Discussion Paper 1008CS - Continued Airworthiness and Safety Improvements for Transport Category Aircraft
The Federal Aviation Regulations (FAR) requirements address concerns pertaining to fuel tank explosions; specifically to the wiring sub system of the fuel system and more generally to aged wiring systems. The FAA has also addressed concerns about aircraft fatigue and airframe cracking via the FAR.
Notice of Proposed Rule Making (NPRM) documentation provided by the FAA explains that fuel tank explosions have been a constant threat with serious aviation safety implications for many years. Since 1960, some 17 aeroplanes have been destroyed as the result of a fuel tank explosion and four fatal aeroplane accidents have been caused by fuel tank explosions since 1989. Safety concerns about wiring systems in aeroplanes were brought to the forefront of public and governmental attention by a 1996 accident over the Atlantic Ocean near East Moriches, New York, involving a 747-131 aeroplane, operated as TWA Flight 800. The probable cause of the TWA Flight 800 accident, in which the aeroplane broke up in flight, was an explosion of the centre wing fuel tank, resulting from ignition of the flammable fuel and air mixture in the tank. The most likely source of ignition energy for the explosion was a wiring failure in the fuel quantity indication system of the centre wing tank.
In response to the fuel tank explosions and the potential for aged wiring systems to cause the ignition of explosions, FAR now have requirements to deal with both matters. In relation to aged wiring, the requirements organise and clarify design requirements and add new certification rules for wiring. Holders of Type Certificates (TC) for certain Transport Category aeroplanes are required to make changes to existing wiring Instructions for Continued Airworthiness (ICA) for incorporation into the aircraft maintenance program. The flow on effect of those changes is that registered operators are also required to incorporate ICA for fuel tank systems into their maintenance/inspection programs.
In relation to fuel tank flammability, the requirements establish performance-based requirements that set acceptable levels of flammability exposure in tanks most prone to explosion or alternatively require the installation of an ignition mitigation means (IMM) in an affected fuel tank.
As well as addressing fuel explosion concerns the FAA have developed regulations to deal with fatigue cracking. Structural fatigue cracking of aging aeroplanes has been a major aviation safety concern for many years. If not detected and repaired, fatigue cracking can eventually lead to catastrophic structural failure and loss of the aeroplane. Damage Tolerance Inspections (DTI) are one way to preclude catastrophic failure resulting from fatigue cracking; but these inspections need to take into account any adverse effects that repairs and alterations may have on fatigue life, or on the capacity to carry out inspections on fatigue critical structures. Currently there is no Australian requirement for repair and alteration designers to provide damage tolerance data to accompany the design or alteration.
In light of that, the FAR now contains requirements for the holders of design approvals to make available to operators, damage tolerance data for repairs and alterations to fatigue critical aeroplane structure.
Comment period now closed.
Contact: Mark Bathie, Project Officer
- DP 1008CS - Continued Airworthiness and Safety Improvements for Transport Category Aircraft
- Annex A - FAA Appendix N to Part 25 – Fuel Tank Flammability Exposure and Reliability Analysis
- Annex B - FAA Appendix M to Part 25 – Fuel Tank System Flammability Reduction Means
- Annex C - Electrical Wiring Interconnection System (EWIS) Instructions for Continued Airworthiness