Fokker50Freighter STC Corp 

13394 E. Control Tower Road 

Englewood, Colorado 80112 

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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 Petition for Exemption from 14 CFR 

25.841(a) and 14 CFR 25.841(b)(6) - Increase in maximum operating altitude for Fokker50Freighter 

aircraft equipped with Large Cargo Door installations 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 Certificate ST01916LA by letter dated March 26, 2012. The 

FAA requested by letter dated June 5, 2012, and again by letter dated January 29, 2013, additional 

information. The intent of this letter is to provide the required information and re-submit the petition for 

exemption. 

The proposed exemption would allow the modified aircraft, with appropriate supplemental oxygen 

systems installed, to operate with cabin pressure altitudes 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 Affected: 

 

14CFR25.841(a) at Amendment 25-87 requires that “Pressurized cabins and compartments to 

be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet 

at the maximum operating altitude of the airplane under 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 preset pressure differential and cabin pressure 

altitude limits are exceeded. Appropriate warning markings 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 means) meets the warning requirement for cabin 

pressure altitude limits if it warns the flight crew 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 operating 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 2006. For structural reasons, the LCD installation 

uses a reduced nominal pressure differential of 4.16 psi versus 5.45 psi for the standard aircraft. This is 

accomplished by modifying the maximum relief value setting on the outflow valves as part of the STC 
Fokker50Freighter STC Corp 

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installation. With the reduced pressure differential, 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 the modified aircraft can then be operated up to the 

standard aircraft certified operating altitude of 25,000 feet. The change in operating procedures to the 

higher flight altitude would result in a maximum cabin pressure altitude of 11,300 feet when the aircraft is 

operated at its maximum altitude of 25,000 feet with the reduced pressurization levels required by the

LCD installation. 

FFF STC recognizes that the risks of hypoxia for exposures above 8,000 feet are higher, especially for the 

general population; however, we believe that current FAA operational regulations show that safety will

not be adversely affected with the limited cabin altitudes 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 14,000 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 time and 

up to 12,000 feet for a maximum of 30 minutes without the use of supplemental oxygen by the flight deck 

crew (14 CFR 135.89 and 14 CFR 121.329). With the 20,000 foot operational limitation removed the 

Fokker 50 with LCD could potentially have a cabin altitude of over 8,000 feet, for slightly more that six 

hours with a fully fueled aircraft and no traffic load. However, with the 4.16 nominal pressure differential 

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 11,300 feet under normal operating conditions. Therefore, it 

is proposed that aircraft operations above 20,000 feet would be conducted with no passengers and only 

the two required crew members plus an optional observer in the jump seat, and that the pilots would use 

supplemental oxygen anytime the cabin altitude is more than 10,000 feet. The aircraft are already certified 

with a supplemental oxygen system meeting the requirements of 14 CFR 25.1441, including quickdonning, full-face-masks for each flight deck member. Furthermore, each crew member is trained in the 

dangers of hypoxia and knowledgeable of oxygen system operations. 

The standard aircraft have an aural and visual alerting 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 system, whenever the cabin altitude exceeds 10,000±800 feet 

a Level 3 alert occurs (the level 3 warning comes on between 9,200 ft and 10,800 ft altitude reading on 

the test-set used per attached Fokker maintenance task 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 light 

marked CABIN ALT will illuminate on the Central Annunciator Panel. There is also a cabin pressure 

differential indicator, with appropriate warning markings, and cabin altitude indicator. The existing 

alerting system activates at a minimum 9,200 foot cabin altitude, which with the 4.16 nominal pressure

differential for the LCD installation equates to approximately a 21,000 foot operating altitude. The 

request for exemption from 14 CFR 25.841(b)(6) is to allow a cabin altitude warning in excess of the 

10,000 feet warning required per the regulation. This relief would prevent activation of the system during 

normal operations above 21,000 feet operating altitudes for the modified aircraft. The FAA has previously 

granted equivalent level of safety (ELOS) findings to other airplane designs for high altitude airport

operations were the cabin pressure may exceed the 10,000 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 feet could be the entire time at cruise altitude which could 

be over six hours. It is judged that ignoring or overriding a warning for an extended period of time is 

unacceptable procedure because it could mask a latent failure of the pressurization system. Therefore, it is 

proposed that the cabin altitude warning would be adjusted to provide an alert whenever the cabin altitude 

exceeds 12,000±500 feet. Fokker50Freighter STC Corp. has worked with Eaton Aerospace, the 
Fokker50Freighter STC Corp 

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manufacture of the pressure warning switch, to ensure 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 activation during normal higher altitude operations. It is 

our believe that the higher 12,000 foot (12,500 maximum 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 12,500 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 feet 

for a maximum of 30 minutes without the use of supplemental 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 in this situation safety will not be adversely affected if the 

following mitigating factors are considered in granting the petition: 

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

• The installation of the LCD installation reduced the aircraft maximum 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 CFR 25.1441 including quickdonning, full-face-masks for each flight crew member is installed in the aircraft. 

• Supplemental oxygen will be used by the required pilots any time 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 flight crew with an aural or visual signal (in 

addition to cabin altitude indicating means) whenever the cabin pressure altitude exceeds an unsafe 

level which is deemed to be 12,000 feet based on 30 minute maximum allowed exposure per Part 121 

and 135 operating rules. 

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

FFF STC feels that granting the petition to allow for relief from the maximum cabin altitude and cabin

pressure warning requirements for the Fokker 50 aircraft equipped with a Large Cargo Door (LCD) per 

STC ST01916LA would be in the public’s interest because; 

• Turbojet aircraft operating at higher altitudes use less fuel which leads to reduced air cargo fares and 

environmental benefits. Fuel savings, and related emissions reductions, of 9.2% are estimated. 

• Aircraft operating at higher altitudes can avoid more weather which improves safety and reduces 

operating costs associated with delays and diversions due to adverse weather conditions. 

• Aircraft operating at higher altitudes have increases safety margins 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 increased 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 requested higher altitude. The letter also summarizes 

feedback from the operators on the improvement on operational flexibility when operating at higher 

altitudes.