Possible Causes of the Boeing 777 Crash Landing at Heathrow?

[Garth]

I find it difficult to believe the fuel could be the source of the problem. It would take an incredible coincidence for both fuel pumps, having worked perfectly for several thousand miles and several hours pumping this fuel, to both suddenly pack up within a few seconds of each other through fuel contamination.

edit One report speaks now of unexpected air in the roller bearing casings of the fuel pumps causing cavitation, could this be due to contaminated fuel?

Both sets of fuel pumps? Simultaneously?

Garth

 

[Art]

Investigators have found no sign of fuel contamination other than from fire fighting equipment but one anomaly during the flight was unusually cold flying conditions of ambient temp -76 C which it is speculated may have led to an increase in the viscosity of the fuel making it harder to pump although the actual fuel temperature never came close to it's freezing point during the flight reaching a lowest temp of -34 C. Samples of the fuel remaining on-board showed it's actual freezing point to be -57 C. Presumably the sensors for measuring fuel temperature are in the fuel tanks so presumably the temperature could be colder in the feed pipes to the pumps and engines?

http://www.iht.com/articles/ap/2008/02/19/europe/EU-GEN-Britain-BA-Crash-Landing.php

A full interim analysis by the AAIB is available here http://www.aaib.dft.gov.uk/cms_resources/S1-2008 G-YMMM.pdf

 

[Andre]

Thanks Art,

The effect of cold on the fuel seems irrelevant.

...and the minimum recorded fuel temperature was -34ºC. The fuel temperature in flight must not reduce to a temperature colder than at least 3ºC above the fuel freezing point of the fuel being used. ...the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC.

To me this looks like a contradiction

The first officer took control for the landing at a height of approximately 780 ft,
in accordance with the briefed procedure,

In my world, taking control, means steering and flying, disengaging autopilots, but:

The airspeed reduced as the autopilot attempted to maintain the ILS glide slope and by 200 ft the airspeed had reduced to about 108 kt. The autopilot disconnected
at approximately 175 ft.

We discussed this intensely, stretching a glide and losing speed is an error. But why was the autopilot allowed to do that as the copilot was flying? Or not?

Detailed examination of both the left and right engine high pressure fuel pumps revealed signs of abnormal cavitation on the pressure-side bearings and the outlet ports. This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel.

There you go, with all redundancy. What causes the damage to occur independently? Wear down? Or was the fuel in the pipes exposed to extreme cooling during a large part of the cruise phase?

There was a region of particularly cold air, with ambient temperatures as low as -76ºC,

Curiously enough the tropopause temperature is normally higher in winter time.

 

[FredGarvin]

The story doesn't specify what is meant by "Investigators have found no sign of fuel contamination other than from fire fighting equipment." Is this a visual inspection or were detailed chemical analyses performed?

A decreased temperature does help with the amount of air that can be entrained in the fuel. A rapid increase in temperature would then release that air. It's definitely a possibility. The effect of temperature on hydrocarbon Ostwald coefficient is

C_T=.3*\left[ e^{\frac{.639(700-T)}{T}*ln(3.33C_o)}\right]

The Ostwald coefficient,C_o=.095 for 273K and 1 atm.

I don't buy the viscosity argument. Anyone who has ever seen a round of certification testing for an aircraft fuel pump will tell you that.

 

[<s>radix</s>]

yep

Wow, that's really too bad.

 

[Astronuc]

Curiously enough the tropopause temperature is normally higher in winter time.

Did this flight fly at an unusually high altitude? Or were there other flights above this one? Were there other flights from Beijing that went to other places in Europe, e.g. Scandanavia or Berlin or Paris, which flew similar distances or higher or more northerly, or did this flight hit a perculiarly cold mass of air?

 

[Andre]

Did this flight fly at an unusually high altitude? Or were there other flights above this one?

No, it flew around the most favorite flight levels FL 340 -400. (times 100 feet with standard pressure altitude 29.98 inch or 1013.2 hPa). The tropopause starts somewhere at those levels above which the temperature stabilizes.

Were there other flights from Beijing that went to other places in Europe, e.g. Scandanavia or Berlin or Paris, which flew similar distances or higher or more northerly, or did this flight hit a perculiarly cold mass of air?

Yes it was much colder than normal. And there was one other unusual thing:

During the descent, from Flight level (FL) 400 the aircraft entered the hold at Lamborne at FL110; it remained in the hold for approximately five minutes, during which time it descended to FL90.

Normally ATC manages to avoid holding patterns, the longer period of time it stayed at lower levels helped warming the fuel again.

So how about the next scenario?

Although the fuel in the tanks never reached critical low temperatures, it may have done so in the fuel pipes, causing partial freezing, which damaged both fuel pumps a bit.

The hyper cold fuel (Freds scenario) during the prolongued flight period increased the entraining of air in the fuel, also facilitated by the pressurizaton of the fuel tanks.

The longer period in the approach fase caused the fuel to warm somewhat longer as normal, causing a super saturation condition.

The damaged fuel pumps may have facilitatedhttp://pdf.aiaa.org/preview/CDReadyMASM07_1064/PV2007_337.pdf , decreasing the fuel flow.

 

[FredGarvin]

Ah crud. I always get this backwards. Air solubility increases with an increase in temperature. That kills my theory. CO2 is the gas that solubility increases with a decrease in temp. This is why I don't design fuel systems Although, I think I like Andre's mention of an increased demand on the low side of the pumps. A sufficient demand would cause the low side to go down so much as to match the vapor pressure and cavitation starts.

 

[Astronuc]

777's power loss concerns aviation officials
http://www.usatoday.com/travel/flights/2008-02-26-777_N.htm

Re: 777's power loss concerns safety officials.
USA Today (2/27, Levin) reports that the recent crash of a Boeing 777 at Heathrow Airport because of power loss "has transfixed the world's aviation safety experts. Not only has the cause so far eluded accident investigators, but the potential impacts are enormous." Bernard Loeb, a former chief investigator for the U.S. National Transportation Safety Board, said, "This is an extremely significant investigation. You cannot have a loss of power in both engines." It is still "too early to indict the 777's safety," experts contend, "but the details of the crash at least raise the possibility that designers overlooked a vulnerability in the engines, the fuel system or the electronics." So far, "one of the few viable clues" investigators have found "is the 'abnormal' wear found on fuel pumps on each engine. The damage indicated the pumps may have run dry." However, investigators "said that finding just raises more questions. How is it possible for fuel flow to be blocked nearly simultaneously in two separate fuel tanks?"

Well, avaiation safety authorities are taking this very seriously.

 

[FredGarvin]

Well, avaiation safety authorities are taking this very seriously.

I will say that, despite being a government agency, The FAA does take every crash/mishap extremely seriously. They definitely do not screw around.

 

[Astronuc]

It's worrisome that "'abnormal' wear found on fuel pumps on each engine" indicates that "the pumps may have run dry." On both pumps - simultaneously! Redundancy - two separate tanks and fuel systems, didn't overcome whatever common failure mode is responsible.

So the have to be wondering - can it happen again, and what is the potential to affect all aircraft?

 

[Moonbear]

It's worrisome that "'abnormal' wear found on fuel pumps on each engine" indicates that "the pumps may have run dry." On both pumps - simultaneously! Redundancy - two separate tanks and fuel systems, didn't overcome whatever common failure mode is responsible.

So the have to be wondering - can it happen again, and what is the potential to affect all aircraft?

Have they considered inspecting the fuel pumps on a similar aircraft to see if this "abnormal wear" is a flaw in the pump design that had been developing for some time until reaching a critical failure, or if it really was something due to unique and immediate circumstances of the fuel delivery just prior to the crash?

 

[Andre]

Have they considered inspecting the fuel pumps on a similar aircraft to see if this "abnormal wear" is a flaw in the pump design that had been developing for some time until reaching a critical failure, or if it really was something due to unique and immediate circumstances of the fuel delivery just prior to the crash?

Be assured that all B-777 fuel pumps are being checked right now. Reaction on this kind of thing is immediately and adequate. Also if they had found any other pump with that damage all unchecked aircraft are grounded automatically. Since that didn't seem to happen, they probably did not find similar problems.

Running completely dry would have caused engine flame outs for sure, that didn't seem to happen but again cavitation with air bubbles is a posibility or perhaps fuel with a too high viscosity due to extreme low temperatures during transport in the fuel pipes.

 

[Art]

And finally a result;

Ice in fuel caused Heathrow 777 crash


Nasty chill provoked reduced fuel flow

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."

http://www.theregister.co.uk/2008/09/04/heathrow_777_verdict/

Full report here http://www.aaib.dft.gov.uk/sites/aaib/cms_resources/G-YMMM Interim Report.pdf

 

[edward]

? There are several jet fuel anti -icing products on the market.

 

[wolram]

flow to both engines was restricted; most probably due to ice within the fuel feed system.

Most probably? sure that inspires confidence, if i ever fly again i will ask if this aircraft comes with addatives.