Possible Causes of the Boeing 777 Crash Landing at Heathrow?

[Garth]

Just to get the facts right as far as we know at present.

The United Kingdom's Air Accident Investigation Board (AAIB) report copied from the Professional Aviation Maintenance Association.

Initial Report Update 23 January 2008

Since the issue of the Air Accidents Investigation Branch (AAIB) 1st Preliminary Report on Friday 18th January 2008 at 1700 hrs, work has continued on all fronts to identify why neither engine responded to throttle lever inputs during the final approach. The 150 tonne aircraft was moved from the threshold of Runway 27L to an airport apron on Sunday evening, allowing the airport to return to normal operations.

The AAIB, sensitive to the needs of the industry including Boeing, Rolls Royce, British Airways and other Boeing 777 operators and crews, is issuing this update to provide such further factual information as is now available.

As previously reported, whilst the aircraft was stabilised on an ILS approach with the autopilot engaged, the autothrust system commanded an increase in thrust from both engines. The engines both initially responded but after about 3 seconds the thrust of the right engine reduced. Some eight 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.

Recorded data indicates that an adequate fuel quantity was on board the aircraft and that the autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.

All possible scenarios that could explain the thrust reduction and continued lack of response of the engines to throttle lever inputs are being examined, in close cooperation with Boeing, Rolls Royce and British Airways

This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles.

Further factual information will be released as and when available.

So not a complete engine shutdown, but loss of thrust at a crucial time, and affecting both engines, but not exactly simultaneously.

And dated 26 Jan 08

Investigators in 777 Crash Looking at Fuel Systems

By Dominic Gates
Seattle Times aerospace reporter

Investigators studying last week's crash landing of a British Airways Boeing 777 said Wednesday they are looking closely at the possibility that the accident was caused by an interruption in the flow of jet fuel to the engines.

The cause of the Jan. 17 crash at London's Heathrow airport has not been established. Even if it is a fuel-flow interruption, that doesn't necessarily exonerate Boeing.

But this focus of the investigation seems to increase the possibility of an external cause such as contamination of jet fuel loaded before the 777 took off from Beijing.
http://seattletimes.nwsource.com/html/nationworld/2004144384_777crash25.html

Garth

 

[Garth]

I was just coming into Heathrow yesterday and I noticed they had fitted some new runaway lights...http://www.outpost.pwp.blueyonder.co.uk/Z-Private/runway.gif

Garth

 

[Astronuc]

I was just coming into Heathrow yesterday and I noticed they had fitted some new runaway lights...

Garth

Clever! :rofl: Good think I had just swallowed the coffee.

 

[Art]

Latest on the crash,

No engine defect on crash plane
The engines of a jet that crash-landed at Heathrow Airport had no mechanical defects, investigators have said.

The Air Accidents Investigation Branch (AAIB) added the black box data recorder showed nothing wrong with the major aircraft systems.

But there was some damage to the fuel pumps, and some small items of debris were found in the fuel tanks.

All 136 passengers and 16 crew on the British Airways flight from Beijing survived the incident on 17 January.

Not giving any cause of the accident, the AAIB said it was carrying out a full examination and analysis of the entire Boeing 777 aircraft and engine fuel system.

http://news.bbc.co.uk/2/hi/uk_news/england/london/7251435.stm

The focus now is on the fuel pumps but don't these work independently pumping from separate tanks? If so what are the chances of both sets failing within seconds of each other?

 

[mgb_phys]

the fuel pumps but don't these work independently pumping from separate tanks? If so what are the chances of both sets failing within seconds of each other?

If it was a random failure of a component in the pump then very low - if it was a failure of a control system common to both of them then it is higher.

It's like the argument for allowing twin engine aircraft, like the 777, to fly over water - the chance of a random fault happening on both engines within a short time is very low. A failure of a common system that took out both engines is just as likely to happen on a 4 engine aircraft so having 4 engines is no advantage.

 

[FredGarvin]

It's impossible to design in redundancy in the fuel you carry. That is one of the few "components" on an aircraft that doesn't have a back up. If it goes bad, things get ugly.

BTW..Thanks for the update, Art. I was just wondering about this.

 

[russ_watters]

If one engine fails to power up as expected, would the flight control system back off on the other one to keep trim?

 

[Moonbear]

I was just coming into Heathrow yesterday and I noticed they had fitted some new runaway lights...


http://www.outpost.pwp.blueyonder.co.uk/Z-Private/runway.gif

Garth

:rofl: Good idea! :rofl: :rofl:

 

[Cyrus]

If one engine fails to power up as expected, would the flight control system back off on the other one to keep trim?

Why would it do that? It could just give it more rudder. The entire point of fly by wire is that if one thing fails, you can use all the other flight surfaces to compensate and fly as normal.

 

[Astronuc]

It's impossible to design in redundancy in the fuel you carry. That is one of the few "components" on an aircraft that doesn't have a back up. If it goes bad, things get ugly.

It's amazing that it happened toward the end of the flight and not sooner. Those folks are lucky that it happened just at the airport as opposed to over a populated area.

I supposed that as the fuel is consumed, the density/concentration of crud/contaminants builds up.

Fred's right. The fuel is common to all the tanks. The only recourse would be to carry extra onboard fuel from a different source, which means carrying extra mass.

Now they have to figure out where the fuel came from. Is it one source, or more than one, and was it a one time event. The same source of fuel could affect other aircraft.

On the plane side, it would seem that a more robust filter system is needed.

 

[russ_watters]

Why would it do that? It could just give it more rudder. The entire point of fly by wire is that if one thing fails, you can use all the other flight surfaces to compensate and fly as normal.

Dunno, just speculating.

 

[lisab]

Now they have to figure out where the fuel came from.

The jet flew direct from Beijing, China, where it had last refueled.

Maybe the fuel had melamine in it.

 

[FredGarvin]

It's amazing that it happened toward the end of the flight and not sooner. Those folks are lucky that it happened just at the airport as opposed to over a populated area.

Indeed. I was thinking the same thing myself. Someone was watching over them that's for sure.

Now they have to figure out where the fuel came from. Is it one source, or more than one, and was it a one time event. The same source of fuel could affect other aircraft.

The chain of custody shouldn't be too difficult to follow. Most major airports have large pipeline systems direct from a terminal. I am going to guess that the terminal and pipeline system will be ok. If it weren't then there would have been a lot of reports of aircraft with bad fuel. There are a lot of precautions taken in the US and Europe regarding the proper handling of fuel. My guess is that there was a serious breech of procedure in this area. There are a lot of things that could go wrong between offload from the pipeline to the offload to the aircraft. The items that most quickly come to mind are improper amounts or improper fuel additives, an exceptionally large amount of water in the truck carrying the fuel and all out contamination of the truck by some other chemical. I don't want this to turn into another China bashing thread, but we all know that the Chinese are notorious for cutting corners.

On the plane side, it would seem that a more robust filter system is needed.

I'm not so sure about that. Aircraft fuel systems are usually pretty well filtered and strained. 10 micron filters are standard on engines and strainers are usually used quite a bit. I am guessing that this was something a filer could not have prevented.

 

[Astronuc]

The chain of custody shouldn't be too difficult to follow. Most major airports have large pipeline systems direct from a terminal. I am going to guess that the terminal and pipeline system will be ok. If it weren't then there would have been a lot of reports of aircraft with bad fuel. There are a lot of precautions taken in the US and Europe regarding the proper handling of fuel. My guess is that there was a serious breech of procedure in this area. There are a lot of things that could go wrong between offload from the pipeline to the offload to the aircraft. The items that most quickly come to mind are improper amounts or improper fuel additives, an exceptionally large amount of water in the truck carrying the fuel and all out contamination of the truck by some other chemical. I don't want this to turn into another China bashing thread, but we all know that the Chinese are notorious for cutting corners.

One other possibility could be a microbial contamination in the fuel storage systems, especially if there is build up of moisture. I could be inadequate procedures, or not following procedures, or cutting corners on quality of fuel.

 

[Art]

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?

 

[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 :tongue2: 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.