Ice in fuel caused Heathrow 777 crash

Nasty chill provoked reduced fuel flow
By Lester Haines $B"*(B More by this author
Published Thursday 4th September 2008 16:37 GMT
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The Air Accidents Investigation Branch (AAIB) has concluded that the
17 January crash-landing of a Boeing 777 at Heathrow was probably
caused by "ice within the fuel feed system" which restricted flow to
the engines.

BA038 (G-YMMM), after a routine flight from Beijing, suffered reduced
thrust in both engines while coming into land and fell short of the
runway. The AAIB explained earlier this year:

The first officer took control for the landing at a height of
approximately 780 ft, in accordance with the briefed procedure, and
shortly afterwards the auto-throttles commanded an increase in thrust
from both engines. The engines initially responded but, at a height of
about 720 ft, the thrust of the right engine reduced. Some seven
seconds later, the thrust reduced on the left engine to a similar
level. The engines did not shut down and both engines continued to
produce thrust at an engine speed above flight idle, but less than the
commanded thrust. The engines failed to respond to further demands for
increased thrust from the autothrottles, and subsequent movement of
the thrust levers fully forward by the flight crew.

Following examination of the aircraft, the AAIB was able to report:
"The high pressure (HP) fuel pumps from both engines have unusual and
fresh cavitation damage to the outlet ports consistent with operation
at low inlet pressure.

"The evidence to date indicates that both engines had low fuel
pressure at the inlet to the HP pump. Restrictions in the fuel system
between the aircraft fuel tanks and each of the engine HP pumps,
resulting in reduced fuel flows, is suspected."

Quite what caused this restricted flow proved a bit of a poser, but
the AAIB now confirms (pdf):

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 AAIB, while describing the incident as "the first known occurrence
of this nature in any large modern transport aircraft", stresses: "All
aviation fuel contains water which cannot be completely removed,
either by sumping or other means. Therefore, if the fuel temperature
drops below the freezing point of the water, it will form ice. The
majority of flights have bulk fuel temperatures below the freezing."

Among its recommendations attached to the latest report, the AAIB
urges "that the Federal Aviation Administration and the European
Aviation Safety Agency, in conjunction with Boeing and Rolls-Royce,
introduce interim measures for the Boeing 777, powered by Trent 800
engines, to reduce the risk of ice formed from water in aviation
turbine fuel causing a restriction in the fuel feed system". (R)
Bootnote

*During its investigation, the AAIB noted that during flight BA038
"there was a region of particularly cold air, with ambient
temperatures as low as -76$B!k(BC, in the area between the Urals and
Eastern Scandinavia".
http://www.theregister.co.uk/2008/09...w_777_verdict/

On Sep 5, 12:56*pm, Weatherlawyer wrote:

Ice in fuel caused Heathrow 777 crash

*During its investigation, the AAIB noted that during flight BA038
"there was a region of particularly cold air, with ambient
temperatures as low as -76°C, in the area between the Urals and
Eastern Scandinavia".http://www.theregister.co.uk/2008/09...w_777_verdict/

Here is a pretty good example of the thermocline:
http://www.woksat.info/etcqg09/qg09-1326-b-xx.html

The official report (incidentally, it's not titled "Cobblers") can be
accessed he

http://www.aaib.dft.gov.uk/cms_resou...m%20Report.pdf

Jack

wrote in message
...
The official report (incidentally, it's not titled "Cobblers") can
be
accessed he

http://www.aaib.dft.gov.uk/cms_resou...m%20Report.pdf


.... I was hoping you might pop in Jack and give us a sensible view: as
aircraft must have been penetrating the stratosphere for many years
and experiencing these sort of temperatures, it seems to a layman that
this would have cropped up before - any tales from your experience on
these things?

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

On Sep 5, 2:55*pm, "Jack )"
wrote:
The official report (incidentally, it's not titled "Cobblers") can be
accessed he

http://www.aaib.dft.gov.uk/cms_resou...m%20Report.pdf

I am going to read that but I went off a bit previous when I read the
part in the article about temperatures.

Looking at a picture of the wreck it seems some tank damage might have
occurred. What sort of filters do these things have and do the crew
have to trip the fuel bleeds for each run?

If it was running regular flights to China from Britain over the year,
it would have vented a lot of water in. Not that the tank would be
standing long. I don't think they switch off the engines except for
servicing, do they?

On 5 Sep, 15:27, "Martin Rowley"
wrote:
it seems to a layman that this would have cropped up before - any tales from your experience on these things?

I never had any problems. Minimum outside temperatures were specified
(I can't recall the numbers now - ten years ago since I last flew.)
Although I did fly in the cold stratosphere, I was mainly on short
haul flights with rarely more than 1.30 hours in those temperatures

The report is quite technical but basically the presumption is that
blocks of ice formed between the fuel tanks and the high pressure
pumps that feed the actual engines. There is seemingly no direct
evidence for that scenario, but simulations of blockages in the
appropriate part of the fuel lines produced exactly the symptoms
experienced on that flight. The engine automatics were trying to
supply the correct fuel, namely increased fuel flow, then it cut back
as the blockage prevented the right amount, increased again and
finally the fuel control system could no longer cope; the engines were
starved although they didn't actually cut out, merely didn't give
enough power.

It is probable that in the longer term, fuel systems in later aircraft
models will have to be redesigned (with more line heaters, etc) but in
the short term, existing aircraft might simply have to operate under
more restrictive conditions, ie limited duration in extreme cold.

One throwaway comment in the report is that this particular problem
might not be confined to Boeing 777s but many types could be
vulnerable. It wouldn't stop me flying with a reputable carrier
(indeed, I'm off to Malta with BA at the end of October). But I might
be more concerned about flying with some operators. No names
mentioned but the grapevine suggests it's only a matter of time.......

Jack

On Sep 5, 4:26 pm, "Jack )"
wrote:
On 5 Sep, 15:27, "Martin Rowley"

wrote:
it seems to a layman that this would have cropped up before - any tales from your experience on these things?

I never had any problems. Minimum outside temperatures were specified
(I can't recall the numbers now - ten years ago since I last flew.)
Although I did fly in the cold stratosphere, I was mainly on short
haul flights with rarely more than 1.30 hours in those temperatures

The report is quite technical but basically the presumption is that
blocks of ice formed between the fuel tanks and the high pressure
pumps that feed the actual engines.

There is seemingly no direct evidence for that scenario, but
simulations of blockages in the appropriate part of the fuel lines
exactly the symptoms produced experienced on that flight.

The engine automatics were trying to supply the correct fuel flow,
then it cut back with the blockage, increased again and finally the
fuel control system could no longer cope.

The engines were starved although they didn't actually cut out,
merely didn't give enough power.

It is probable that in the longer term, fuel systems in later aircraft
models will have to be redesigned (with more line heaters, etc) but in
the short term, existing aircraft might simply have to operate under
more restrictive conditions, ie limited duration in extreme cold.

One throwaway comment in the report is that this particular problem
might not be confined to Boeing 777s but many types could be
vulnerable.

With the problem occurring at just the wrong time producing the
spectacle, I assume it is a fairly frequent occurrence (statistically
speaking) at altitudes where corrections can be made.

At 7000 feet for example, you get the ability to switch tanks that
isn't available while you are still wondering: "WTF?" at 700 feet?

On 5 Sep, 23:01, Weatherlawyer wrote:

At 7000 feet for example, you get the ability to switch tanks that
isn't available while you are still wondering: "WTF?" at 700 feet?

Not quite as the blockage (as far as I understand) was immediately
upstream of the High Pressure Pump that feeds the engine and
downstream of the "Collector Tank" (or whatever the equivalent is on a
B777) so changing fuel tank sources wouldn't have helped.

However - out of interest - it used to be established practice in
small piston engined aircraft to warm the engine every so often during
a long descent, but I think nowadays, fully throttle-closed descents
are discouraged. Turbine aircraft DO however, descend for long
periods with power levers at idle. I did make the point on PPrune
(the Professional Pilots Rumour Network) that routinely testing engine
response during the descent while still at height might give early
warning of problems ahead if that response is abnormal. The crew
would at least have time to prepare for a forced landing; they didn't
in the Heathrow accident. There are hints in the report that
something along these lines might be considered.

The only real ice effect I encountered regularly when flying was Sun
Dogs (parhelia). These are seen frequently in/near cirrus clouds.
Other meteorological phenomenon that I certainly observed from the air
far more often than from the ground were the Green Flash, the Glory
and full-circle rainbows. Good old days! Nowadays, my excietment is
limited to emptying the rain gauge of the 20 mms that fell last
evening in about 30 minutes. But the grass has dried at a phenomenal
rate and I was able to give it a mow this morning. It had grown an
estimated 1 centimetre in two days.

Jack