British Airways 777 Accident at Heathrow Likely Caused by Ice Buildup

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.

08_01_18 ba777accident

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.

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17 Comments on "British Airways 777 Accident at Heathrow Likely Caused by Ice Buildup"

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The Traveling Optimist
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The Traveling Optimist
Astounding. I immediately recalled the 13-hour flight from Beijing to Chicago I flew several years ago and asked myself how those circumstances, polar route included, didn’t add up to an accident like this. Is the fuel flow system or tolerance levels different on P&W engines versus Rolls Royce? I also immediately thought of every Continental flight between EWR and NRT/HKG and ask why those flights haven’t been affected. HUGE fuel burn on take-off, low burn right at the midpoint of the flight, over the Arctic and high burn again holding in to NRT or vectoring in to HKG. And I… Read more »
Jennifer
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Living in a high-desert region myself, we are constantly aware of the need to keep our car tanks 1/2-full or higher in the winter due to condensation build-up. It doesn’t suprise me that there was ice in the fuel tanks, only that there would be a piece large enough to cause complete blockage.

It does make me wonder,however, what the US airlines will do this winter with fuel prices soaring; and with pilots being told to use minimum safe fuel amounts….

Less fuel
= more area for condensation
= more potential for larger ice pieces

Wonko Beeblebrox
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Agreed. It is interesting reading…

Anon Coward
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I haven’t RTFA yet but I wonder: under what circumstances would an aircraft use far less fuel than average during climb yet far more fuel than average during descent? Uncrowded/more flexible or fuel-optimized airspace at origin (Beijing) and crowded/more tightly controlled/un-fuel-efficient airspace at destination?

Anon Coward
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Might have answered my own question: unfavorable winds on approach to London?

Anon Coward
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-and a pilot who wants to save fuel on the uphill?

The Traveling Optimist
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The Traveling Optimist
“Step” climbing in flight occurs for any number of reasons: 1) Overflying Traffic. Think of sitting at a metered on ramp waiting to join traffic on your local interstate. Gotta wait your turn to take-off much less maneuver in to the fast lane. 2) Authorized vs. Restricted Airspace. For any flight between Europe and Asia that, I believe, covers much of Russian air space. Even over the United States certain flight levels are restricted for military access only (although most of these are too high for commercial jets to reach anyway). 3) Airframe Operating Characteristics (Or The Engines). On older… Read more »
Anon Coward
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The pumps had marks from cavitation. That makes me think that they were completely starved on the intake side, i.e. issue was not water replacing fuel but no intake at all due to blockage. Report seems to lean towards slow buildup of a blockage far upstream which was not sufficient to impede flow of fuel at the slow rate during most of the flight. The partial blockage is then dislodged by a sudden increase in flow, the blockage then flows to a narrower choke point, at which time it causes a total blockage.

Wonko Beeblebrox
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Anon Coward:

Sounds like you just described a heart attack.

Maybe that’s the best paradigm for this accident: dual simultaneous heart attacks caused by blockages of fuel(blood) to the engines(heart) at some chokepoint.

The Traveling Optimist
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The Traveling Optimist

Along with the cavitation, indicating stress on the valves from working in a vacuum, I wonder if they also reviewed the linings of the feedlines for scarring or other indicators of heavy (blockage) particles scratching and dragging along the insides up to a clear point of solid implanting.

materials man
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Fuel is lighter than water or ice. Cold is going to get into the tanks mainly through the bottom. My guess is that any water in the fuel is going to build up like frost on the tank bottom, looking something like edge-on snowflakes. This slush, though stuck to the bottom, could be fragile, so that when something happened and fuel sloshed around, it could come loose and block a mesh intended to keep crap out of the fuel lines. If fuel from the plane tested dry, it could well be because most of the water had been frozen out… Read more »
Texan
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Thanks and good job to the BA Pilots and boeing engineers.. Everyone safe..

BedBugDogNYCd
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138bt
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????????

exit right
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777 down early in san fran, struck a perimeter structure before landing.
?

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