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.comReference: 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 Dig ital 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 FailureReference: 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 TH IS 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 EXE RCISE 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 MS HWG WORKING GROUP 17

8 DEI METHOD APPLIED TO A350 XWB 21Reference: 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 23Reference: 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 wi th 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 applicatio n of the rules, since aircraft pr esently in the fleet and those in production whose certification wa s applied for before the rule became effective are not required to meet this requirement, even though a simple operatio nal 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 y ears with no relevant se rvice 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 si tuation 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 requi rements of 14 CFR 25.841(a)(2)(i) and (ii) and (3) for all system and structural failure ev ents. 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 A 1 m 2 (A 1 : defined in Appendix I. Airbus believes, based on fleet service experience, that thes e are rare events (as described later on in this petition). This new aircraft complies with the latest FAA requirements and, therefore, offers a significant ly 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, wi thout adversely affecting the safety of the passengers.

• Approval for flight at FL 430 will enable the A 350 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 mo re 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. Calc ulations 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 thes e 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 conditio ns 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 th at the worst case hypothetical scenario (i.e., 1/3 rotor disk fragment striking airplane crown or keel t angentially) 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 pres sure 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 envelop e 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 t hat 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 A ssessment (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 exempt ion 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 require ments of 14 CFR 25.841(a)(2)(i) and (ii) and (3) for all system and structural failure events. An ex emption 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 A 1 m 2 (A 1 : defined in Appendix I of this petition for exemption).



### 1-6 REASONS FOR REQUESTING EXERCI SE OF THE PRIVILEGES OF THE EXEMPTION OUTSIDE THE UNITED STATES

The exemption sought pertains to design requirem ents 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 t he United States. Exercise of the privileges of the exemption outside the United States is theref ore 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 st ructural failure events, there are some cabin decompressions caused by UERF events for whic h A350 XWB cabin pressure altitudes and/or exposure time would exceed those limits. An exempt ion 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 A 1 m 2 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 8,00 0 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 occ upants will not be exposed to a cabin pressure altitude that exceeds the following after decom pression 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 ma rket impact on the Airbus A350 XWB family when operated in the USA airspace. Airbus further submit s that the difficulties, which Airbus A350 XWB aircraft has experienced in meeting these requirement s, 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 evaluati ng the cabin decompression include a 1/3 fan disc tangentially penetrating the pressurized fuse lage (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 impossi ble 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 fusela ge) as required indirectly by FAR 25.841(a)(2)(ii).

In addition, when considering pilot reaction time a nd 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 t he 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 ar e 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 penal ized with an U.S. registered A350 XWB due to higher fuel burn and reduced range.

Furthermore, limiting the maximum cruise flight le vel 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, addi tional 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 a llow operation in the less congested airspace above 40,000 feet will enhance safety. Previ ously certified large transport ai rcraft incorporating established design practices have operated at altitudes in e xcess of 40,000 feet for more than 20 years. Historically, relatively few accide nts 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 passenger s 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 2 nd and 3 rd generation turbofan engines. For 3 rd 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 avera ge 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 Ad visory 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 consens us 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 31 st , 2003 and recommended the use of the so-called Decompression – Severity – Index (DSI) – meth od to assess the physiological effects of a decompression event on aircraft occ upants. 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 f light for the period of time until a new rule is available. This memorand um details how the FAA will evaluat e 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 M aximum 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 A 1 m 2 (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 Rati o 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 le vel 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 desi gn features incorporated that will further reduce the risk for occupants to be exposed to low cabin pr essure conditions resulting from system failures. Those new design features are:

- A multi-redundant fresh air supply that will re nder 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 signifi cant 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 ca bin for at least two percent of the occupants able to provide an average flow of 3 litres of o xygen 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 ma ximum 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 ex cess of 34,000 feet. Compliance with the rule ensures the flight crew will retain the abilit y 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 o xygen system provides an adequat e protection against hypoxia as the masks are correctly used as shown dur ing 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 petiti on, along with the FAA approved oxygen masks and associated oxygen flow rates with which the A 350 XWB is equipped, are adequate to protect the passengers from adverse c onsequences of a worst case cabin decompression.Reference: V21RP1005510 

### 11 3 STATISTICAL INFORMATION ON CABIN DECOMPRESSION

Transport category airplanes certified prior to intr oduction of Amendment 25-87 in 1996 have been certified up to altitudes of 45,000 feet. Those aircraft have been certificated by FAA to oper ate 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. Indus try 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 a ll 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 depressu risation 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. Neve rtheless, 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

U n k n o w n

P r e ss .

S

y s. Door/S e a l

St ru

c t ur

a l Proced u ral

E n g in

e

S h o ot

d o w n / Bo

m b # 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

U n k n o w n

P r e ss .

S

y s. Door/S e a l

St ru

c t ur

a l Proced u ral

E n g in

e

S h o ot

d o w n / Bo

m b # Ev/Hr # Ev/FltReference: V21RP1005510 

### 12 Table 3-1: Most Significant Pressurization Events. Note: Based on a Total of 2866 pressurization events reported since 1959. Transport Category Ai rplanes over 60,000 lbs. Source: MSHWG working group report 34,000 39,000 1 F/A PASSED OUT FROM 2 - 3 MINUTES, 1 F/A GRAYED OUT, NUMEROUS PSGR RECEIVED EAR BLOCKS, ONE PSGR SEVERELY HE WAS HOSPITALIZED FOR 3 DAYS DEPRESSUR E - CABIN ALT 34 000 05/05/66

18,000 38,000 CABIN CREW STEWARD BECAME HYPOXIC DEPRESSUR E - CABIN ALT=18,000 FT 04/24/63 34,000 39,000 Aircraft decompressed to 34,000 ft in 26 seconds. Two F/A 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 altit udes abo ve 25,000ft for

more than 2 minutes. One passenger ejected ENGINE FAILURE 11/03/73 28,000 43,100 SEVERAL CREW MEMBERS WERE HOSPITALIZED DUE TO 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 30,000 30,000 CAPTAIN DONNED A PORTABLE OXYGEN MASK WENT INTO MAIN DECK CARGO AREA . LOST CONSCIOUSNESS AND ULTIMATELY DIED OF HY POXIA PILOT HYPOXIA 02/09/89 19,000 31,000 ONE PASSENGER DIED BEFORE LANDING EMERGENCY DESCENT/FATALIT Y 11/03/77 10,000 31,000 ELDERLY LADY ON OXYGEN. TRANSPORTED TO HOSPITAL WHERE SHE LATER DIED. CAB PRESS LOSS - PAX ILLNESS 03/09/89 20,000 33,000 STUDENT FLIGHT ENGINEER BECAME INCAPACITATED HOSPITALIZED BUT DI ED SOON THEREAFTER. AEROEMBOLISM SUPERNUMERY CREW FATALIT Y 11/11/82 13,800 35,000 71 YR OLD MAN WITH CARDIAC PROBLEM DIED OTHER 05/07/75 Cabin Altitude Event Altitude Remarks Title Date

34,000 39,000 1 F/A PASSED OUT FROM 2 - 3 MINUTES, 1 F/A GRAYED OUT, NUMEROUS PSGR RECEIVED EAR BLOCKS, ONE PSGR SEVERELY HE WAS HOSPITALIZED FOR 3 DAYS DEPRESSUR E - CABIN ALT 34 000 05/05/66

18,000 38,000 CABIN CREW STEWARD BECAME HYPOXIC DEPRESSUR E - CABIN ALT=18,000 FT 04/24/63 34,000 39,000 Aircraft decompressed to 34,000 ft in 26 seconds. Two F/A 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

more than 2 minutes. One passenger ejected ENGINE FAILURE 11/03/73 28,000 43,100 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 30,000 30,000 CAPTAIN DONNED A PORTABLE OXYGEN MASK WENT INTO MAIN DECK CARGO AREA . LOST CONSCIOUSNESS AND ULTIMATELY DIED OF HYPOXIA PILOT HYPOXIA 02/09/89 19,000 31,000 ONE PASSENGER DIED BEFORE LANDING EMERGENCY DESCENT/FATALIT Y 11/03/77 10,000 31,000 ELDERLY LADY ON OXYGEN. TRANSPORTED TO HOSPITAL WHERE SHE LATER DIED. CAB PRESS LOSS - PAX ILLNESS 03/09/89 20,000 33,000 STUDENT FLIGHT ENGINEER BECAME INCAPACITATED HOSPITALIZED BUT DI ED SOON THEREAFTER. AEROEMBOLISM SUPERNUMERY CREW FATALIT Y 11/11/82 13,800 35,000 71 YR OLD MAN WITH CARDIAC PROBLEM DIED OTHER 05/07/75 Cabin Altitude Event Altitude Remarks Title Date

CAPTAIN WAS SUCKED PARTWAY OUT OF COCKPIT SUFFERED FACIAL BRUISES, FRACTURED ELBOW, WRIS T AND THUMB, AND FROSTBITE 19,000 19,000 SOME FLIGHT ATTENDANTS WERE LYING ON THE FLOOR. RAPID DECOMPRESSION 11/23/93 20,000 20,000 ONE CREW MEMBER FAINTED FROM LOSS OF OXYGEN WHILE AIDING PASSENGERS WITH THEIR MASKS. DEPRESSURIZATION AT 20,000 FT 04/11/60 23,000 23,000 INJURIES SUSTAINED BY THE SURVIVORS WERE CAUSED BY THE EVENTS ASSOCIATED WITH THE DECO MPRESSION, 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 24,000 24,000 ONE FLIGHT ATTENDANT WAS LOST OVERBOARD DURING THE DECOMPRESSION. ANOTHER FLIGHT ATTENDANT AND 7 PASSENGERS RECEIVED SERIOUS INJURIES OF L ACERATIONS, SKELETAL FRACTURES AND CONCUSSIONS. FUSELAGE OPENED IN FLIGHT 04/28/88 25,000 25,000 F/A FELL TO FLOOR, UNCONSCIOUS FO R ABOUT 15 SECONDS AND SUSTAINE MINOR LEG, HEAD AND HAND INJURIES. ATB/EXPLOSIVE DECOMPRESSION 09/17/79 8,500 28,000 TWO PASSENGERS AND ONE FLIGHT ATTE NDANT WERE UNCONSCIOUS FOR A SHORT PERIOD. ATB/PRESSURIZATION LOST 03/03/87

17,000 17,000 CAPT PARTLY EJECT THRU WINDSHD 06/10/90 19,000 19,000 SOME FLIGHT ATTENDANTS WERE LYING ON THE FLOOR. 11/23/93 20,000 20,000 ONE CREW MEMBER FAINTED FROM LOSS OF OXYGEN WHILE AIDING PASSENGERS WITH THEIR MASKS. DEPRESSURIZATION AT 20,000 FT 04/11/60 23,000 23,000 INJURIES SUSTAINED BY THE SURVIVORS WERE CAUSED BY THE EVENTS ASSOCIATED WITH THE DECO MPRESSION, 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 24,000 24,000 ONE FLIGHT ATTENDANT WAS LOST OVERBOARD DURING THE DECOMPRESSION. ANOTHER FLIGHT ATTENDANT AND 7 PASSENGERS RECEIVED SERIOUS INJURIES OF L ACERATIONS, 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 8,500 28,000 TWO PASSENGERS AND ONE FLIGHT ATTE NDANT WERE UNCONSCIOUS FOR A ATB/PRESSURIZATION LOST 03/03/87 08/26/84 CBN PRESS FAILURE 1 PAX SUFFERED HEART ATTACK

10,000 37,000 FOUR PASSENGERS AND THREE F/A LOST CONS CIOUSNESS FOR APPROXIMATELY 2 TO 3 MINUTES DUE TO RAPID DECOMPRESSION 03/18/99

30,000 37,000 3 F/A'S COLLAPSED FROM LACK OF O XYGEN, AND ONE LAVATORY PSGR. BECAME HYPOXIC. DEPRESSURED CABIN ALT=30,000 FT 08/21/83

20,000 35,000 CAPITAIN AND SENIOR F/A LOST CONSCIOUSNESS RAPID DECOMPRESSURIZATI ON 08/13/98

25,000 35,000 ONE (OR TWO) F/A'S ''CONVULSED AND LOST CONSCIOUSNESS.'' ONE F/A BECAME HYPOXIC. DEPRESSURIZATION 10/03/74

26,000 35,000 TWO WOMEN PSGR FAINTED AND WERE A TTENDED TO BY AN ONBOARD DOCTOR DEPRESSURED CABIN ALT=26,000 FT 07/22/81

29,000 35,000 FEMALE PAX LOST CONSCIOUSNESS NO PULSE NOR BREATHI NG. F/A ADMISTERED HEART MESSAGE AND MOUTH RESUSCITATION. DEPRESSURED CABIN ALT>29,000 FT 07/05/78

22,001 33,000 CAPITAIN, FLIGHT ENGINEER AND LEAD FLIGHT ATTENDANT ALL BECAME UNCONSCIOUSNESS DUE TO HYPOXIA LOSS OF CABIN PRESSURE 05/12/96

33,000 33,000 CLIMBING TO FL350 WITH SUPP OXYG EN,THE CAPT BECAME INCAPACITATED NITROGEN NARCOSIS (BENDS) TAKEN TO A HOSPITAL SERIOUS CONDITION PILOT INCAPACITATION 03/15/94Reference: 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 Fragm ent 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 wi th increasing engine generation is demonstrated.

Table 4.1 from the AIA report. Commercial High By pass Turbofan Disk Uncontainment Statistics, 1969-2006

Figure 4.3 of the AIA report cit ed above amends these general findings by the MSHWG with respect to the UERF occurrence above an aircraft al titude of 25000 ft. For the statistical ensemble constructed from 1 st and 2 nd generation engines, less than 10% of the UERF events occur at an altitude above 25000 ft. For 2 nd generation engines alone, no event above 25000 ft has been reported, while for 3 rd generation engines, no UERF events at all have been reported up to now.

Figure 4.3 from AIA report. Aircraft altitude ve rsus cumulative percentage of UERF eventsReference: 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 decompre ssion events caused by UERF are Extremely Improbable.



### 4 UNCONTAINED ENGINE ROTOR FAILURES (UERF)

4-1 GENERAL
Current design practices for large commercial transp orts 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 f eature exposes a large area to potential penetration of the pressurized vessel in the event of an UERF.

Smaller commercial airplanes have typically atta ched the engines to the rear fuselage. The aftpressurized bulkhead has been placed immediately forwar d 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 de compression 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 ft 2 (‘Albuquerque event’), or 0,418 m 2

. It must be noted that a UERF event on an A350 XWB re sulting 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 Appendi x 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 Interi m Policy on High Altitude Cabin Decompression (ref. ANM-03-112-16) would be met if a hole with a geometric area of 4.5 ft 2 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 CAL CULATING 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 li near 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 A 1 (A 1 defined in Appendix I). It will be shown in section 6 that for all hole sizes smaller than A 1 FAR 25.841(a)(2)(i) and (ii) limits can be met entirely by A350 XWB. Please note that in the following the A 1 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 fu selage holes” are addressed and covered by the engine rotor burst analysis (FAR 25.903(d)).

The largest hole size in the pressurized fuse lage resulting from the model described above corresponds to A 3 m 2 (in accordance to CS /FAR 25.365(e)(1) ). A 3 is defined in Appendix I of this petition.

It will be shown in section 9 of this petition t hat for a geometrical hole size as large as A 3 m 2 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 Turb ine Engine Uncontained Rotor Events 1969 – 2006 And Small Fragment Threat Characterization, Volume 1, January 2010). The given probability of occurrence averages over 2 nd and 3 rd generation turbofan engines.

• For 2 nd and 3 rd generation turbofan engines, the AIA repo rt 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 A 1 m 2 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 hole s 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 mo st UERF debris have enough energy to penetrate the fuselage but not enough to create a second hole fo r 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 A 1 are structurally not survivable (e.g. t angential 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 Fi gure 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 bypa ss ratio engines that will have following design features/maintenance procedures in order to minimi ze 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 pr otections 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 dev eloping material more robust to thermal and mechanical stress. The A350 XWB engines will prof it 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 Figu re 3, Appendix I. Following assumptions have been made:



### • For A350 XWB, like on all other Airbus program s, the flight manual will recommend to use the autopilot for the emergency des cent. 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, unrelat ed 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 hy draulic 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 st ructure, systems and other engines function enable a V mo /M mo rate of descent. For the autopilot des cent the speed is slightly below that value: V mo /M mo – 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 experienc e has shown that in particular in terms of spoiler deflection angle margins are required in order to address uncertai nties in terms of buffeting and handling qualities. Therefore, a margin of 10% has been added to t he 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 de flection angle is automatically determined by the flight control system and does not require specif ic pilot actions (common descend procedure for all A350 XWB weight configurations).



### • The maximum aircraft weight at maximum operating alti tude (43,000ft) is W1***. This value depends on the engine thrust and buffet onset and is therefor e 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) provi de 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.
***) W 1 and W 2 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 pressu re time history have been investigated and are shown in Figure 4, Appendix I. For holes smaller than A 1 , the cabin pressure altitude does not exceed 2 minutes above 25,000 ft, wherea s it requires a hole bigger than A 2 to exceed 40,000 ft cabin altitude.
The provided data are based on the envelope emerge ncy descent profile, which represents the worst case for time spent above 25,000/40,000ft cabin altitude.

A 1 , A 2 are defined in figure 5 of Appendix I.



### 7 RESULTS OF THE ARAC MSHWG WORKING GROUP

The MSHWG report recommended the use of a m eans of compliance to the requirements of 25.841(a) via an approach designated as the Depressu risation Exposure Integral method (DEI). The method defines the envelope of vulnerability of passengers for permanent physiological harm during a cabin depressurisation. The method assumes cons ervatively that passengers are not provided with supplemental oxygen.

The MSHWG discovered that when the decomp ression data from decompression studies (References 4, 5 and figure 7-1) were evaluated via t he use of an alveolar partial pressure of oxygen (i.e., a lung pressure) time integral, a trend became clea r. There is a direct correlation of the integral in that as the value of the integral increase s, 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 measur e 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 fe et [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 refere nce conditions. DEI below 30 mmHg and 60 mmHg provides a measure of the severi ty of the depressurisation event.

Figure 6 of Appendix I presents the cabin altitude - time history of a depressurization event resulting from a A 3 m 2 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 DEI3 0/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 ) + 57.625Reference: 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 exis ts. The MSHWG found that a value of 2,700 mmHgsec for the DEI30 represents the threshold at whic h 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 Allow able Values” for DEI30 and DEI60 in order to address uncertainties of the method. The “Maxim um Allowable Values for DEI30 and DEI60 equal 80% of the “Maximum Never to Exceed Value”.

As further corroboration, the MSHWG utilized the dat a from the experiment by Dr. Hans Clamann as reported in “An Analysis of the Oxygen Protecti on Problem at Flight Altitudes Between 40,000 and 50,000 Feet, Final Report”, prepared for the Federa l 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 airp lane 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 resulti ng 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 A 350 XWB AND COMPARISON WITH FAA INTERIM POLICY ON HIGH ALTI TUDE 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 A 3 m 2 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 second s 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 A 3 m 2

,

• 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: 2,1 60 mmHg-sec, DEI60: 12,000 mmHg-sec)Reference: V21RP1005510 

### 22 9 SYSTEM FAILURES

Derived from the A350 XWB System Safety Assessment s 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 pr essurized fuselage. All resulting leakage / hole sizes are below the critical hole size of A 1 . 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 f light 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 si mple analysis shows that the tyre burst generated hole would be smaller than the critical hole size of A 1 m 2 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 whee l 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 A 1
.



### 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” (A 1 ) as one parameter for the definition of the crack inspection intervals.

It is assumed that propagation of a crack from string er to stringer and frame to frame leading to the total loss of a skin panel is prevented by sc heduled 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 gove rning 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 o ffers a significantly higher level of safety than previously certified tran sport category aircraft under FAR 25. Th e exemption requested herein will not adversely affect safety to the occupants.
Authorization for flight at FL430 will enable the A350 XW B to compete fairly with other existing aircraft that are subject to older amendments without causing any adverse e ffects 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 affect ing 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 ad verse effects to the passengers.

• Serve the public interest via the use of the ne w 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 employ ment 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 new er, more efficient airplanes flying at higher altitudes will burn less fuel with corresponding less emis sions 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 promot ing a non-level playing field.Reference: V21RP1005510 

### 24 REFERENCE DOCUMENTS

• "Factors Influencing the Time of Safe C onsciousness (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 Protec tion 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 Decomp ression in Supersonic Transport Aircraft", Aerospace Medicine, J.B. Brierley an d A.N. Nicholson, August 1969.

• "Neurological Sequelae of Prolonged Decompress ion", Aerospace Medicine, A.N. Nicholson and J.R. Ernsting, April 1967.

• "An Analysis of the Oxygen Protection Probl em at Flight Altitudes Between 40,000 and 50,000 Feet, Final Report", prepared for the Fe deral 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 – A cceptable 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. Brierl ey”, Brain Research, 22 (1970) 327-345.

• “Hypoxia and Performance De crement”, William F. O’Conn or, 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 C abin: Protection Against Hypoxia”, H. Marotte, C.Toure, J.M. Clere, and H. Vieillefond, Av iation, Space and Env. Medicine, Jan. 1990.

• “Effects of Decompression on Operator Perfor mance”, William F. O’Co nnor, Ph. D., George E. Pendergrass, DOT/FAA/AM 66-10, April, 1966.

• “Behaviour of Naïve Subjects During Decomp ression: An Evaluation of Automatically Presented Passenger Oxygen Equipment”, Chis holm DM, Billings CE, Bason R, Aerospace Medicine, February 1, 1974.

• “Physiologically Tolerable Decompressi on 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 1996Reference: 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 Operat ion”, Advisory Circular- FAA, 1996

• AIA Committee Report On High Bypass Ratio Turbine Engine Uncontained Rotor Events 1969 – 2006 And Small Fragment Threat Charac terization, Volume 1, January 2010.