Fuel Tank Safety
BACKGROUND
This subject was an aspect of the Jet A/Jet B fuel controversy referred to in the History Section of this website; indeed it was a Jet B fuel tank explosion which brought the Group into being. The subject has remained of continuing interest.
As a result of the two accidents referred to in the History Section, and the attendant publicity generated by this Group and others, the tendency among some airlines to switch to the cheaper but less safe Jet B was arrested, though not entirely eliminated. Accordingly Jet A became re-established as the majority civil jet fuel, and for the following two and a half decades an almost unblemished record of fuel tank safety was achieved. Explosions did occur in tanks exposed to extensive post-crash fire, but the record for undamaged tanks in non-fire conditions was good. It was always recognised, however, that some operations would be conducted at temperatures above the Jet A flashpoint (38C typically) and therefore control of ignition sources inside the tank, from aircraft systems or from lightning, would be essential.
In 1990, in warm weather conditions, a centre tank explosion occurred on the ground in a B737, resulting in several fatalities. A similar accident, to another B737, occurred some years later. But in 1996 a B747 (TWA 800) was destroyed in flight, with the loss of all occupants, by an explosion in the empty centre fuel tank. This accident, and the two B737 events, revealed a situation which had not been fully appreciated hitherto, namely, that centre tanks of some aircraft were being heated by nearby air conditioning equipment, and were thus more vulnerable to an internal ignition source during the early stages of flight. Although the exact ignition source in the TWA 800 accident was not identified, several Airworthiness Directives were issued by the FAA and other Authorities to reduce possible ignition sources.
In November 2005, the FAA issued an NPRM to control the flammability of fuel tanks - 'REDUCTION OF FUEL TANK FLAMMABILITY IN TRANSPORT CATEGORY AIRPLANES', DOCKET FAA-2005-22997.
This proposes a limit on the acceptable percentage time that the fuel tank can be 'flammable' (as defined by the lower flammability level - approximately equivalent to the fuel flashpoint at sea-level). If above this 'limit', tanks will need a positive means to reduce flammability (e.g. some form of inerting) or to mitigate the effects of an ignition (e.g. foam filled tanks). But the NPRM does not directly address the unwanted heating of fuel tanks. Accordingly, in its response to the NPRM, the Group has highlighted this issue and made a number of adverse comments on the other proposals. The FAA's response is still awaited. At the end of 2005, EASA amended CS-25 to include a new requirement, together with Advisory material, to prevent the unwanted heating of fuel tanks. Under this requirement, the tank design seen on the B737 and B747 would appear not to be acceptable and the Group recently raised this question in a letter to EASA. Whilst the FAA are intending to mandate retrofitting action to counter the flammability issue, EASA remains undecided following a Regulatory Impact Assessment and is undertaking a further study, which should reach a decision by the third quarter 2008.
Finally, the Group is aware that environmental issues are now coming more to the fore and notes that research into alternative forms of fuel is well underway. The Group will be watching this development with interest but any operational environmental benefits must not be off set by even greater emissions created during the production process of any such new fuel.
