Reference: V21RP1005510

Petition for Exemption from 14 CFR 25.841(a)(2) and (3) 

Petitioner: Airbus SAS 

 1 Rond Point Maurice Bellonte 

 31707 Blagnac Cedex 

 France 

Contact in France: Yves Regis 

 Vice President, Product Integrity (EA) 

 33.(0)5.61.93.30.28 

 Yves.Regis@airbus.com

 

 
Reference: V21RP1005510 2

ACRONYMS 

AC Advisory Circular 

A/C Aircraft 

AECMA European Association of Aerospace Industries 

AFA Association of Flight Attendants 

AIA Aerospace Industry Association 

AMM Aircraft Maintenance Manual 

ALPA Air Line Pilots Association 

ARP Airspace Recommended Practice 

ATA Air Transport Association 

ARAC Aviation Rulemaking Advisory Committee 

CAMI FAA Civil Aerospace Medical Institute 

CFR Code of Federal Regulations 

CPCS Cabin Pressure Control System 

DEI Depressurisation Exposure Integral 

DSI Depressurisation Severity Index 

FAA Federal Aviation Administration 

FADEC Full Authority Digital Engine Control 

FAR Federal Aviation Regulation 

FL Flight Level 

GAMA General Aviation Manufacturing Association 

HBR High Bypass Ratio (Engines) 

ICAO International Civil Aviation Organisation 

JAA Joint Aviation Authorities 

JAR Joint Aviation Requirements 

LBR Low Bypass Ratio (Engines) 

MSHWG Mechanical Systems Harmonization Working Group 

RVSM Reduced Vertical Separation Minimum 

SAE Society of Automotive Engineers 

UERF Uncontained Engine Rotor Failure 
Reference: V21RP1005510 3

Table of Contents 

ACRONYMS 2 

1 SUMMARY OF INFORMATION REQUIRED BY 14 CFR 11.81 5 

1-1 SPECIFIC SECTIONS OF THE FAR FROM WHICH WE SEEK 

EXEMPTION: 5 

1-2 EXTENT OF RELIEF SOUGHT, AND REASONS: 5 

1-3 REASONS WHY GRANTING THIS REQUEST WOULD BE IN THE 

PUBLIC INTEREST 5 

1-4 REASONS WHY GRANTING AN EXEMPTION WOULD NOT 

ADVERSELY AFFECT SAFETY 6 

1-5 SUMMARY FOR FEDERAL REGISTER 7 

1-6 REASONS FOR REQUESTING EXERCISE OF THE PRIVILEGES OF 

THE EXEMPTION OUTSIDE THE UNITED STATES 7 

2 SUPPORTING INFORMATION 7 

2-1 TEXT OF FAR 25.841(A), AT AMENDMENT 87 7 

2-2 GENERAL DISCUSSION – GROUNDS SUPPORTING EXEMPTION 8 

3 STATISTICAL INFORMATION ON CABIN DEPRESSURISATION 11 

4 UNCONTAINED ENGINE ROTOR FAILURES 13 

4-1 GENERAL 14 

4-2 IN SERVICE EVENTS 14 

4-3 AIRBUS DEBRIS MODEL FOR CALCULATING HOLE SIZES IN THE 

PRESSURIZED FUSELAGE FOLLOWING AN UNCONTAINED 

ENGINE ROTOR FAILURE 14 

4-4 IMPLEMENTED MEANS TO REDUCE UERF ON A350 XWB 16 

5 A350 XWB DESCENT PERFORMANCE 16 

6 INFLUENCE OF HOLE SIZE ON CABIN PRESSURE TIME HISTORY 17 

7 RESULTS OF THE MSHWG WORKING GROUP 17 

8 DEI METHOD APPLIED TO A350 XWB 21 
Reference: V21RP1005510 4

9 SYSTEM FAILURES RESULTING IN CABIN DECOMPRESSION 22 

10 OTHER FAILURES LEADING TO DECOMPRESSION 22 

10-1 TYRE BURST 22 

10-2 WHEEL FAILURE 22 

10-3 LOSS OF ANTENNA AND DOOR SEALS 22 

10-4 LOSS OF WINDOW OR WINDSHIELD 22 

10-5 FUSELAGE STRUCTURE 22 

11 PUBLIC INTEREST 23 

REFERENCE DOCUMENTS 23 
Reference: V21RP1005510 5

1 SUMMARY OF INFORMATION REQUIRED BY 14 CFR 11.81 

1-1 SPECIFIC SECTIONS OF THE FAR FROM WHICH WE SEEK EXEMPTION 

Exemption from strict compliance with parts of the requirements of 14 CFR 25.841(a)(2)(i) and (ii) and 

(3) is sought for the Airbus A350 XWB aircraft family. 

1-2 EXTENT OF RELIEF SOUGHT, AND REASONS 

Relief is sought from the requirements applied by FAA that compliance with 14 CFR 25.841(a)(2)(i) 

and (ii) and (3) be shown for some uncontained engine rotor failure (UERF) scenarios. All UERF 

scenarios have been analysed and for most of them compliance with the rule can be shown. The 

design of modern aircraft with wing mounted engines does not permit compliance with the cited 

requirements for all uncontained engine rotor failure scenarios. As a practical matter, compliance can 

only be achieved for such aircraft by restricting flight ceilings. Such flight restrictions pose an 

unnecessary additional burden on the public by causing unnecessary additional fuel burn and 

associated additional cost to the flying public, additional high altitude pollutant emissions, and 

increased air traffic congestion with the associated increase in risk. These adverse effects are 

accompanied by inconsistent application of the rules, since aircraft presently in the fleet and those in 

production whose certification was applied for before the rule became effective are not required to 

meet this requirement, even though a simple operational limitation on permissible flight ceiling could 

be mandated to achieve compliance with the rule. This inconsistent application of this requirement 

results in unfair competitive advantage to such older designs, which have been certificated by FAA for 

operation above FL390 (39,000 ft) for more than 35 years with no relevant service difficulties having 

been encountered in the world fleet. Further, the risk to passenger safety that arises from the 

requested exemption is at a minimum and well within the range of uncertainty of the data available 

which purports to show the safety benefit of the rule. Granting this exemption does not set a 

precedent, as the facts and circumstances of this situation are similar to those that led FAA to grant 

e.g. Boeing 787-8 Exemption No. 8857.

1-3 REASONS WHY GRANTING THIS EXEMPTION WOULD BE IN THE 

 PUBLIC INTEREST 

• The A350 aircraft fully complies with the requirements of 14 CFR 25.841(a)(2)(i) and (ii) and 

(3) for all system and structural failure events. An exemption is requested for cabin 

decompressions that can occur from uncontained engine rotor failures that result in large 

holes in the fuselage (i.e., those holes with a geometric area exceeding A1 m2 (A1: defined in 

Appendix I. Airbus believes, based on fleet service experience, that these are rare events (as 

described later on in this petition). This new aircraft complies with the latest FAA 

requirements and, therefore, offers a significantly higher level of safety than pre certified 

transport category aircraft. 

• Approval for operation at FL 430 will enable the air traffic system to provide more capacity, 

and hence more aircraft separation and safety, without adversely affecting the safety of the 

passengers. 

• Approval for flight at FL 430 will enable the A350 to compete fairly with other existing aircraft 

that are not subject to the same requirements, without causing any adverse effects to the 

passengers. 

• Approval for operation at FL 430 will also serve the public interest via the use of the newest 

generation of engines available today, and permit them to operate where they offer lower 

emissions, and higher fuel efficiency. 
Reference: V21RP1005510 6

• Approval for flight at FL 430 provides for more economical operation of the A350, reducing 

the cost to the travelling public. 

1-4 REASONS WHY GRANTING AN EXEMPTION WOULD NOT ADVERSELY 

 AFFECT SAFETY 

The A350 XWB complies fully with all FAA requirements regarding cabin decompression, except 

some extremely rare UERF event scenarios. Calculations can be used to show an increase in 

theoretical “risk” to passengers if these extremely rare scenarios are followed. However, an 

examination of the basis for these risk assessments as described in the available physiological data 

shows that this very small theoretical increase in risk is far smaller than, and certainly within the 

variability of, results from examination of available physiologic data. There is, therefore, no basis for 

assessing a real increase in risk from the conditions that prevail with a grant of this exemption. 

After an introduction and background discussion in Section 2, statistical information described in 

Section 3 of this petition shows that the worst case hypothetical scenario (i.e., 1/3 rotor disk fragment 

striking airplane crown or keel tangentially) has never occurred. 

Section 4 discusses UERF events in general and details the Airbus model used for the damage 

analysis. 

Section 5 discusses the A350 XWB descent performance. To account for uncertainties in terms of 

buffet and handling qualities, a 10% margin has been included in the analysis. Section 6 details the 

effects of hole size on cabin pressure history, showing the analysis that enables us to conclude that 

all but the most extreme scenarios of hole size permit compliance with FAR 25.841(a)(2)(i)(ii) and (3). 

Section 7 reviews the conclusions reached by the Mechanical Systems Harmonization Working 

Group (MSHWG). The MSHWG recommends the use of the Depressurisation Exposure Integral 

(DEI) method to define the envelope of vulnerability of passengers subject to decompression. We 

show in Section 8 that for the A350 XWB, DEI values remain well within the limits proposed by the 

MSHWG. Furthermore, we show that during the worst case anticipated depressurization event the 

cabin altitude stays within the limits proposed by the FAA memorandum ANM-03-112-16 (‘Interim 

Policy on High Altitude Cabin Decompression’). 

Section 9 summarizes that all A350 XWB ATA 21 system failures not shown to be Extremely 

Improbable in the corresponding System Safety Assessment (SSA) can only lead to openings in the 

pressurized fuselage for which direct compliance to FAR 25.841 can be shown. 

Section 10 reviews failure cases other than UERF that might lead to a decompression, and none 

prevents full compliance with FAR 25.841(a)(2)(i) and (ii). 

Section 11 summarizes why the grant of an exemption from FAR25.841(a)(2) & (3) is in the public 

interest. 
Reference: V21RP1005510 7

1-5 SUMMARY FOR FEDERAL REGISTER 

The A350 XWB aircraft fully complies with the requirements of 14 CFR 25.841(a)(2)(i) and (ii) and (3) 

for all system and structural failure events. An exemption is requested to 14 CFR 25.841(a)(2)(i) and 

(ii) and (3) for those cabin decompressions which can occur from uncontained engine rotor failures 

(UERF) that result in holes in the fuselage exceeding a geometric area of A1 m2

 (A1: defined in 

Appendix I of this petition for exemption). 

1-6 REASONS FOR REQUESTING EXERCISE OF THE PRIVILEGES OF THE 

 EXEMPTION OUTSIDE THE UNITED STATES 

The exemption sought pertains to design requirements of 14 CFR 25. If granted, the exemption will 

result in an FAA certificated aircraft design that embodies the privileges of the exemption, and the 

operation of these aircraft will include flights outside the United States. Exercise of the privileges of 

the exemption outside the United States is therefore required. The exemption requested does not 

involve a specific ICAO standard, and a filing of a difference with ICAO would not be required. 

2 SUPPORTING INFORMATION 

2-1 TEXT OF FAR 25.841(a) AT AMENDMENT 87 

The following request for exemption for the Airbus A350 XWB family of aircraft is limited to the 

requirements as set forth in FAR 25.841(a)(2)(i) and (ii) and (3), Amendment 25-87. 

Specifically, while the A350 XWB aircraft fully complies with the requirements of 14 CFR 

25.841(a)(2)(i) and (ii) and (3) for all system and structural failure events, there are some cabin 

decompressions caused by UERF events for which A350 XWB cabin pressure altitudes and/or 

exposure time would exceed those limits. An exemption is requested from CFR 25.841(a)(2)(i) and (ii) 

and (3) for those cabin decompressions which can occur from UERF events that result in holes in the 

fuselage exceeding A1 m2 geometric area. 

The current regulatory requirements as contained in FAR 25.841 Pressurized Cabin at Amendment 

25-87 states, 

(a) 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. 

(1) If certification for operation over 25,000 feet is requested, the airplane must be designed 

so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet 

after any probable failure condition in the pressurization system. 

(2) The airplane must be designed so that occupants will not be exposed to a cabin pressure 

altitude that exceeds the following after decompression from any failure condition not shown 

to be Extremely Improbable: 

(i) Twenty-five thousand (25,000) feet for more than 2 minutes; or 

(ii) Forty thousand (40,000) feet for any duration. 

(3) Fuselage structure, engine and system failures are to be considered in evaluating the 

cabin decompression. 
Reference: V21RP1005510 8

2-2 GENERAL DISCUSSION – GROUNDS SUPPORTING THE EXEMPTION

Airbus respectfully submits that compliance with subsection (2) (i) and (ii) and (3) of the above 

referenced regulation will likely have an adverse market impact on the Airbus A350 XWB family when 

operated in the USA airspace. Airbus further submits that the difficulties, which Airbus A350 XWB 

aircraft has experienced in meeting these requirements, are similarly shared by other manufacturers 

of regional and large transport category aircraft with wing mounted engines for which new or 

amended Type Certificates will be sought. 

For aircraft designs incorporating wing mounted engines FAA provides guidance that the “engine 

failures” that should be considered in evaluating the cabin decompression include a 1/3 fan disc 

tangentially penetrating the pressurized fuselage (consistent with CS/FAR 25.903(d)). 

For aircraft designs incorporating wing mounted engines this tangential fuselage strike scenario 

results, even for large aircraft including the Airbus A350 XWB family, in a cabin depressurisation fast 

enough to reach in less than 20 seconds outside ambient pressure in the cabin. 

With currently available technology it is impossible to descend a large passenger aircraft from 

maximum cruise flight level down to 40,000 ft within about 20 seconds (typical time to decompress 

the aircraft resulting from a major hole in the fuselage) as required indirectly by FAR 25.841(a)(2)(ii). 

In addition, when considering pilot reaction time and the weight of a large passenger aircraft it is 

impossible to reach FL250 within 2 minutes (FAR 25.841(a)(2)(i)) if a rapid decompression occurs at 

maximum cruise flight level. 

Consequently, direct compliance to FAR 25.841(a)(2) and (3) could only be achieved for such aircraft 

by restricting flight ceilings. A limitation of the maximum operating altitude would prevent the A350 

XWB from operating competitively and economically as compared to existing aircraft fleets in the USA 

airspace. In order to comply with the 25.841(a)(2) regulation as written, the A350 XWB would have to 

limit their maximum operating altitude to a level that would be far below FL400. The existing airplanes 

can cruise at the higher altitudes where they are more fuel-efficient. Under typical operational 

conditions, a limitation of max flight level would cause either a significant range deficit or a major 

block fuel increase. 

It should also be noted that current CS regulations will enable the Airbus A350 XWB aircraft to fly up 

to a ceiling of 43,000 feet without any operational restrictions for European operators, whereas 

American operators would be operationally penalized with an U.S. registered A350 XWB due to 

higher fuel burn and reduced range. 

Furthermore, limiting the maximum cruise flight level of new generation airplanes to lower altitudes 

will result in higher traffic density, which could lead to greater collision risk if not otherwise mitigated. 

In anticipation of this increased level of risk, additional costs may be imposed on the aviation system, 

in order to maintain an acceptable level of safety. Larger separation minima may be required, 

resulting in more delay, and less growth potential for U.S. economy. 

Airbus respectfully submits that continuing to allow operation in the less congested airspace above 

40,000 feet will enhance safety. Previously certified large transport aircraft incorporating established 

design practices have operated at altitudes in excess of 40,000 feet for more than 20 years. 

Historically, relatively few accidents or incidents have occurred due to rotor burst during cruise. 

It is important to note that decompression incidents that have exposed an aircraft cabin to pressure 

altitude profiles with risk of injury to passengers are very rare events. Decompression events resulting 

from engine rotor burst are even more remote. The AIA Committee Report On High Bypass Ratio 

Turbine Engine Uncontained Rotor Events (ref. [19], 2010)) performed a data reduction on engine 

rotor burst events covering the period from 1969 to 2006 and concludes a rate of 3.5E-8 per engine 

cycle for basic rotor burst events, averaging 2nd and 3rd generation turbofan engines. For 3rd

generation turbofan engines, the basic rotor burst rate is quoted to be less than 2.5E-8 per engine 

cycle. This is supported by a 5 years rolling average that reveals a rate of 2.1 E-8 per engine cycle. 

This indicates why industry history reveals that few cases of catastrophic decompressions at high 

altitude have occurred, and those that have occurred involve in their majority small business jets. 
Reference: V21RP1005510 9

The FAA assigned the Aviation Rulemaking Advisory Committee (ARAC) and the Mechanical 

Systems Harmonization Working Group (MSHWG) the task of addressing various concerns raised by 

the FAR 25.841 regulation and of reaching a consensus among experts from the FAA, JAA, ATA, 

ALPA, AFA and Aircraft Manufacturer members with representation in the AIA, AECMA or GAMA. 

The MSHWG issued their final report in July 31st, 2003 and recommended the use of the so-called 

Decompression – Severity – Index (DSI) – method to assess the physiological effects of a 

decompression event on aircraft occupants. Airbus has used the recommendation of the MSHWG to 

show that, assuming a worst case depressurisation event, physiological effects to the aircraft 

occupants stay well within the recommended limits of the MSHWG. 

 

In 2006 the FAA issued memorandum ANM-03-112-16 that provides an interim policy on 

decompression issues associated with high altitude flight for the period of time until a new rule is 

available. This memorandum details how the FAA will evaluate petitions for exemption from FAR 

25.841, at amendment 25-87. 

The requirements of ANM-03-03-112-16 (FAA policy on High Altitude Cabin Decompression are as 

follows: 

Cabin Pressure Altitude Maximum Total Exposure Time 

[minutes] 

Above 45000 0 

Above 40000 1 

Above 25000 3 

Above 10000 6 

Airbus will show in section 9 of this petition that these requirements are met. 

The current FAR 25.841(a)(3) rule states that engine failures are also to be considered in showing 

compliance, which Airbus did by an A350 XWB cabin decompression analysis. The results of this 

analysis are compiled in Appendix I of this petition. The limits defined in FAR 25.841(a)(2) will only be 

exceeded for geometrical hole sizes exceeding A1 m2

 (compare figures 2, 4 and 5 of Appendix I). 

Different hole sizes in the pressure vessel result from the application of the guidance material (debris 

model, trajectories) contained in AC20-128A. From this, the percentage of trajectories leading to a 

certain hole size can be derived. If this percentage is combined with the basic rate of occurrence for 

UERF taken from the AIA report ‘High Bypass Ratio Turbine Engine Uncontained Rotor Events 1969 

– 2006 And Small Fragment Threat Characterization’ it becomes obvious that an exceedance of the 

altitude / exposure time limits given by FAR 25.841(a)(2) are, from a probabilistic point of view, highly 

unlikely. 

Without any assumptions on the geometry of debris trajectories and based on the statistics only, the 

AIA report concludes in section 6 that all aircraft level effects classified ‘Catastrophic’ according to 

CS/FAR 25.1309 are in ‘the order of the 1E-9 level traditionally interpreted as being Extremely 

Improbable’. 

In addition, the Airbus A350 XWB will have new design features incorporated that will further reduce 

the risk for occupants to be exposed to low cabin pressure conditions resulting from system failures. 

Those new design features are: 

 

- A multi-redundant fresh air supply that will render a cabin depressurisation caused by total 

loss of fresh air nearly Extremely Improbable. 

- A Cabin Pressure Control System (CPCS) design that will have a sophisticated 

control/monitoring architecture. Furthermore, the dissimiliarity principle has been carefully 

incorporated to prevent any occurrence of common mode failures. 
Reference: V21RP1005510 10

- A pair of spoilers that will have an electrical back-up actuation. In case of total loss of all 

hydraulic power in combination with a significant damage of the pressurized fuselage, the 

aircraft will still be able to deploy a pair of spoilers in order to descend quickly to a safe 

altitude. 

Furthermore it is worth noting that current aircraft standards contain already design features in order 

to minimize cabin decompression hazards for occupants: 

- It is today a worldwide standard on all civil transport aircraft to have First Aid Oxygen onboard for occupant hypoxia treatment. As per CS/FAR 25.1443(d) and CS/FAR 121.333(e)(3) 

First Aid Oxygen needs to be carried in the cabin for at least two percent of the occupants 

able to provide an average flow of 3 litres of oxygen per minute for the entire flight after cabin 

depressurisation occurred. The purpose of First Aid Oxygen is to treat those occupants that 

have suffered from hypoxia by providing the maximum possible blood oxygen saturation level 

in order to ensure best possible recovery. 

- FAR 25.1447(c)(3)(ii) requires the flight crew be given pressure demand type oxygen 

dispensing equipment if decompressions that are not extremely improbable may expose the 

flight crew to cabin pressure altitudes in excess of 34,000 feet. Compliance with the rule 

ensures the flight crew will retain the ability to safely operate the airplane following a 

decompression. 

- As per CS/FAR 25.1443(c) passengers and flight attendants need to be supplied with 

supplemental oxygen. Those means have been developed and introduced to sufficiently 

protect cabin occupants against physiological harm of hypoxia. It has been shown by various 

tests that the supplemental oxygen system provides an adequate protection against hypoxia 

as the masks are correctly used as shown during the safety briefing before each flight. 

Section 25.1441(d) requires that the oxygen flow rate and the oxygen equipment used in airplanes 

operating at high altitudes must be approved. The combination of emergency descent rate 

documented in the Appendix I of this petition, along with the FAA approved oxygen masks and 

associated oxygen flow rates with which the A350 XWB is equipped, are adequate to protect the 

passengers from adverse consequences of a worst case cabin decompression. 
Reference: V21RP1005510 11

3 STATISTICAL INFORMATION ON CABIN DECOMPRESSION 

Transport category airplanes certified prior to introduction of Amendment 25-87 in 1996 have been certified 

up to altitudes of 45,000 feet. Those aircraft have been certificated by FAA to operate at altitudes in excess 

of 40,000 ft for some 35 years representing hundreds of millions of flight hours. 

It should be remembered that very few decompression incidents have exposed an aircraft cabin to pressure 

altitude profiles that run the risk of permanent harm to occupants. Industry experience shows that very few 

cases of catastrophic decompressions at high altitude have occurred, notably in small business jets. 

For large transport aircraft the MSHWG has collected all available cabin decompression data. An extract of 

the most severe cases is shown in Table 3-1. Please note that: 

• While fatalities have occurred during cabin depressurisation events (shown in Table 3-1), none are 

attributable to the extremely rare scenario identified by FAR 25.841(a)(2) and (3) 

• Pressurization system faults predominate in the identified causes of decompression events [see Figure 

3-1]. For the A350 XWB, the corresponding System Safety Assessment will demonstrate that system 

faults leading to the scenario identified by FAR25.841(a)(2) are Extremely Improbable. 

• Decompressions due to engine rotor bursts are rare events [see Figure 3-1] 

Figure 3-1: Pressurization Events by Cause (source MSHWG report) 

Current airplanes with wing-mounted engines would not meet the requirements of 25.841(a) as modified by 

Amendment 25-87 at present certified altitudes. Nevertheless, current industry practice maintains an 

excellent level of safety with respect to rapid decompression at high altitudes. 

Pressurization Events per Hour and per Flight, by Cause

0.00E+00

5.00E-07

1.00E-06

1.50E-06

2.00E-06

2.50E-06

3.00E-06

3.50E-06

4.00E-06

Unknown

Press. Sys.

Door/Seal

Structural

Procedural

Engine

Shootdown/Bomb

# Ev/Hr # Ev/Flt

Pressurization Events per Hour and per Flight, by Cause

0.00E+00

5.00E-07

1.00E-06

1.50E-06

2.00E-06

2.50E-06

3.00E-06

3.50E-06

4.00E-06

Unknown

Press. Sys.

Door/Seal

Structural

Procedural

Engine

Shootdown/Bomb

# Ev/Hr # Ev/Flt
Reference: V21RP1005510 12

Table 3-1: Most Significant Pressurization Events. Note: Based on a Total of 2866 pressurization 

events reported since 1959. Transport Category Airplanes over 60,000 lbs. Source: MSHWG working 

group report 

1 F/A PASSED OUT FROM 39,000 34,000

2

-3 MINUTES, 1 F/A GRAYED OUT, NUMEROUS 

RECEIVED EAR BLOCKS, ONE PSGR SEVERELY HE WAS HOSPITALIZED PSGR 

FOR 3 DAYS

DEPRESSUR

E

- CABIN 

ALT 

34 000

05/05/66

CABIN CREW STEWARD BECAME 38,000 18,000

HYPOXIC 

DEPRESSUR

E

- CABIN 

ALT=18,000 

FT

04/24/63

Aircraft decompressed to 34,000 ft in 26 seconds. Two F/A 39,000 34,000

lost c

onsciousness

almost immediately when they stood up. Aircraft occupants 

were e

xposed to 

altitudes above 30,000 ft for about one minute and 

altitudes abo

ve 25,000ft for 

than 2 minutes. One passenger more 

ejected 

ENGINE 

FAILURE

11/03/73

SEVERAL CREW MEMBERS WERE HOSPITALIZED DUE TO 43,100 28,000

EFFECTS OF DECOMPRESSION.

RAPID 

DECOMPRESSIO

N

-

DUCT 

FAIL

02/02/95

CABIN PRESSURIZATION FAILURE. MAINTENANCE ENGINEER 

FOUND DE

AD IN 

CARGO AREA OF 

AIRPLANE

CREW MEMBER DIED 

IN FLT 

12/31/97

CAPTAIN DONNED A PORTABLE OXYGEN MASK WENT INTO MAIN DECK 30,000 30,000

CARGO AREA . LOST CONSCIOUSNESS AND ULTIMATELY DIED OF 

HYPOXIA 

PILOT 

HYPOXIA

02/09/89

ONE PASSENGER DIED BEFORE 31,000 19,000

LANDING

EMERGENCY

DESCENT/FATALIT

Y

11/03/77

ELDERLY LADY ON OXYGEN. TRANSPORTED TO HOSPITAL WHERE SHE 31,000 10,000

LATER DIED. 

CAB PRESS 

LOSS

-PAX 

ILLNESS

03/09/89

STUDENT FLIGHT ENGINEER BECAME INCAPACITATED HOSPITALIZED 33,000 20,000

BUT DI

ED 

SOON THEREAFTER. 

AEROEMBOLISM 

SUPERNUMERY 

CREW FATALIT

Y

11/11/82

71 YR OLD MAN WITH CARDIAC PROBLEM 35,000 13,800

DIED 

05/07/75 OTHER

Cabin 

Altitude

Event 

Altitude

Date Title Remarks

1 F/A PASSED OUT FROM 39,000 34,000

2

-3 MINUTES, 1 F/A GRAYED OUT, NUMEROUS 

RECEIVED EAR BLOCKS, ONE PSGR SEVERELY HE WAS HOSPITALIZED PSGR 

FOR 3 DAYS

DEPRESSUR

E

- CABIN 

ALT 

34 000

05/05/66

CABIN CREW STEWARD BECAME 38,000 18,000

HYPOXIC 

DEPRESSUR

E

- CABIN 

ALT=18,000 

FT

04/24/63

Aircraft decompressed to 34,000 ft in 26 seconds. Two F/A 39,000 34,000

lost c

onsciousness

almost immediately when they stood up. Aircraft occupants 

were e

xposed to 

altitudes above 30,000 ft for about one minute and 

altitudes abo

ve 25,000ft for 

than 2 minutes. One passenger more 

ejected 

ENGINE 

FAILURE

11/03/73

43,100 28,000

DECOMPRESSION.

RAPID 

DECOMPRESSIO

N

-

DUCT 

FAIL

02/02/95

CABIN PRESSURIZATION FAILURE. MAINTENANCE ENGINEER 

FOUND DE

AD IN 

CARGO AREA OF 

AIRPLANE

CREW MEMBER DIED 

IN FLT 

12/31/97

CAPTAIN DONNED A PORTABLE OXYGEN MASK WENT INTO MAIN DECK 30,000 30,000

CARGO AREA . LOST CONSCIOUSNESS AND ULTIMATELY DIED OF 

HYPOXIA 

PILOT 

HYPOXIA

02/09/89

ONE PASSENGER DIED BEFORE 31,000 19,000

LANDING

EMERGENCY

DESCENT/FATALIT

Y

11/03/77

ELDERLY LADY ON OXYGEN. TRANSPORTED TO HOSPITAL WHERE SHE 31,000 10,000

LATER DIED. 

CAB PRESS 

LOSS

-PAX 

ILLNESS

03/09/89

STUDENT FLIGHT ENGINEER BECAME INCAPACITATED HOSPITALIZED 33,000 20,000

BUT DI

ED 

SOON THEREAFTER. 

AEROEMBOLISM 

SUPERNUMERY 

CREW FATALIT

Y

11/11/82

71 YR OLD MAN WITH CARDIAC PROBLEM 35,000 13,800

DIED 

05/07/75 OTHER

Cabin 

Altitude

Event 

Altitude

Date Title Remarks

CAPTAIN WAS SUCKED PARTWAY OUT OF COCKPIT SUFFERED FACIAL 

BRUISES, FRACTURED ELBOW, WRIST AND THUMB, AND FROSTBITE 

RAPID SOME FLIGHT ATTENDANTS WERE LYING ON THE FLOOR. 19,000 19,000 

DECOMPRESSION

11/23/93 

ONE CREW MEMBER FAINTED FROM LOSS OF OXYGEN WHILE AIDING 20,000 20,000 

PASSENGERS WITH THEIR MASKS. 

DEPRESSURIZATION 

AT 20,000 FT 

04/11/60 

INJURIES SUSTAINED BY THE SURVIVORS WERE CAUSED BY THE EVENTS 23,000 23,000 

ASSOCIATED WITH THE DECOMPRESSION, SUCH AS BARO - 

AND CUTS AND ABRASIONS FROM THE FLYING DEBRIS IN THE CABIN. Nine 

passengers ejected. 

CARGO 

DOOR/FUSELAGE 

OPENED IN FLIGHT 

02/24/89 

ONE FLIGHT ATTENDANT WAS LOST OVERBOARD DURING THE 24,000 24,000 

DECOMPRESSION. ANOTHER FLIGHT ATTENDANT AND 7 PASSENGERS 

RECEIVED SERIOUS INJURIES OF LACERATIONS, SKELETAL FRACTURES AND

CONCUSSIONS. 

FUSELAGE OPENED 

IN FLIGHT 

04/28/88 

F/A FELL TO FLOOR, UNCONSCIOUS FOR ABOUT 15 SECONDS AND SUSTAINE 25,000 25,000 

MINOR LEG, HEAD AND HAND INJURIES. 

ATB/EXPLOSIVE 

DECOMPRESSION 

09/17/79 

TWO PASSENGERS AND ONE FLIGHT ATTENDANT WERE UNCONSCIOUS FOR A 28,000 8,500 

SHORT PERIOD. 

ATB/PRESSURIZATION 

LOST 

03/03/87 

CAPT PARTLY EJECT 17,000 17,000 

THRU WINDSHD 

06/10/90 

11/23/93 SOME FLIGHT ATTENDANTS WERE LYING ON THE FLOOR. 19,000 19,000 

ONE CREW MEMBER FAINTED FROM LOSS OF OXYGEN WHILE AIDING 20,000 20,000 

PASSENGERS WITH THEIR MASKS. 

DEPRESSURIZATION 

AT 20,000 FT 

04/11/60 

INJURIES SUSTAINED BY THE SURVIVORS WERE CAUSED BY THE EVENTS 23,000 23,000 

ASSOCIATED WITH THE DECOMPRESSION, SUCH AS BARO - TRAUMA TO EARS, 

AND CUTS AND ABRASIONS FROM THE FLYING DEBRIS IN THE CABIN. Nine 

passengers ejected. 

CARGO 

DOOR/FUSELAGE 

OPENED IN FLIGHT 

02/24/89 

ONE FLIGHT ATTENDANT WAS LOST OVERBOARD DURING THE 24,000 24,000 

DECOMPRESSION. ANOTHER FLIGHT ATTENDANT AND 7 PASSENGERS 

RECEIVED SERIOUS INJURIES OF LACERATIONS, SKELETAL FRACTURES AND

CONCUSSIONS. 

FUSELAGE OPENED 

IN FLIGHT 

04/28/88 

25,000 25,000 

MINOR LEG, HEAD AND HAND INJURIES. 

ATB/EXPLOSIVE 

DECOMPRESSION 

09/17/79 

ATB/PRESSURIZATION TWO PASSENGERS AND ONE FLIGHT ATTENDANT WERE UNCONSCIOUS FOR A 28,000 8,500 

LOST 

03/03/87 

08/26/84 CBN PRESS FAILURE 1 PAX SUFFERED HEART ATTACK 

37,000 10,000 FOUR PASSENGERS AND THREE F/A LOST CONSCIOUSNESS FOR APPROXIMATELY 2 TO 3 

MINUTES 

DUE TO RAPID 

DECOMPRESSION 03/18/99 

3 F/A'S COLLAPSED FROM LACK OF OXYGEN, AND ONE LAVATORY PSGR. 37,000 30,000 

BECAME HYPOXIC.

DEPRESSURED CABIN 

ALT=30,000 FT 

08/21/83 

CAPITAIN AND SENIOR F/A LOST CONSCIOUSNESS 35,000 20,000 RAPID 

DECOMPRESSURIZATI ON 08/13/98 

35,000 25,000 ONE (OR TWO) F/A'S ''CONVULSED AND LOST CONSCIOUSNESS.'' ONE F/A 

BECAME HYPOXIC. 10/03/74 DEPRESSURIZATION 

TWO WOMEN PSGR FAINTED AND WERE ATTENDED TO BY AN ONBOARD DOCTOR 35,000 26,000 DEPRESSURED CABIN 

ALT=26,000 FT 07/22/81 

35,000 29,000 FEMALE PAX LOST CONSCIOUSNESS NO PULSE NOR BREATHING. F/A ADMISTERED 

HEART MESSAGE AND MOUTH RESUSCITATION.

DEPRESSURED CABIN 

ALT>29,000 FT 

07/05/78 

CAPITAIN, FLIGHT ENGINEER AND LEAD FLIGHT ATTENDANT ALL BECAME 33,000 22,001 

UNCONSCIOUSNESS 

DUE TO HYPOXIA 

LOSS OF CABIN 

PRESSURE 

05/12/96 

33,000 33,000 CLIMBING TO FL350 WITH SUPP OXYGEN,THE CAPT BECAME INCAPACITATED 

NITROGEN NARCOSIS (BENDS) TAKEN TO A HOSPITAL SERIOUS CONDITION PILOT 

INCAPACITATION 

03/15/94 
Reference: V21RP1005510 13

Table 4.1 from the AIA report (AIA Committee Report On High Bypass Ratio Turbine Engine 

Uncontained Rotor Events 1969 – 2006 And Small Fragment Threat Characterization, Volume 1, 

January 2010) provides the statistics of basic UERF events in the worldwide fleet between 1969 and 

2006. A clear decrease of the rotor burst rate with increasing engine generation is demonstrated. 

Table 4.1 from the AIA report. Commercial High Bypass Turbofan Disk Uncontainment Statistics, 

1969-2006 

Figure 4.3 of the AIA report cited above amends these general findings by the MSHWG with respect 

to the UERF occurrence above an aircraft altitude of 25000 ft. For the statistical ensemble 

constructed from 1st and 2nd generation engines, less than 10% of the UERF events occur at an 

altitude above 25000 ft. For 2nd generation engines alone, no event above 25000 ft has been 

reported, while for 3rd generation engines, no UERF events at all have been reported up to now. 

Figure 4.3 from AIA report. Aircraft altitude versus cumulative percentage of UERF events 
Reference: V21RP1005510 14

It has to be noted that in the frame of this petition, the statistical material presented has to be 

understood as supplemental information only. Nevertheless, it will be used elsewhere to support the 

demonstration that high altitude cabin decompression events caused by UERF are Extremely 

Improbable. 

4 UNCONTAINED ENGINE ROTOR FAILURES (UERF) 

4-1 GENERAL 

Current design practices for large commercial transports have evolved such that placement of the 

engines is accomplished via attachments to pylons located underneath the wings. While permitting 

benefits to stability and airplane performance, this feature exposes a large area to potential penetration 

of the pressurized vessel in the event of an UERF. 

Smaller commercial airplanes have typically attached the engines to the rear fuselage. The aftpressurized bulkhead has been placed immediately forward of the rotor burst zone. Traditionally this 

has limited pressurized fuselage penetrations to no more than a few small fragments. This concept 

might be safer in terms of cabin depressurisation but it provides a higher risk of engine debris hitting 

the second engine due to its proximity. 

4-2 IN SERVICE EVENTS 

The MSHWG report provides analysis on past decompression events caused by Uncontained Engine 

Failures and lists the resulting total hole area in the fuselage. 

For high bypass engine events, the maximum hole size is 4.5 ft2 (‘Albuquerque event’), or 0,418 m2

. 

It must be noted that a UERF event on an A350 XWB resulting in the same hole size would not lead 

to a cabin altitude in excess of 40000 ft, compare figures 4 and 5 of Appendix I. The associated 

probability of occurrence using the data of Appendix I, figure 2 is below 1E-9/FH, i.e. Extremely 

Improbable, using the most conservative basic UERF rate of occurrence. 

Accounting for the A350 XWB descent performance as depicted in figures 3 and 5 of Appendix I 

reveals that the requirements of the FAA Interim Policy on High Altitude Cabin Decompression (ref. 

ANM-03-112-16) would be met if a hole with a geometric area of 4.5 ft2

 would be assumed in the 

A350 XWB fuselage at FL 430 (exposure time above 25000 ft cabin pressure altitude below 3 

minutes). 

4-3 AIRBUS DEBRIS MODEL FOR CALCULATING HOLE SIZES IN THE 

 PRESSURIZED FUSELAGE FOLLOWING UERF 

Airbus has developed a geometric analysis in order to determine the hole size distribution for an A350 

XWB pressurized fuselage following an UERF. The Airbus model is based on following assumptions: 

- The model is based on fragment sizes and trajectories as defined in FAA AC20 -128A. 

- One third disc fragments are assumed to have infinite energy, thus able to create two fuselage 

holes (one hole when impacting and the second when escaping the fuselage, except for a 

tangential hit). 

- The generated hole size is defined by the linear cut of a 1/3 disc fragment going through the 

fuselage multiplied by the disc thickness. 

- The linear cut resulting from a given 1/3 of disc fragment is variable according to the location 

of the impact on the fuselage. 

- All engine discs - fan, compressor and turbine discs - are considered. It is assumed that the 

failure rate of all discs is identical. 
Reference: V21RP1005510 15

- Only the impact on the pressurized fuselage is considered. The damage of the unpressurised 

fuselage (mainly the centre wing box) is not part of this model. 

Figure 1 of Appendix I shows the results provided by the Airbus Debris Model: 

• Only around 16% of all possible trajectories can hit the pressurized fuselage. 

• Only around 5% of all trajectories can generate a hole size that is bigger than A1 (A1 defined in 

Appendix I). It will be shown in section 6 that for all hole sizes smaller than A1 FAR 25.841(a)(2)(i) 

and (ii) limits can be met entirely by A350 XWB. Please note that in the following the A1 hole size 

will be called “Critical Hole Size”. 

It is worth noting that a certain amount of holes are catastrophic from structure point of view. 

However, those cases have not been excluded from hypoxia consideration and remain part of the 

model. Please note that those “catastrophic fuselage holes” are addressed and covered by the 

engine rotor burst analysis (FAR 25.903(d)). 

The largest hole size in the pressurized fuselage resulting from the model described above 

corresponds to A3 m2

 (in accordance to CS/FAR 25.365(e)(1) ). A3 is defined in Appendix I of this 

petition. 

It will be shown in section 9 of this petition that for a geometrical hole size as large as A3 m2 the 

requirements of the FAA Interim Policy as well as the DEI limits proposed by the MSHWG are met 

(compare also figure 6 of Appendix I). 

It is possible to correlate the data of Figure 1 of Appendix I with following probability data known from 

in-service events: 

• The probability of an UERF event is 3.5E-8 occurrences per engine cycle (ref. 19: AIA 

Committee Report On High Bypass Ratio Turbine Engine Uncontained Rotor Events 1969 – 

2006 And Small Fragment Threat Characterization, Volume 1, January 2010). The given 

probability of occurrence averages over 2nd and 3rd generation turbofan engines. 

• For 2nd and 3rd generation turbofan engines, the AIA report quotes that from 12 UERF events 

none has occurred above 25000 ft. Assuming one event has occurred leads to a conditional 

probability of 1/13 for an UERF above 25000 ft. 

 

By using those data it is possible to establish a relationship between hole size and the likelihood of 

occurrence as shown in Figure 2, Appendix I. From this figure it can be concluded that the probability 

of having a hole bigger than the Critical Hole Size (geometrical hole size for which the cabin altitude 

would stay just below 25000 ft) of A1 m2

 is less than 1E-9 per flight hour, even if the probability to 

have a UERF in cruise / at altitude is neglected. 

It should be highlighted that for the Figures 1 and 2 of Appendix I it has been assumed that all engine 

debris have infinite energy thus creating two holes per trajectory. But, based on observations that 

have been made on various reports (e.g. reports from the Naval Aviation Systems Team - FAA 

Technical Center), Airbus has concluded that most UERF debris have enough energy to penetrate 

the fuselage but not enough to create a second hole for escaping the fuselage. In addition, it is worth 

noting that for both figures even those trajectories had been considered which have to cut massive 

structural elements on their way to the fuselage. In reality most of such debris would never reach the 

fuselage since it would be stopped before doing so. 

This is supported by the AIA committee report that provides extensive material from in-service 

experience on shielding effects provided by aircraft skin and structure. 

Furthermore, a certain portion of the holes with a size bigger than the critical hole size of A1 are 

structurally not survivable (e.g. tangential fuselage strike). Please note that both figures include those 

cases, which is very unrealistic but conservative. 
Reference: V21RP1005510 16

As stated above the probability figures shown in Figure 2 of Appendix I are based on engine data of 

different turbofan generations. 

4-4 IMPLEMENTED MEANS TO REDUCE UERF ON A350 XWB 

The A350 XWB will have third generation high bypass ratio engines that will have following design 

features/maintenance procedures in order to minimize the occurrence of uncontained engine failures: 

• The engines will be equipped with a fan containment ring that reduces the “uncontainment” risk at 

fan level. The fan is considered to have the highest damage potential compared to turbine and 

compressor stages. The containment test of a fan blade forms part of engine certification. 

• Several FADEC protections like over speed protections and vibration indications will be 

implemented in order to reduce the risk of abnormal engine behaviour (including UERF). 

• Engines are regularly inspected externally and internally to detect damages of the 

turbomachinery. After foreign object ingestion by the engine such as debris, bird and ice the AMM 

procedure requires checks in order to evaluate the damage. 

• Engine manufacturers have made progress in developing material more robust to thermal and 

mechanical stress. The A350 XWB engines will profit from those material improvements, which 

will further contribute to a reduced probability of rotor burst events. 

5 A350 XWB DESCENT PERFORMANCE 

The A350 XWB descent capability is shown in Figure 3, Appendix I. Following assumptions have been 

made: 

• For A350 XWB, like on all other Airbus programs, the flight manual will recommend to use the 

autopilot for the emergency descent. This will relieve workload from the flight crew and provides a 

smooth start for the emergency descent in order to avoid head injuries in the cabin. The provided 

curves in Figure 3 of Appendix I represent autopilot descents.

• In the event of an uncontained engine failure, it is assumed that other, unrelated system or structural 

failures do not occur at the same time. This means for the scenario of a worst case damage of the 

pressurized fuselage it can be assumed that all hydraulic systems are available (except the hydraulic 

pumps linked to the failed engine) and no spoiler has been damaged, as detailed in FAA memorandum 

ANM-03-112-16 (‘Interim Policy on High Altitude Cabin Decompression’). 

• The airplane is designed such that in the event of an UERF the airplane structure, systems and other 

engines function enable a Vmo/Mmo rate of descent. For the autopilot descent the speed is slightly below 

that value: Vmo/Mmo– 5 kts. 

• The curves of figure 3 in Appendix I are based on data that have been established by simulation and 

wind tunnel tests. Past experience has shown that in particular in terms of spoiler deflection angle 

margins are required in order to address uncertainties in terms of buffeting and handling qualities. 

Therefore, a margin of 10% has been added to the calculated values, which means that the descent 

time of figure 3 represents 110% of the calculated time. 

It should be noted that: 

• The maximum spoiler deflection angle depends on aircraft weight (the heavier the aircraft the lower is 

the spoiler deflection angle). The maximum spoiler deflection angle is automatically determined by the 

flight control system and does not require specific pilot actions (common descend procedure for all 

A350 XWB weight configurations). 

• The maximum aircraft weight at maximum operating altitude (43,000ft) is W1***. This value depends on 

the engine thrust and buffet onset and is therefore subject to minor changes during the aircraft 

development process. 
Reference: V21RP1005510 17

• The longest emergency descent time that is required to descend down to 10,000 ft is for an A350 XWB 

that weighs W2***. 

• Both descent profiles (for W1 and for W2) provide the envelope for the A350 XWB emergency descent 

time. All other configurations in terms of weight and altitude require less descent time. 

• The provided descent profiles are based on ISA temperature conditions. 

***) W1 and W2 are defined in Appendix I and are also shown in figure 3 of Appendix I.

6 INFLUENCE OF HOLE SIZE ON PRESSURE - TIME HISTORY 

The hole size and its effects on the cabin pressure time history have been investigated and are 

shown in Figure 4, Appendix I. For holes smaller than A1, the cabin pressure altitude does not 

exceed 2 minutes above 25,000 ft, whereas it requires a hole bigger than A2 to exceed 40,000 ft 

cabin altitude. 

The provided data are based on the envelope emergency descent profile, which represents the worst 

case for time spent above 25,000/40,000ft cabin altitude. 

A1, A2 are defined in figure 5 of Appendix I. 

7 RESULTS OF THE ARAC MSHWG WORKING GROUP 

The MSHWG report recommended the use of a means of compliance to the requirements of 

25.841(a) via an approach designated as the Depressurisation Exposure Integral method (DEI). The 

method defines the envelope of vulnerability of passengers for permanent physiological harm during 

a cabin depressurisation. The method assumes conservatively that passengers are not provided with 

supplemental oxygen. 

The MSHWG discovered that when the decompression data from decompression studies 

(References 4, 5 and figure 7-1) were evaluated via the use of an alveolar partial pressure of oxygen 

(i.e., a lung pressure) time integral, a trend became clear. There is a direct correlation of the integral 

in that as the value of the integral increases, there is an increasing likelihood of permanent 

physiological damage. 

For the DEI method the first step was obtaining a relationship called the Depressurisation Severity 

Index (DSI). This relationship provides a measure of the severity of the depressurisation to 

atmospheric total pressure and was determined from published data (Reference 7) and via 

calculation (see figure 7-2). The MSHWG proposed the lung partial pressure of oxygen time integral 

method to assess physiological risk to the occupants and called this measure the Depressurisation 

Exposure Integral (DEI). The integral is based on the period of time in seconds that the lung partial 

pressure of oxygen is below 30 mmHg or 60 mmHg [see Figure 7-3]. 

Note that historically FAA has referenced 10,000 feet [approximately equivalent to DSI of 60 mmHg] 

and 25,000 feet [approximately equivalent to DSI of 30 mmHg] as critical points in the cabin pressure 

altitude, and these were selected as reference conditions. DEI below 30 mmHg and 60 mmHg 

provides a measure of the severity of the depressurisation event. 

 

Figure 6 of Appendix I presents the cabin altitude - time history of a depressurization event resulting 

from a A3 m2

 hole and depicts the resulting values for DEI 30 and DEI 60. Both values are well below 

the threshold recommendation defined by the MSHWG. 
Reference: V21RP1005510 18

Figure 7-1: Pressure time history for the altitude chamber experiments conducted in references 4 and 

 5. 
Reference: V21RP1005510 19

 

Figure 7-2: DSI versus cabin pressure 

Figure 7-3: Integral – DEI30/DEI60 

Note : The Figure 7-3 graphically depicts the DEI30/DEI60 for an event where the cabin pressure 

reaches 51,000 feet pressure altitude. The integral is calculated based upon the difference in DSI and 

the given reference condition (i.e., 30 mmHg). 

DETERMINATION OF DEPRESSURIZATION SEVERITY INDEX (DSI) 

AS A FUNCTION OF AIRCRAFT CABIN PRESSURE

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

CABIN PRESSURE (mmHg)

DEPRESSURIZATION SEVERITY INDEX (mm Hg)

For Cabin Pressure above 411.89 mmHg 

DSI = 0.1752 * (Cabin Pressure)- 30.176

For Cabin Pressure between 47.0 mmHg and 291,995 mmHg

 DSI = 16.964 * Ln(Cabin Pressure ) - 65.312

For Cabin Pressure between 291,995 mmHg and 411.89 mmHg 

DSI = 0.0004441 * (Cabin Pressure )2

 - 0.2209 * (Cabin Pressure ) + 57.625
Reference: V21RP1005510 20

Figure 7-4: DEI60 values. 

Figure 7-5: DEI30 values 

As noted in the MSHWG report, it was observed that a direct correlation of the DEI to increasing 

likelihood of permanent physiological harm exists. The MSHWG found that a value of 2,700 mmHgsec for the DEI30 represents the threshold at which permanent physiological harm started on tested 

animals (Maximum Never to Exceed Value). The same was observed for DEI60 at 15,000 mmHg-sec 

[see Figures 7-4 and 7-5]. 
Reference: V21RP1005510 21

The MSHWG defines in its report “Maximum Allowable Values” for DEI30 and DEI60 in order to 

address uncertainties of the method. The “Maximum Allowable Values for DEI30 and DEI60 equal 

80% of the “Maximum Never to Exceed Value”. 

As further corroboration, the MSHWG utilized the data from the experiment by Dr. Hans Clamann as 

reported in “An Analysis of the Oxygen Protection Problem at Flight Altitudes Between 40,000 and 

50,000 Feet, Final Report”, prepared for the Federal Aviation Agency, Contract FA-955, by Blockley 

and Hanifan, February 20, 1961 (Reference 6). Dr Clamann utilized a chamber to simulate a rapid 

decompression from 9,800 feet to 49,200 feet (pressure altitude) and then repressurised the chamber 

at a rate of 410 feet per second (simulating an airplane rate-of-descent). He did not use supplemental 

oxygen but breathed air at the chamber pressure. It was reported that he retained consciousness 

during the entire, albeit short, event. The resulting integral for this exposure was 1215 mmHg-sec and 

substantiates Dr Clamann’s statement that consciousness was maintained throughout the 

decompression. (See Reference 6). 

8 DEI METHOD APPLIED TO A350 XWB AND COMPARISON WITH FAA 

 INTERIM POLICY ON HIGH ALTITUDE CABIN DECOMPRESSION 

Figure 6 of Appendix I shows the envelope descent profiles of section 5 and the corresponding cabin 

pressure time-history following a sudden cabin depressurisation caused by a A3 m2 hole in the 

pressurized fuselage for an A350 XWB at FL 430. The emergency descent is performed by the 

autopilot, which is triggered 17 seconds after the “Excessive Cabin Altitude” warning appears. 

The 17 seconds include crew recognition time for decompression, oxygen mask donning time and the 

triggering of the autopilot. The value of 17 seconds is provided in the AC 25-20 and has been 

confirmed by the MSHWG. 

For the envelope profile the Depressurisation Exposure Integral (DEI) was calculated per the method 

recommended by the MSHWG. 

From Appendix I, figure 6, the following is concluded: 

For the worst case anticipated geometrical hole size of A3 m2

, 

• the exposure of aircraft occupants to cabin pressure altitudes in excess of 40000 ft (but 

below 45000 ft) is less than 1 minute which is in line with the FAA Interim Policy on high 

altitude decompression 

• the exposure of aircraft occupants to cabin pressure altitudes in excess of 25000 ft is less 

than 3 minutes which is in line with the FAA Interim Policy on high altitude decompression 

• The exposure of aircraft occupants to cabin pressure altitudes in excess of 10000 ft is less 

than 6 minutes which is in line with the FAA Interim Policy on high altitude decompression 

• the Decompression Exposure Integral (DEI) values stay well below the limits recommended 

by the MSHWG* 

* MSHWG recommendation (DEI30: 2,160 mmHg-sec, DEI60: 12,000 mmHg-sec) 
Reference: V21RP1005510 22

9 SYSTEM FAILURES 

Derived from the A350 XWB System Safety Assessments all failures or failure conditions have been 

identified that can result in a cabin depressurisation which cannot be shown to be Extremely 

Improbable. Appendix I lists the corresponding failure conditions, together with their probability range 

and the resulting geometrical leakage size in the pressurized fuselage. All resulting leakage / hole 

sizes are below the critical hole size of A1. Hence, for the applicable failure conditions identified, direct 

compliance to FAR 25.841 (a)(2) can be shown. 

10 OTHER FAILURES LEADING TO DECOMPRESSION 

10-1 TYRE BURST FAILURE 

According to historic data, the tyre burst occurs in flight occasionally but rarely compared to ground 

operation. The ground loads are not applicable in flight and for this condition tyres are extremely 

robust. 

However, the energy of a tyre burst occurring on a landing gear of an A350 XWB might be sufficient 

enough to perforate the pressurized fuselage, but simple analysis shows that the tyre burst generated 

hole would be smaller than the critical hole size of A1 m2

 which means that the limits of FAR 

25.841(a)(2) are not exceeded by tyre burst. 

10-2 WHEEL FAILURE 

Wheel failure that could depressurise the pressure vessel does not occur in flight condition. Very 

stringent requirements are applied to the wheel design and tests. In addition, the MSHWG has 

investigated the events and failure modes of wheel failures and has recommended that those events 

can be excluded to happen during cruise. In any case, a retracted wheel rim release in flight would 

lead to an opening in the pressure vessel that is much smaller than the critical hole size of A1. 

10-3 LOSS OF ANTENNA AND DOOR SEALS 

Losses of antenna and door seals have a negligible influence on A350 XWB cabin pressurization. 

The effects of losing those components are covered by the margins that have been added on the 

“normal” air leakage of the pressurized fuselage. 

10- 4 LOSS OF WINDOW OR WINDSHIELD 

AC 25-20 states that total loss of a window due to fatigue failure may be considered Extremely 

Improbable if the window is designed fail safe and capable of withstanding full cabin pressure in 

conjunction with external aerodynamic pressure with a factor of safety of 1.5 for the primary panel 

alone and 1.15 for the fail-safe panel alone (for aircraft approved for pressure altitudes up to 

45,000ft). The A350 XWB windows and windshields comply with those requirements, hence the loss 

of windows and windshield is not considered in this dossier. 
Reference: V21RP1005510 23

10-5 FUSELAGE STRUCTURE 

AC 25-20 states that the cabin altitude/time history should not exceed the limitations of FAR 

25.841(a) after the maximum pressure vessel opening resulting from an initially detectable crack 

propagating for a period encompassing four normal inspection intervals. Airbus will consider the 

“Critical Hole Size” (A1) as one parameter for the definition of the crack inspection intervals. 

It is assumed that propagation of a crack from stringer to stringer and frame to frame leading to the 

total loss of a skin panel is prevented by scheduled maintenance programs, and therefore does not 

occur. 

The application of probability of structural failure does not form part of the basic structural design 

approach, and therefore probability of structural failure should not be considered in establishing 

compliance to the subject rule. The regulations governing structures are intended to render structural 

failures extremely improbable by virtue of choice of design loads, margins of safety, testing, and 

required maintenance programs. 

11 PUBLIC INTEREST 

The A350 XWB aircraft fully complies with FAR 25.841, amendment 87, except with regard to the 

narrow exemption requested herein. This new aircraft offers a significantly higher level of safety than 

previously certified transport category aircraft under FAR 25. The exemption requested herein will not 

adversely affect safety to the occupants. 

Authorization for flight at FL430 will enable the A350 XWB to compete fairly with other existing aircraft 

that are subject to older amendments without causing any adverse effects on passengers. 

Approval for flight to 43,000 feet will: 

• Enable this new aircraft to fly at FL430, creating more additional lines of aircraft separation in 

US airspace without adversely affecting the safety of the passengers. 

• Enable the Airbus A350 XWB to compete fairly with other existing aircraft that are not subject to 

this amendment without causing any adverse effects to the passengers. 

• Serve the public interest via the use of the new generation engines in the market, as they offer 

lower operational cost and higher fuel efficiency. 

Airplane manufacturers and suppliers will benefit from a granted exemption 

• They will benefit from a competitive aircraft able to find its market. More efficient airplanes that will 

give continuity to their business. 

• Their employees will benefit by sustained employment in the airplane manufacturing industry. 

• The public will benefit due to the availability in the future of newer, more efficient airplanes with the 

possibility of less expensive fares (higher altitudes means less fuel consumption). 

• Newer airplanes should also be more reliable reducing the inconvenience of delays and flight 

cancellations to the public. 

• Finally, there is a benefit to the public in that newer, more efficient airplanes flying at higher altitudes 

will burn less fuel with corresponding less emissions of harmful gases and particulates, and the 

airspace will be more effectively utilized. 

Airplanes not certificated under the FAR would be able to achieve lower operating costs because they could 

be designed to higher maximum certified altitude. This may give a competitive advantage to non-FAA 

certificated airplanes, thus promoting a non-level playing field. 
Reference: V21RP1005510 24

REFERENCE DOCUMENTS 

• "Factors Influencing the Time of Safe Consciousness (TSU) for Commercial Jet Passengers 

Following Cabin Decompression" by Dr. James Gaume, Aerospace Medicine, April 1970. 

• "Sudden High Altitude Cabin Decompression Immediately Threatens Safety of Aircraft Crew 

and Passengers" by Dr Stanley Mohler, Flight Safety Foundation, Human Factors & Aviation 

Medicine, December 1994. 

• “Concepts Providing for Physiological Protection After Aircraft Cabin Decompression in The 

Altitude Range of 60,000 to 80,000 Feet above Sea Level”, Robert P. Garner, DOT/FAA/AM99/4, Office of Aviation Medicine, February, 1999. 

• "Neurological Study of Simulated Decompression in Supersonic Transport Aircraft", 

Aerospace Medicine, J.B. Brierley and A.N. Nicholson, August 1969. 

• "Neurological Sequelae of Prolonged Decompression", Aerospace Medicine, A.N. Nicholson 

and J.R. Ernsting, April 1967. 

• "An Analysis of the Oxygen Protection Problem at Flight Altitudes Between 40,000 and 

50,000 Feet, Final Report", prepared for the Federal Aviation Agency, Contract FA-955, by 

Blockley and Hanifan, February 20, 1961. 

• "Fundamentals of Aerospace Medicine", Roy L. DeHart, second edition, Williams & Wilkons, 

1996, Table 5.12, Respiratory Gas Pressures and Gas Exchange Ratios, pg 91. 

• “Prevention of Hypoxia – Acceptable Compromises”, Aviation, Space, and Environmental 

Medicine, J. Ernsting, March 1978. 

• “A Behavioural and Neuropathological Study of the Sequelae of Profound Hypoxia”, A.N. 

Nicholson, Susan A. Freeland, and J.B. Brierley”, Brain Research, 22 (1970) 327-345. 

• “Hypoxia and Performance Decrement”, William F. O’Connor, Ph. D., Jim Scow, M.D., 

George Pendergrass, Capt., USAF, DOT/FAA/AM 66-15, May, 1966. 

• “Performance of a Continuous Flow Passenger Oxygen Mask at an Altitude of 40,000 Feet”, 

Robert P. Garner, DOT/FAA/AM-96/4, February 1996. 

• “Rapid Decompression of a Transport Aircraft Cabin: Protection Against Hypoxia”, H. Marotte, 

C.Toure, J.M. Clere, and H. Vieillefond, Aviation, Space and Env. Medicine, Jan. 1990. 

• “Effects of Decompression on Operator Performance”, William F. O’Connor, Ph. D., George 

E. Pendergrass, DOT/FAA/AM 66-10, April, 1966. 

• “Behaviour of Naïve Subjects During Decompression: An Evaluation of Automatically 

Presented Passenger Oxygen Equipment”, Chisholm DM, Billings CE, Bason R, Aerospace 

Medicine, February 1, 1974. 

• “Physiologically Tolerable Decompression Profiles for Supersonic Transport Type 

Certification”, Stanley R. Mohler M.D. and P.V. Siegel M.D., DOT/FAA/AM 70-12, July 1970. 

• “SAE AIR 4770 – Report by the Committee on Uncontained Turbine Engine Rotor Events” – 

Data period 1984 through 1989, Aerospace Information Report, 8 April 1996 
Reference: V21RP1005510 25

• AC 20-128A – “Design Considerations for Minimizing Hazards caused by Uncontained 

Turbine Engine and Auxiliary Power Unit Rotor Failure”, Advisory Circular- FAA, 1997 

• AC 25-20 – “Pressurization, Ventilation and Oxygen Systems Assessment for Supersonic 

Flight including High Altitude Operation”, Advisory Circular- FAA, 1996 

• AIA Committee Report On High Bypass Ratio Turbine Engine Uncontained Rotor Events 

1969 – 2006 And Small Fragment Threat Characterization, Volume 1, January 2010.