On 17 January 2008, British Airways flight 38 was on final approach to Heathrow after a flight from Beijing when both engines failed to respond to thrust commands. The Boeing 777 landed on the grass just short of runway 27L. All 152 people aboard survived, with one serious injury.
What happened
At about 720 feet and two miles out, the autothrottle commanded an increase in thrust. The right engine responded briefly then rolled back. Seconds later the left engine did the same. Both were running, but neither would produce the commanded power.
Captain Peter Burkill and First Officer John Coward had seconds. Coward, flying, lowered the nose slightly to maintain speed. Burkill retracted the flaps from 30 to 25 degrees, reducing drag and extending the glide just enough. The aircraft cleared the airport perimeter fence and the instrument landing approach lights and came down on the grass, sliding onto the threshold.
The landing gear collapsed and separated, absorbing energy, and a fuel tank ruptured without igniting. The aircraft was written off.
The cause: something new
The investigation by the UK Air Accidents Investigation Branch had to explain a dual engine rollback with no mechanical fault, no fuel contamination and no evidence of anything wrong with the engines themselves.
The answer was ice. Water is always present in jet fuel in small quantities, dissolved and suspended. In the extremely cold conditions of the flight — a long cruise over Siberia at unusually low fuel temperatures — ice crystals had formed and accumulated in the fuel system.
On the approach, when fuel flow increased sharply for the first time in hours, the accumulated ice released and restricted the fuel-oil heat exchangers on both engines simultaneously. Fuel flow was choked. The engines did not fail; they were starved.
This mechanism had not been recognised. It required a specific combination of long exposure to very cold fuel, low fuel flow for an extended period, and then a sharp demand.
What changed
The heat exchanger on the affected engine type was redesigned to prevent ice accumulation from restricting flow, and the modification was mandated. Operating procedures were introduced for long flights in very cold conditions, requiring periodic increases in fuel flow to shed any accumulation before it can build.
Fuel system icing research was expanded significantly, because BA38 revealed that a well-understood substance behaved in a way nobody had modelled at the extremes of the operating envelope.
The accident is also cited for the flap decision. Reducing flap from 30 to 25 on short final is not a procedure; it was a judgement made in seconds that extended the glide by a distance that turned out to be the difference between the grass and the approach lights.