TWA 800, 17 July 1996

On 17 July 1996, TWA Flight 800 — a Boeing 747-131 — broke apart off Long Island shortly after departing New York. All 230 people aboard died.

The cause

The NTSB determined the probable cause to be an explosion of the centre wing fuel tank, from ignition of a flammable fuel and air mixture inside it.

The ignition source was never established with certainty. The most likely candidate the investigation identified was a short circuit outside the tank that allowed excessive voltage to reach it through the wiring of the fuel quantity indication system.

The underlying condition mattered more than the spark. The centre tank was nearly empty, and had been warmed by the air conditioning packs beneath it while the aircraft sat on the ground in summer heat. A nearly empty warm tank contains a large volume of fuel vapour in air — which, unlike liquid fuel, is explosive.

What changed

This is the clearest case of a 747 accident producing a design change applied across the whole industry.

Regulators concluded that the answer was not only to chase every possible ignition source but to remove the flammable mixture. In 2008 the US Department of Transportation issued a final rule requiring fuel tank inerting — pumping nitrogen-enriched air into the tank so the vapour above the fuel cannot burn.

The rule applied to centre wing tanks on new passenger and cargo aircraft, and to passenger aircraft built through most of the 1990s. Inerting systems are now standard.

Wiring inspection standards for ageing aircraft were also substantially tightened as a result.

Related

• All 747 accidents
• The 747 fuel system — including the centre wing tank
• The 747-100