Development of a FAA Special Condition for Flammability

Reduction Systems

Ivor ThomasConsultant to FAA

1-425 455 1807fuelsguy@msn.com

Background

• With the development by the FAA of a concept for a practical inerting system, the Boeing Company elected to move forward with the development and certification of such a system on a 747.

• The FAA and EASA determined that the current FAR’s did not address such a system adequately and therefore harmonized Special Conditions (SC) were needed.

• Special conditions were created jointly with EASA, TCCA and FAA.

Special Conditions Objectives• As the Boeing Company was moving forward

with a Nitrogen Generation System for the 747, the FAA and EASA had to address a number of issues in the SC,– What level of flammability reduction was needed– How to define and determine tank flammability– How to determine a satisfactory O2 level to be

considered Inert – How to set standards that were performance based

and did not impose a single solution– How to address any new hazards that inerting may

present– Ensuring harmonization of the special conditions

Fundamental Features of the SC

Objectives: 1) Reduce Flammability to an acceptable level, 2) Address warm day specific risk– Fleet average exposure less than 3% of operational

time - all flight phases combined• 3% divided approximately equally into

~1.5 % for system performance limitations~1.5% for times when system is unavailable

– Fleet exposure on days over 80 deg F when system is available

• For ground and climb phases, tank flammable less than 3% of operational time

Fundamental Features of the SC• Fleet Average Exposure less than 3% of Operational Time

– This provides a means of regulating the fleet average exposure to a level commensurate with the historical level of unheated wing tanks.

• The division of the 3% into two approximately equal sub-requirements is intended to ensure that the flammability requirement is not degraded by system unreliability, while allowing the design to be focused on normal operation rather than being designed for a “worst case” flight.

– One 1.5% sub-requirement allows the design to be sized for normal operation and extreme cases, such as emergency descents, can be included in this allowance.

– The second 1.5% recognizes that the system is intended to be a single thread system and there will be occasions when the system is unavailable, both when the system has failed but has not been recognized as failed, and occasions when the system is known to be inoperative

– The design must ensure the overall requirements are met, by addressing performance, reliability and system health monitoring

Fundamental Features of the SC

• Fleet exposure on warm days - over 80 deg F when system is functional

• On the ground, tank flammable less than 3% of time on ground• In climb, tank flammable less than 3% of time in climb

– This requirement was included in the SC to ensure that the system addresses the high flammability exposure of hot days on the ground and in climb.

– In principle, a system could meet the average 3% requirement only, by addressing the higher altitude conditions and by being very reliable.

– This is considered unacceptable because this system would not address the conditions that were present on the most recent three fuel tank explosion accidents.

– The “80 Deg F warm day” conditions ensure that the system protects the airplane on warm days, again accepting the concept of a single thread system.

Fundamental Features of the SC

• Use of the ARAC Monte Carlo model approach is required– SC limits certain parameters for consistency

• Temperature variation on ground and cruise• Overnight temperature drop• Fuel Flash point variation• Flammability envelope

– based on 1 joule spark (Equivalent to envelope used by ARAC)

– Applicant can use airplane specific data where appropriate and substantiated by data

• Airplane/Engine parameters• Mission distribution

Fundamental Features of the SC

• The SC is performance based, and not limited to a specific design solution.

• The fundamental requirement is a level of flammability as determined by a standard approach.

• Alternative solutions can be addressed by showing that the flammability targets are met.

Additional Features of the SC

• Although the SC is performance based, inerting is one way of satisfying the requirements.

• The tank is considered inert if the O2 level is below 12% for altitudes from Sea Level to 10,000 ft, and then increasing linearly to below 14.5% at 40000 ft. – Linear extrapolation above 40,000 ft is acceptable.

• Thus the tank is considered non-flammable if either the tank is outside the flammability envelope for the fuel on board, or the O2 level satisfies the inert definition above.

Additional Features of the SC

• In the descent case, localized concentrations above the inert level are allowed. – This is judged to be acceptable based on the very

lean conditions of the entering air (no hydrocarbons present and cold temperature) and the time needed to mix the incoming air with the existing tank ullage.

– Under these conditions, the tank is considered non-flammable during descent provided the tank was non-flammable at the entry into the descent.

– If the tank is above 12% O2 at touch down, the time after touch down when the tank is flammable is counted towards the determination of flammability exposure.

Additional Features of the SC

• In the case of a multi-bay tank, i.e. one where individual bays of the tank are separated from each other by walls with very little air or fuel communication, – Conditions can occur where one or more bays may be

at a higher O2 level than the rest of the bays.

– Under these conditions, the tank is considered flammable if the O2 level is above the inert level in any bay and the temperature of the tank is in the flammable range

Harmonization

• The FAA, EASA and TCCA have worked closely in developing the harmonized SC,– The FAA proposed SC and the EASA

equivalent were developed in harmony with TCCA and released for comment as virtually identical text.

– The comment periods for both were the same, and FAA and EASA have reviewed, and disposed of, comments together.

– The final SC is harmonized with EASA.

Summary

• The SC is performance based with flammability levels intended to provide a significant improvement in safety.

• Inerting is one acceptable means of complying with the SC and is addressed in the SC, but other systems may be proposed to meet the SC.

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