabin a ltitude of over 8,000 feet, for slightly more that six hours with a fully fueled aircraft and no traffic l oad. However, with the 4.16 nominal pressure differ ential a 10,000 foot cabin would not be exceeded until the aircraft altitude is greater than approximately 23 ,000 feet, and the cabin altitude should not exceed 11,3 00 feet under normal operating conditions. Therefor e, it is proposed that aircraft operations above 20,000 f eet would be conducted with no passengers and only the two required crew members plus an optional obse rver in the jump seat, and that the pilots would us e supplemental oxygen anytime the cabin altitude is m ore than 10,000 feet. The aircraft are already cert ified with a supplemental oxygen system meeting the requi rements of 14 CFR 25.1441, including quickdonning, full-face-masks for each flight deck membe r. Furthermore, each crew member is trained in the dangers of hypoxia and knowledgeable of oxygen syst em operations.

The standard aircraft have an aural and visual aler ting system (Integrated Alerting Unit (IUA) and separate Central Annunciator Panel (CAP)) which is unchanged by the existing STC modification with respect to cabin altitude warnings. With this syste m, whenever the cabin altitude exceeds 10,000±800 f eet a Level 3 alert occurs (the level 3 warning comes o n between 9,200 ft and 10,800 ft altitude reading o n the test-set used per attached Fokker maintenance t ask 21-33-02-720-815-A). The Level 3 alert consists of a continuous triple chime audio signal plus two red master warning lights and additionally a red li ght

marked CABIN ALT will illuminate on the Central Ann unciator Panel. There is also a cabin pressure differential indicator, with appropriate warning ma rkings, and cabin altitude indicator. The existing alerting system activates at a minimum 9,200 foot c abin altitude, which with the 4.16 nominal pressure differential for the LCD installation equates to ap proximately a 21,000 foot operating altitude. The request for exemption from 14 CFR 25.841(b)(6) is t o allow a cabin altitude warning in excess of the 10,000 feet warning required per the regulation. Th is relief would prevent activation of the system du ring normal operations above 21,000 feet operating altit udes for the modified aircraft. The FAA has previou sly granted equivalent level of safety (ELOS) findings to other airplane designs for high altitude airport operations were the cabin pressure may exceed the 1 0,000 feet warning requirement of 14 CFR 25.841(b)(6), however, these warnings are of short duration. For this proposal the time above the threshold of the cabin altitude warning at 9,200 fe et could be the entire time at cruise altitude whic h could be over six hours. It is judged that ignoring or ov erriding a warning for an extended period of time i s unacceptable procedure because it could mask a late nt failure of the pressurization system. Therefore, it is proposed that the cabin altitude warning would be a djusted to provide an alert whenever the cabin alti tude exceeds 12,000±500 feet. Fokker50Freighter STC Corp . has worked with Eaton Aerospace, the

## Fokker50Freighter STC Corp

Page 3 of 4 manufacture of the pressure warning switch, to ensu re the sensor design tolerance of ±500 feet is achievable. This level would still provide warning of a pressurization failure which could potentially

result in an unsafe cabin altitude but not cause ac tivation during normal higher altitude operations. It is our believe that the higher 12,000 foot (12,500 max imum with sensor tolerances) warning level provides an acceptable level of safety based on FAA general aviation operational rules that permit aircraft to operate with cabin pressure altitudes of up to 12,5 00 feet for an unlimited amount of time and commuter/on-demand and air carrier operating rules which allow cabin pressure altitude up to 12,000 fe et for a maximum of 30 minutes without the use of supp lemental oxygen by the flight deck crew.

Fokker50Freighter STC Corp. recognizes the risks of hypoxia for the general population are greater for cabin altitudes over 8,000 feet but believes that i n this situation safety will not be adversely affec ted if the following mitigating factors are considered in gran ting the petition:

• The standard airplane is type certificated per TCDS A-817 to o perate at altitudes up to 25,000 feet.

• The installation of the LCD installation reduced the aircraft m aximum relief value setting for the outflow valves for structural reasons only (as related to the LCD installation) and this maximum pressure differential is not changed or exceeded with the higher altitude operations.

• The modified aircraft would be limited to no passengers and a maximum of three flight crew members who are trained in the dangers of hypoxia and knowledgeable of oxygen system operations for operations exceeding a cabin altitude of 8,000 feet.

• A supplemental oxygen system meeting the requirements of 14 CF R 25.1441 including quickdonning, full-face-masks for each flight crew member is instal led in the aircraft.

• Supplemental oxygen will be used by the required pilots any t ime the cabin altitude exceeds 10,000 feet as required by Part 121 and 135 operating rules.

• The revised annunciation for cabin altitude will alert the fligh t crew with an aural or visual signal (in addition to cabin altitude indicating means) whenever the cabin pr essure altitude exceeds an unsafe level which is deemed to be 12,000 feet based on 30 minute maxi mum allowed exposure per Part 121 and 135 operating rules.

• The quantity of supplemental oxygen carried shall be sufficien t for operational requirements.

FFF STC feels that granting the petition to allow f or relief from the maximum cabin altitude and cabin pressure warning requirements for the Fokker 50 air craft equipped with a Large Cargo Door (LCD) per STC ST01916LA would be in the public’s interest bec ause;

• Turbojet aircraft operating at higher altitudes use less fuel wh ich leads to reduced air cargo fares and environmental benefits. Fuel savings, and related emissions red uctions, of 9.2% are estimated.

• Aircraft operating at higher altitudes can avoid more weather whi ch improves safety and reduces operating costs associated with delays and diversions due to adv erse weather conditions.

• Aircraft operating at higher altitudes have increases safety margi ns for terrain avoidance.

• A larger range of operating altitudes for aircraft allows more flexibility for air traffic control routing and avoidance of restricted airspace which improves the efficiency for the national airspace system

and the individual aircraft.

• The above benefits of operating the Fokker50Freighter at higher altitude improve the marketability of the conversions which can lead to more sales and potentially in creased employment.

• The increase in maximum cabin altitude and cabin pressure warning altitude requirements does not adversely affect safety if the above mitigating factors are taken.

Attached is a letter from Aircraft Conversion, the parent company of FFF STC responsible for sales and marketing of the Fokker50Freighter, which provides specific estimates of fuel and emissions reduction that would be achieved when operating at the reques ted higher altitude. The letter also summarizes feedback from the operators on the improvement on o perational flexibility when operating at higher

altitudes.