From the start, the British Airways Boeing 777 crash at London Heathrow felt like a rare event - perhaps a one-off. But is it actually likely to happen again?
After all, like most big jets of its generation, the 777 has had a pretty trouble-free history since service entry in 1995, and aviation is now such a mature industry that, surely, there are few surprises left for the operator or air traveller?
But the Heathrow accident has provided some surprises. The UK Air Accident Investigation Branch's latest interim report says ice particles in the fuel system is the only likely cause of this event, and adds that the behaviour of water in fuel below certain temperatures that are regularly experienced is an unknown quantity. The Branch explains: "When the fuel temperature reduces to approximately ‑18°C (0°F), the ice crystals adhere to each other and become larger. Below this temperature little is known about the properties of ice crystals in fuel and further research may be required to enable the aviation industry to more fully understand this behaviour." The fuel in G-YMMM's tanks was -22degC on the fateful final approach.
So what is the risk that an event like this will recur, perhaps with a much worse outcome? If the fuel supply restriction that caused G-YMMM to crash-land just inside Heathrow's boundary had occurred 30 seconds earlier, the aircraft might have hit a light industrial site and crossed two busy roads before coming to rest.
That said, there is a fair amount of evidence in the AAIB's latest interim report to show that the risk is very low and could be lowered further by the use of ice-inhibiting fuel additives widely used in military aviation. The report narrative confirms that, since every component in the aircraft's fuel system and in the engines is known beyond doubt to have performed as it should have done in terms of functionality, and that "waxing" of the fuel at that temperature would not have been an issue, ice particles or crystals clogging the fuel system is the only explanation for what happened.
Why it occurred on this and not on other flights is not yet completely clear, but it seems likely to have been the particular combination of a long fuel tank cold-soak at lower than usual temperatures, combined with an extended period of commanded low fuel flow until high power was demanded just before the unwanted power reduction on final approach.
Working with Boeing, the AAIB was able to mine data from 13,000 Rolls-Royce-powered 777 flights. Among these, they found 118 flights during which the fuel temperature profile for the whole flight was at or below that for G-YMMM's final trip, and others in which the fuel flows demanded were close to the profile for this flight, yet they did not experience any problems with fuel supply to the engines.
Because there was no fuel supply system component malfunction in G-YMMM, one of the AAIB's recommendations is: "The Federal Aviation Administration and the European Aviation Safety Agency should take immediate action to consider the implications of the findings of this investigation on other certificated airframe / engine combinations." That means everything flying, not just 777s.
Meanwhile it has taken 17.5 million flying hours and 3.9 million flights by all 777s for this one event to occur. Specifically for
So all but exceptionally nervous fliers should be able to feel safe in the 777.