The US National Transportation Safety Board (NTSB) has proposed far-reaching recommendations to reduce the risk of explosion in fuel tanks. The safety board warns that all airliner fuel tanks are vulnerable to explosion under certain conditions.

Suspicions about fuel-tank safety have existed for many years, admits the NTSB in a document based on concerns raised by the Trans World Airlines (TWA) Boeing 747-100 crash near Long Island, New York, USA, on 17 July, 1996, in which 230 people died. Fuel-tank explosions have been a factor in three other fatal accidents, the NTSB reveals.

Although it emphasises that the cause of the TWA centre-wing fuel-tank (CWT) explosion has not yet been determined, the NTSB gives details of an August 1996 flight-test replicating flight TWA800, and notes that the conditions caused "...an explosive fuel-air mixture to exist in the tank ullage [the space in the tank not filled by liquid fuel]". In the TWA800 747's CWT, the temperature was about 46¹C, says the NTSB. Fuel-air vapour becomes potentially explosive as fuel-tank temperature is between about 45¹C and the "auto-ignition" point of 226¹C -temperatures which reduce with altitude. Given these conditions, says the NTSB, all that is needed for explosion is "an energy source sufficient for ignition".

The NTSB recommends that the US Federal Aviation Administration invoke measures "precluding the operation of transport category aircraft with explosive fuel-air mixtures in the fuel tanks". Although all the accidents quoted in the paper involve Boeings, the implication here and in the report's generic descriptions of the issue is that it involves all airliner types.

The NTSB, however, has declined to clarify the point, as has Airbus Industrie, saying that it has received no instructions on the subject. Boeing has warned the FAA that any action will have implications for "the entire aviation industry". The FAA says that it will consider the NTSB's recommendations carefully. These include:

- incorporating systems for filling the fuel-tank ullage with an inert gas like nitrogen;

- banning heat-generating equipment from proximity to fuel tanks, or improving tank insulation. The safety board notes that heat-producing air-conditioning units are situated close to 747 CWTs;

- until such modifications can be incorporated, all available measures for loading cold fuel and keeping it cold should be adopted;

- in the 747, an "appropriate" minimum amount of fuel should be kept in the CWT;

- in aircraft with a heat-source near any fuel tank, tank-temperature should be displayed.

The NTSB quotes accidents to support its argument, including :

- the crash of an Iranian air force Boeing 747-100 approaching Madrid, Spain on 9 May, 1976: following a reported lightning-strike, there was an explosion and airborne separation of the right wing;

- an accident to an Avianca 727 near Bogota, Colombia, on 27 November, 1989. A small bomb under a passenger seat exploded. The NTSB says: "The bomb explosion did not compromise the structural integrity of the aircraft [but] punctured the CWT and ignited the fuel-air vapours in the ullage," destroying the aircraft;

- an 11 May Philippine Airlines 737-300 accident at Manila Airport, where the CWT exploded on the ground, killing eight people. The NTSB notes that the ambient temperature was 35¹C and the aircraft had been parked in the sun.

Source: Flight International