Possible Causes of the Boeing 777 Crash Landing at Heathrow?

[Cyrus]

Grant, how fast do blocked Pitot and static ports take to melt when flying in above freezing conditions, if you don't have pitot heat available (in something like a 172)?

 

[Garth]

I think the pilots would have noticed if their altimeters still read 33,000 feet on final due to pitot blockage that occurred at altitude. Flying in lower air at above freezing temperature will melt pitot ice most of the time. Its the reason why your told to fly lower if you suspect icing.

I would have thought pitot tube icing would have affected the airspeed indicator...

Not knowing the particular configuration of this 777 I am not sure, but would not the pitot tube have electric de-icing in any case?

The PFD would indicate radio altitude, which of course would not be affected by icing.

As a pilot, I find such statements as:

A touching example of faith placed in modern software, I hope it doesn't prove to be a delusion.

Aircraft avoidance, GPS maps with ground speed, multiple redundancies, in air weather and terrain maps, full glass panel displays with systems information, you name it -all thanks to modern software.

And as a pilot I would prefer not to place total faith in electronics, especially the software.

I was at a lecture at Queen Mary College, (University of London), about the Space Shuttle in the 1980's, pre-Challenger disaster, and asked why a recent launch had had to be postponed because there had been zero wind velocity at the Cape. The answer was that the computers on board had been designed with the rest of the craft around 1975 and they had 1970's memory shortage. As there was seldom zero wind velocity at the Cape they were programmed for a wind speed of 10 - 70 knots. (If I remember correctly).

I asked, "Why not replace the computers with modern ones with more memory?"
The answer came back, "Because we trust the software!"

The software in modern computers is so complex that, although it can be de-bugged and tested to the n^th degree, you can never be sure that you have got rid of all the bugs.

My intuition is that the investigation will discover that this incident was caused by one of those undiscovered bugs, but then what do I know?

Garth

 

[Cyrus]

I would have thought pitot tube icing would have affected the airspeed indicator...

Yes, sorry. I switched the two by accident.

Not knowing the particular configuration of this 777 I am not sure, but would not the pitot tube have electric de-icing in any case?

Im sure it would on an A/C that large.

And as a pilot I would prefer not to place total faith in electronics, especially the software.

As this is the first accident on a 777 in 12 years of service, its a very very good system and A/C.

I was at a lecture at Queen Mary College, (University of London), about the Space Shuttle in the 1980's, pre-Challenger disaster, and asked why a recent launch had had to be postponed because there had been zero wind velocity at the Cape. The answer was that the computers on board had been designed with the rest of the craft around 1975 and they had 1970's memory shortage. As there was seldom zero wind velocity at the Cape they were programmed for a wind speed of 10 - 70 knots. (If I remember correctly).

I asked, "Why not replace the computers with modern ones with more memory?"
The answer came back, "Because we trust the software!"

The software in modern computers is so complex that, although it can be de-bugged and tested to the n^th degree, you can never be sure that you have got rid of all the bugs.

My intuition is that the investigation will discover that this incident was caused by one of those undiscovered bugs, but then what do I know?

Garth

I don't know how you are comparing 1975 software with modern fly-by-wire systems. Thats over 33 years of improvements. I don't doubt it was a bug, as I said so myself. But blanket statements such as 'faith in electronics' makes no sense. A large portion of airliners are fly by wire for many years. It works, over and over, again and again. One time in 12 years something goes wrong and people get all worked up. Amazing.

I would venture that the space shuttle would have a fly by wire system as, if not more, complicated than airliners today.

 

[grant9076]

Grant, how fast do blocked Pitot and static ports take to melt when flying in above freezing conditions, if you don't have pitot heat available (in something like a 172)?

This is an excellent question and I honestly do not know the answer.

However, I think that the 2 biggest factors which affect this are the type/severity of the icing, and the total air temperature TAT. In turn, the TAT is a function of the static air temperature and the true airspeed.

On a side note: Although lower altitudes are generally warmer, this is not always the case, especially with a strong temperature inversion like the one I encountered 3 days ago on the descent into Dallas Fort Worth. The TAT was showing 11 degrees celsius at 12,000 ft (although we were doing 300 Knots indicated airspeed). By the time we rolled out on the ILS course at around 3,500 ft, the TAT was showing 3 degrees celsius.

This brings up another side note. We turned on the engine anti-ice at 12,000 ft because engine icing can occur at total air temperatures as warm as 10 degrees celsius (depending on the engine) if there is any type of visible moisture (including clouds or visibility less than a mile) present.

A third note: Every 777 pilot that I know describes it as possibly the best designed aircraft that he/she has ever flown.

 

[Garth]

I don't know how you are comparing 1975 software with modern fly-by-wire systems. Thats over 33 years of improvements. I don't doubt it was a bug, as I said so myself. But blanket statements such as 'faith in electronics' makes no sense. A large portion of airliners are fly by wire for many years. It works, over and over, again and again. One time in 12 years something goes wrong and people get all worked up. Amazing.

I would venture that the space shuttle would have a fly by wire system as, if not more, complicated than airliners today.

I was simply saying that NASA were sure of the simple 1970's software because the lines of code could be de-bugged 'by hand'. Modern software is so complex involving millions of lines of code that you cannot be sure that every bug has been dealt with.

I am sure modern fly-by-wire systems have been robustly tested and reliable, however we will see whether this case is an example of an unknown bug being missed until now.

Garth

 

[Garth]

Another possibility is that the emergency was caused by another electronic device, such as a mobile phone (Hi love I'm almost home!), interfering with the normal operation of the flight control computers.

Garth

 

[Cyrus]

Could you explain more on that please?

 

[grant9076]

I just recently saw the latest press release from the UK Air Accidents Investigation Branch. The report indicates that there was adequate fuel in the tanks, and that the autothrottles and the electronic engine controls functioned perfectly as advertised.

They are now investigating all possible causes including the entire fuel flow pathway from the tanks to the engine fuel nozzles.

http://www.aaib.dft.gov.uk/latest_n..._17_january_2008___initial_report_update.cfm"

 

[chroot]

The MythBusters did an interesting show on cell phones aboard aircraft. Quite surprisingly, basic nav radios, the kind you'd find in Cessnas, are DRAMATICALLY affected by cell phone signals. On the other hand, they were unable to get a properly shielded jet aircraft's avionics to respond in any way at all to cell phone signals. They weren't legally allowed to use a cell phone while in a flying aircraft, though, so they had to do their tests while powered up and parked on the tarmac.

Either way, I'd put the odds of a cell phone causing a crash at about one in a billion.

- Warren

 

[Cyrus]

The point I was getting at is that a cell phone would affect navigation. That does not play a roll in the pitot probe, nor the physical connection between the autopilot and engines. I don't see how that could possibly be an issue.

BUT, thanks for that warren. I didnt know it was an affect on the nav that much in a cessna.


EDIT: You know, open up your sport's pilot catalog. They sell cell phone adapators to hook up to your headset. Hmmmmmmmmmmmmmmmmm...

http://www.sportys.com/acb/showdetl.cfm?&did=19&product_id=7145

http://www.sportys.com/terryc/images/4149al.jpg

 

[Art]

How do you reconcile these two statements?

So to say that there is an 'unresolved computer glitch' which could cause a catstrophe at any time is nothing more than uninformed nonsense

I don't doubt it was a bug, as I said so myself.

 

[grant9076]

How do you reconcile these two statements?

As I mentioned in my previous post, the evidence clearly shows that there is no electronic malfunction. The investigation is now directed at fuel blockage/contamination.

I think that the point that you are trying to make with this post is now irrelevant.

 

[Cyrus]

How do you reconcile these two statements?

I was very clear in pointing out that each 777 is as unique as the next in terms of the combination of engine and avionics packages and a problem in one particular A/C is not necessarily common to all aircraft of that type. I also mentioned Airworthiness directives being issued if it were common to many A/C in the past 12 years.

I also said, it was either a computer issue, or possibly fuel starvation.

I blame those British Rolls engines. They should have put Good American G&E engines.

 

[Cyrus]

As I mentioned in my previous post, the evidence clearly shows that there is no electronic malfunction. The investigation is now directed at fuel blockage/contamination.

I think that the point that you are trying to make with this post is now irrelevant.

Im guessing the tanks on large aircraft use crossfeeding; but, I would expect each tank to have at least two selector valves incase one becomes blocked. I would expect the tanks to become blocked either because the fuel expanded at a valve and froze or the fuel was contaminated. Could the fuel really have been that contaminated though? Its very unlikely. He should have used his fuel sample cup pre-flight

 

[russ_watters]

As I mentioned in my previous post, the evidence clearly shows that there is no electronic malfunction. The investigation is now directed at fuel blockage/contamination.

I think that the point that you are trying to make with this post is now irrelevant.

Read it again. The article says the autothrottle was working properly both before and after the reduction in speed. It doesn't say it was working properly during the reduction in throttle. I think that means it is still an open question.

 

[Cyrus]

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.

Not really an open question. I think you misread it. If it was working even after the thrust was reduced, its not the computers fault, or it would have spooled back up.

 

[grant9076]

Read it again. The article says the autothrottle was working properly both before and after the reduction in speed. It doesn't say it was working properly during the reduction in throttle. I think that means it is still an open question.

Actually Russ, I saw an even more detailed report at my airline's website in the section for pilots only. Although the information concerning the accident is not sensitive, it is in a security restricted area and I am thus unable to share the link with anyone here.

However, the information that I gathered showed what happened with the throttles and the fuel control systems during the incident. It showed that the Electronic Engine Controls (EEC's) functioned exactly as advertised. The data records also showed that the fuel control valves opened to the proper settings for the commanded thrust during the incident. However, the records showed that only a fraction of the fuel flow was received (just enough to keep the engines slightly above idle). This is why the investigation is now turned to fuel blockage/contamination.

This is an extremely brief synopsis of the info that I saw but it is why I no longer consider it to be an open question. P.S. The engine's responses were not exactly identical but similar to each other (according to the records). I did not see any indications of cross feeding.

 

[Garth]

Thank you Grant for that inside information.

If it was a fuel blockage the problem would be to explain how both independent fuel systems, from two independent wing tanks and one common central tank, dried up at the same time.

Not knowing the detail of the 777 fuel tank installation I am dependent on others', such as your own, expertise. Would there be a second set of fuel control valves, close to each fuel tank, ready to shut off the fuel in case of emergency? If so might these have been all inadvertently activated by an electronic glitch?

Garth

 

[grant9076]

Not knowing the detail of the 777 fuel tank installation I am dependent on others', such as your own, expertise. Would there be a second set of fuel control valves, close to each fuel tank, ready to shut off the fuel in case of emergency? If so might these have been all inadvertently activated by an electronic glitch?

A very good question. However, I doubt that it would be the case and here is why:

For every boeing aircraft that I have flown, each one has a separate fire detection and protection system for each engine. In addition, each fire detection system has 2 separate and independent loops along with a fault monitoring system which disables the faulty loop. If it detects a fire, it warns the pilots with audiovisual indications and unlocks the fire handle which allows the pilot to pull the fire handle as part of the fire protection. Otherwise, the pilot will have to manually disengage the locking devise for the fire handle in order to pull it. Pulling the fire handle will cutoff fuel, hydraulics, and bleed air in addition to tripping the respective generator off line.

The chances of all 4 separate and independent fire detection loops (2 per system) failing simultaneously is more remote by several orders of magnitude than the fuel in both wing tanks being contaminated. In addition, the chances of the fire handles pulling themselves or the results happening without them being pulled is doubtful.

On a side note, I did have doubts about an Electronic Engine Control (EEC) malfunction from the very beginning and here is why:

Although I have not flown the 777, I know that the B-737-800's EEC's are very similar in operation to those of the B-777.

For the B-737-800:
1. Each engine has its own separate and independent EEC.
2. Each EEC in turn has 2 separate and independent channels with a self monitoring system which disables the faulty channel.
3. If both normal channels fail, the EEC has 2 alternate modes.
4. The EEC's are designed to never share single source data.
5. In the alternate modes, the EEC's will command thrust settings which are always greater than or equal to those commanded by the normal modes for the same throttle position.

I also consider the chances of all of these failing simultaneously to be remote.

 

[Garth]

Yet obviously something happened to both engines/fuel systems simultaneously.

Garth

 

[FredGarvin]

Question for Grant...Did your source say that the fuel flow rate went down or did it say that the same fuel rate was reported, but it had no effect?

I'm going with foreign matter or water in the fuel system. Not knowing the plumbing makes it tough to figure out, but I can not fathom a control system issue with what has been said. There is no way it's a FADEC problem either.

 

[Astronuc]

How much fuel would have been left? I wonder if toward the end of the flight, contaminants could have built up to the point where they mucked up the engine.

But wouldn't the fuel supply (in Beijing) have affected other aircraft? Why just that one?

I presume the fuel, the tanks and the engines will be thoroughly analzyed.

 

[Cyrus]

Ohhhhhhh its from Beijing... UNLEADED FUEL ONLY.

 

[grant9076]

Question for Grant...Did your source say that the fuel flow rate went down or did it say that the same fuel rate was reported, but it had no effect?

It was a drop in fuel flow rate.

How much fuel would have been left? I wonder if toward the end of the flight, contaminants could have built up to the point where they mucked up the engine.

But wouldn't the fuel supply (in Beijing) have affected other aircraft? Why just that one?

I do not know how much fuel was on board. These are all very good questions and I have some of the same thoughts. Although the report that I saw is much more detailed than the press release by the UK AAIB, I am sure that it is a synopsis of an even more detailed report. There are obviously still a great deal of missing pieces to this puzzle.

Ohhhhhhh its from Beijing... UNLEADED FUEL ONLY.

I briefly had the same thought.

P.S. If I stop posting in the next 24 or so hours, it is probable because I got called to go fly a sequence.

 

[FredGarvin]

How much fuel would have been left? I wonder if toward the end of the flight, contaminants could have built up to the point where they mucked up the engine.

I seem to recall that most airlines require a specific amount of fuel to be in reserve for every flight to handle emergencies. It should not have been anywhere near empty. Even if it were, the protocols and requirements for proper fuel handling would prevent any large scale contamination that we would be looking at here.

But wouldn't the fuel supply (in Beijing) have affected other aircraft? Why just that one?

I'm thinking that there could have been an omission on the part of the airline to include an anti icing inhibitor, i.e. PRIST (FSII) into the fuel load the aircraft took on. It could be as simple as that. This does go back to my question to Grant in which he mentioned the fuel flow. I can't imagine a catastophic failure of something in the fuel system that would contaminate things to the point of clogging filters and reducing fuel flow to that point.

I presume the fuel, the tanks and the engines will be thoroughly analzyed.

Oh. You know it. They're going to do a ton of testing. As well as tearing apart the engine themselves. When any aircraft that is in this kind of accident that has our engines on it, they get sent back to us for disassembly and a thorough inspection.

 

[grant9076]

I'm thinking that there could have been an omission on the part of the airline to include an anti icing inhibitor, i.e. PRIST (FSII) into the fuel load the aircraft took on.

I don't know what type of fuel that they used but if it is Jet A1, then they shouldn't have to worry icing inhibitors because the presence of the icing inhibitor is what makes it Jet A1. However, if they used Jet A then the icing inhibitor will be absent as you mentioned.

The thing to remember here is that I only operate these airplanes (not design them). Therefore, there are many cases where you will know more than I do.

 

[Art]

This crash seems like a real 'who done it' With both engines operating on independent systems the only common denominator seems to be the fuel but even if it was contaminated what are the chances of both engines experiencing the identical effect within seconds of each other after taking off and flying 1000s of miles without a problem? Are there any other common denominators? Given that eyewitnesses said the engines were roaring as it passed them is it possible they did have the thrust but some part of the flight configuration was wrong regarding flaps or something?

 

[FredGarvin]

the presence of the icing inhibitor is what makes it Jet A1.

I can's say that I agree with that. Do you have any kind of reference I can look at that states that? I would appreciate it. The only difference I am aware of between Jet-A and A1 is that A1 has a slightly lower freezing point and is used extensively in Europe. It was decided to use Jet-A in the US because of the higher freezing point, a slightly larger percent could be manufactured in any given time.

 

[Astronuc]

I guess I have to wonder why icing would be a problem just before landing. Why not at cruise altitude where the atmosphere is colder?

Given that eyewitnesses said the engines were roaring as it passed them is it possible they did have the thrust but some part of the flight configuration was wrong regarding flaps or something?

The flaps/pitch were probably OK, but the plane needs a certain amount of thrust.

I've been on flights where the engines power up a little just a few km from the end of the runway, while on others the plane just glides in gently, without additional power.

 

[grant9076]

I can's say that I agree with that. Do you have any kind of reference I can look at that states that? I would appreciate it. The only difference I am aware of between Jet-A and A1 is that A1 has a slightly lower freezing point and is used extensively in Europe. It was decided to use Jet-A in the US because of the higher freezing point, a slightly larger percent could be manufactured in any given time.

You are exactly right. I was thinking of JP-8. Also, our minimum operating fuel temperatures for Jet A1 and Jet A are -43 degrees celsius and -37 degrees celsius respectively. So you are correct there too.

 

[grant9076]

Given that eyewitnesses said the engines were roaring as it passed them is it possible they did have the thrust but some part of the flight configuration was wrong regarding flaps or something?

This is a very valid question. The likely reason is because the aircraft was well below glidepath and therefore much closer to the witnesses than it would normally be. The thing to remember is that these engines (even at low power settings) are loud enough to cause permanent hearing damage if you are close enough to them. So it is quite normal in this scenario for the people to hear them as being louder than normal.

 

[Andre]

But then again, there is also this passenger who seemed to have heart the engines roaring:

http://www.timesonline.co.uk/tol/news/uk/article3207393.ece

Antonio De Crescenzo, 52, from Naples, said there was little warning that the plane was in difficulty. “We were coming into land but the plane felt like it should have been taking off. The engines were roaring and then we landed and it was just banging.

 

[Garth]

But then again, there is also this passenger who seemed to have heart the engines roaring:

http://www.timesonline.co.uk/tol/news/uk/article3207393.ece

That report also repeats the statement:

There appeared to have been a catastrophic loss of power affecting both engines. The cockpit electronics may also have failed, leaving only the battery-powered airspeed indicator and altimeter operating.

They lost avionics as well.

If that is so, say due to a power cut, would that have also affected the engine control systems?

If the engines were "roaring" on landing it indicates the problem may have been a temporary one and power came back on albeit too late.

The B-777 descent rate quoted in Andre's earlier post #38 would suggest the plane ought to have crashed onto houses before the airport perimeter, the fact that it didn't would be explained by the engines regaining power.

If they did come back at the last second, thus averting that disaster, their escape was closer than ever!

Garth

 

[Andre]

The (temporary?) loss of electrical power may have affected the fuel boost pumps temporarily. Just speculating.

In the same report quoting again:

Martin Green, another airport worker, told Sky News: “It came in at a very high angle and just dropped like a stone — I would estimate 200ft.

“It seemed to be flying fairly slow and it had a very high angle of attack. The nose was high up in the air, which is very unusual.”

Engines are installed so that the thrust vector passes underneath the centre of gravity that means by giving power, the torque causes the aircraft to pitch up. Also with many fast aircraft, Grant can confirm this for his category, under these conditions, slow speed and no significant thrust, the authority to pitch up to those very high angles of attack is simply not available without that engine torque in the aircraft I know. However if the engines 'roar' into max power, they also provide that required nose up torque to get there.

Just a thought. The investigators need to find out.

 

[grant9076]

I guess I have to wonder why icing would be a problem just before landing. Why not at cruise altitude where the atmosphere is colder?

This is an excellent point.
It is possible that icing could have been a problem both in cruise and just before landing. It could be that the globules (or whatever form it may take) of frozen water did not migrate to the fuel boost pump intakes until during the descent. This is highly speculative but I have landed in warm climates and discovered (during the post flight walk around) frosting on the lower wing surfaces due to super cold fuel in the tanks. This indicates that the fuel temperature is well below zero degrees celsius and it is quite the norm after a long flight. FredGarvin is likely to know more about fuel icing than I do.

They lost avionics as well.

If that is so, say due to a power cut, would that have also affected the engine control systems?

I did not remember reading about any loss of avionics. That news report using the term "may" indicates that they probably got it from a passenger because the pilots would know for sure whether or not they lost avionics. If it is from a passenger or flight attendant, then the normal load shedding feature will explain it. All of the boeing aircraft that I have flown are designed to remove power from non-essential equipment (passenger seat lighting, galley power, etc.) if there is a risk of overloading the generators (engine problems, etc). This will leave passengers nervous and in the dark, but the pilots will have all of the electrical equipment that they need.

Each electronic engine control (EEC) has its own separate generator which is operational even at extremely low RPM's (windmilling), and the engines would not have time to spool down to this low of an RPM. In other words, the EEC's are designed to still be working long after the main generators have lost their exciter fields. Besides, the data records showed that the EEC's were fully functional throughout the event. However, it is possible that the engines may have quit and restarted. The more modern boeing aircraft have a feature where the EEC for a particular engine will energize both ignition systems for that engine if it senses an uncommanded drop below idle N2 RPM (or N3 if it's a triple spool engine). The net result is that if an engine quits without the pilot shutting it down, the EEC for that engine will automatically keep trying to restart it.

If the engines did actually fail and then restart, this would be further evidence that the EEC's were functional.