We finally have an “Interim Report” on what happened to the BA 777 that lost power and crash-landed at London/Heathrow in January of this year. According to the UK’s Air Accidents Investigation Branch (AAIB), it was probably due to the buildup of ice in the fuel system that blocked the flow of fuel, but that’s not exactly certain. The AAIB report on aircraft G-YMMM (21 pages, PDF) provides some extremely interesting reading. I’d recommend curling up with it this weekend if you have the time.
In short, here’s what they think happened:
The investigation has shown that the fuel flow to both engines was restricted; most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment; although, G-YMMM was operated within the certified operational envelope at all times.
The interesting thing here is that they really aren’t sure what happened, but they’ve reached this conclusion through a process of elimination. Everything appeared to function as expected, but there was reduced fuel flow. What caused it? That’s where the speculation begins.
They do know that the aircraft was flying in unusually cold conditions but not cold enough to cause “fuel waxing” which is when fuel would freeze. They know that there is naturally-occurring water that builds up in fuel over time, and this would freeze at those temps, but they didn’t find anything excessive. What could have happened is that small bits of ice built up over time and were jarred into the unfortunate position of blocking the fuel flow. What would have jarred them?
Well, when the airplane was descending, it had to power up a couple times for holding and to speed up to remain lined up with the runway. This of course, happens all the time, but it was also just the type of event that could have knocked ice crystals into a bad spot. It sounds like it was truly an amazing coincidence that these events resulted in an aircraft landing short of the runway and being written off.
The AAIB examined 13,000 777 flights powered by Rolls-Royce engines and found the following:
- Of the 13,000 flights, only 118 had takeoff fuel temps below the 28 degrees F found on this flight
- On the approach, only 70 flights had fuel at or below the -8 degrees F found on this flight
- Only 10% of the flights examined had fuel flows of less than 10,000 pound of fuel per hour (pph) for the step climbs after departure (this flight never exceeded 8,896 pph)
- Only 10% of the flights examined had fuel flows of more than 10,000 pph during the approach phase (this one was more than 12,000 pph)
So as you can see, the combination of low fuel temps, low fuel flow early in the flight and high fuel flow toward the end may have doomed this aircraft. Had it happened in any other phase of flight, the ice would have disappeared quickly enough that it would have been easy to recover. This was, as usual, a series of things going wrong that combined to create a nasty accident.
The AAIB recommends requiring airlines use measures to reduce the risk of ice formation. This could include things like using fuel additives that lower the freezing point, but it didn’t actually specify what should happen as of yet, as far as I can tell. Initially these recommendations are only for Rolls-Royce powered aircraft, but they’re going to review other engines and aircraft types to see if it might be necessary elsewhere.
As I said above, this is really a fascinating read. I’d recommend taking it home with you this weekend.
