On 17 January 2008, British Airways Flight 38 crash-landed short of the runway at Heathrow Airport. At the time, the newspapers were filled with mad speculation about the cause of the accident, ranging from computer failures, bird strikes, electronic glitches, low quality fuel and wind shear.
Within two days I had had enough of the media frenzy. I wrote about the incident with some frustration at the time:
There is a lot of speculation going on in the press and, worse, a lot of speculation being presented as fact. The words “Absolute nonsense!” have become a frequent sound in our household as we read the articles in the popular press. I know accident reports aren’t everyone’s thing, but in the interests of understanding what happened, here is a quick run-down , based on the initial report.
Following an uneventful flight from Beijing, China, the aircraft was established on an ILS approach to Runway 27L at London Heathrow.
Nothing out of the ordinary until the final approach: that is the plane has already done its initial descent. The stewerdesses are buckled up and the plane is coming in to land. This is the hectic bit of the flight. A pilot friend of ours jokes that he gets paid for hours of boredom with a little bit of excitement at the beginning and the end. Some of the newspaper reports seem to almost imply that planes land themselves as they bandy about terms like Instrument Landing System and auto-thrust. Planes can land themselves in certain circumstances but it’s rough, we still prefer real people to put the planes onto the ground. It really is a safe assumption that the pilots would be giving the situation their full attention.
Initially the approach progressed normally, with the Autopilot and Autothrottle engaged, until the aircraft was at a height of approximately 600 ft and 2 miles from touch down.
At 600 feet they would be forty seconds away from anticipated touchdown. Then, something went wrong.
At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond.
This is the loss of power: equivalent to putting your foot down and your car not accelerating.
Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond.
The pilots immediately realised there was a problem and manually pushed the levers and got the same effect: no power.
Forty seconds is not a lot of time to make decisions and the ones we are hearing about in the press seem to have been sensible. Put the auxiliary power on. Don’t mess about with changing control, let the co-pilot land. Keep the plane in the air for as long as possible. Get the damn plane clear of the road.
The aircraft stopped on the very beginning of the paved surface of Runway 27L.
It seems likely that, given the time they had to make a decision, the pilots simply focused on getting that plane to the runway. They didn’t but they made it to the ground and cleared the perimeter fence: well done! There’s not a lot of options when you are in a 777 with no power, 600 feet above the ground.
The big question is: Why did the power fail? Any comment on that is complete speculation at the moment: the only information so far is why it didn’t.
The Air Accidents Investigation Branch (AAIB) released its final report a few months ago and I was remiss in not reporting back. But now we finally have the specifics of what happened and a logical view of the cause.
The AAIB has determined that the loss of power (specifically a reduction in thrust) was caused by a restricted fuel flow to both engines.
Speculation at the time included the possibility that there was water in the fuel, since the lack of response from the engine was consistent with fuel starvation but it was clear that the plane had fuel. They now believe that the restrictions were caused by ice in the fuel feed pipes.
Note FOHE is the Fuel Oil Heat Exchanger.
The investigation identified the following probable causal factors that led to the fuel flow restrictions:
1) Accreted ice from within the fuel system released, causing a restriction to the engine fuel flow at the face of the FOHE, on both of the engines.
2) Ice had formed within the fuel system, from water that occurred naturally in the fuel, whilst the aircraft operated with low fuel flows over a long period and the localised fuel temperatures were in an area described as the ‘sticky range’.
3) The FOHE, although compliant with the applicable certification requirements, was shown to be susceptible to restriction when presented with soft ice in a high concentration, with a fuel temperature that is below -10°C and a fuel flow above flight idle.
4) Certification requirements, with which the aircraft and engine fuel systems had to comply, did not take account of this phenomenon as the risk was unrecognised at that time.
Point 4 is important as it clarifies that there were no certification safety requirements missing from the aircraft.
Boeing have confirmed that they have taken steps to help prevent fuel restrictions caused by ice in the fuel, although the circumstances that led to the situation are described as unique.
A large number of the recommendations from the AAIB are focused on safety in the event of a crash, based on the results of this incident where there was insufficient time to brief the cabin crew. Most of the passengers were not aware of any issue until the plane was on the ground.
It’s good news that we understand more about ice and have conclusive studies about some of the rarer conditions in which it can appear. Still, not quite as exciting as the popular press seemed to be hoping for at the time, eh? Searching for additional details for this blog post, I noticed a rather subdued reaction to the results, especially in contrast with the original frenzy.
Oh well, mustn’t grumble.