British Airways 777 Accident at Heathrow Likely Caused by Ice Buildup

777, Accidents/Incidents, British Airways

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

  1. 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 believe they operate Rolls engines.

    Then I ask myself how ice crystals could form a cap so solid and thick as to block even waxed fuel from entering a pipeline that is surely at least, what, one inch in diameter to feed the mix chamber of an engine with at least an 11-foot mouth??

    I don’t know the dimensions of a fuel line on the largest engines yet built. I simply find myself caught between “Stranger things have happened” and “They can’t be serious!” The series of events they describe pretty much affect, in my mind’s eye, nearly every 777 flight longer than six hours: heavy burn on take-off, fuel conservation management at the highest and naturally coldest altitudes followed by heavy consumption in warmer and moister air on approach and landing. And with three makes of engines on this one airframe, is it in fact a design issue with just the one or should every manufacturer be checked?

    I don’t seek blame or guilt, just logic, even in the most implausible circumstance. A cap of ice over the fuel valve during one of the heaviest moments of consumption involving a heavy corrosive? Not quite logical. I look forward to the final investigation.

  2. 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

  3. Optimist – One thing I think I left out here is that the initial takeoff wouldn’t have been affected because it used fuel from the main tank and the levels were high. It could only have been the step climbs that came later that would have caused this, possibly. But since they didn’t get that much fuel flow during those climbs, it was less likely to happen.

    I agree though. They seem far from certain here, but it’s the best guess they have right now. I too am looking forward to the final word on this one. BTW, they only called out the Rolls engine because that’s what this was on. They are reviewing other engines to see if it could be a problem as well, but they didn’t jump to that conclusion yet. It seems like all engines would be affected, but what do I know?

    Jennifer – Same kind of thing as Optimist says. How did such a big piece of ice get together to block the fuel flow so much? That’s a huge question that they have to answer.

  4. 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?

  5. It’s probably just the way the pilots were flying the plane. There were step climbs after departure, but the low fuel flow probably means the pilots chose to go it slow. On descent, I’m not sure. They could have been lower than they needed to be and had to climb in a hurry. I’m not sure, of course.

  6. “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 widebodies, such as the 747, anywhere from the first four to six hours of flight are below the maximum planned altitude until enough fuel is burned off to allow the wing surface to sustain flight at thinner/higher altitudes.

    Once an airplane is light enough and is already traveling at better than 500 knots it doesn’t take much juice to “bump/step” up another 5,000 feet from FL280 to FL330. It’s had a several hour running jump at it and is not working nearly as hard as hauling MTOW off the ground at sea level (LAX, ORD, JFK, NRT, HKG, etc).

    Fuel consumption at approach and landing is also understandable considering more throttle commands are required in this phase of operation than any other time. Even the most remote and widely spread runway pattern in the world rarely allows a pilot to glide at idle from cruise all the way to the threshold for any of the reasons below.
    a) Traffic. Holding patterns, approach vectors in multiple airfield airspace, possible go-arounds, etc.
    b) Weather. Changes in approach patterns up to and including reversing runways, micro-bursts and diversions to alternate fields.
    c) Noise Abatement Flight Rules.
    d) Denser air. Takes more fuel to run and respond at lower, warmer altitudes.

    All of this leads me to believe that either of the two step climbs over Russia would not have formed ice in the valves sufficient to jeapordize the plane up to a full five hours later upon approach and landing. The ice cap required to cover the valve at the tank would have to have been particularly thick and formed over time. If it were formed over time the flight would have experienced less and less PPH to the engines for the rest of the flight, to the point of becoming another Air Canada Glider by the time it got to London. The flight crew would surely have noticed such a steady decline in fuel flow long before engaging the initial at Heathrow. This didn’t happen, and no ice blockage was found over the mesh valve covering or at the mix chamber.

    Further, ice crystals even in waxed fuel would simply have been melted and burned through as it would have been surrounded by high enough concentrates of flammable fuel to not be an issue.

    Dissolved water, molecules attached to fuel, would thin the fuel, the same as ice in a mixed drink but the potency of the fuel would remain strong enough to negate the effect. Here again, analysts found insufficient levels of water to allow this as a factor in bringing the plane down. The fuel wasn’t diluted enough by dissolved water.

    Entrained water, like pockets of oil in vinegar, would have been treated the same as ice crystals or dissolved water. It would have been surrounded enough by usable fuel to have not been an appreciable factor. Again, I point to the report’s findings as evidence of this, along with the statement that the aircraft operated entirely within normal operating parameters from beginning to end.

    That, to my amateur mind, leaves on possibility: floating water. Where the report allowed for up to 5 ltrs of water in each tank for the amount of fuel boarded in China, I also use “Jennifer’s” statement above to forward the hypothesis that enough free-floating water got in to the mix chamber ahead of usable fuel at just the worst time as to nearly shut the engines down. There was so much WATER in the mix chamber that there was an appreciable gap in time and delivery from the last usable fuel before enough pure fuel in good quantity could get in to the mix chamber to keep the engines going, thus, idling the engines to the point of no response and downing the plane to the point of no return.

    In both engines at the same time?! The scavenge system had emptied the center tank well back in to the cruise of the flight, taking the product there from 800kg down to zero and bringing on the two wing tanks for the duration of the flight. Here is where my theory comes in to (no pun intended) rough water.

    Depending on where the tank valves are placed, either at the very bottom of a low tank gradient (angled for flow) or somewhere above the floor line of that tank itself. I honestly cannot remember if treated petroleum products such as Jet-A or Fuel#3 are heavier or lighter than pure, free floating water. Either way, though, based on the amount of fuel left in the tanks at the time of the approach, if water is heavier and the valves are at the bottom of the tank, there was enough water present in both to get in to the mix chamber before fuel could compensate. If water is lighter and the valve is raised above the floor, since free floating water is NOT surrounded by or bonded with the fuel itself, it siphons in to the chamber independently, again cutting off the energy source long enough to keep the engines running.

    The sumping procedure, if I recall, occurs during ground maintenance, not in flight. And sumping removes heavier particles along with free water but is not intended to be activated on tanks loaded with $3/gallon fuel. That’s throwing the baby out with the bathwater. So….

    a) Sumping would not have been performed in flight.
    b) Investigators found less than 2 liters of free water in the tanks after the crash where analysts allow for up to five liters of water per tank on a load such as the one onboarded in PEK.
    c) Ice in waxed fuel is surrounded by usable fuel
    d) Dissolved water is bonded and outnumbered by usable fuel molecules
    e) Entrained water is also again surrounded by usable fuel.
    f) Free floating water does what it wants unless there is a straining mechanism in the fuel system design to shift water towards the overflow valves instead of down to the engine feed lines.
    g) Fuel valves, to my mind, would be wide open during approach and landing to allow for maximum power. Throttle retardation would restrict fuel at the engine intake not the tank point of outflow. This also disallows the possibility of the fuel valve in the tank being closed or restricted naturally or by ice build-up during the most critical phase of flight. While driving a car, you let up off the gas to slow the car down but you don’t close the fuel line from the tank into the engine.

    So, hopefully this was not too boring or laborious, I still don’t see ice build-up as the reason this airplane came down. I see either a water “hiccup” occurring under freak conditions in both tanks simultaneously. Either that or, for some truly strange reason, the tank valves DID close and the scavenge system reversed itself back to the center tank for engine feed, a tank that was already empty.

    I flew the eastbound of this same route in 2000 and now wonder if it may have been the same plane. Thank you for your indulgence.

  7. 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.

  8. 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.

  9. 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.

  10. 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 of it during the flight.

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