Heathrow Triple 7 crash update

[Saitek]

http://news.bbc.co.uk/1/hi/england/london/7251435.stm

[Xyn_Air]

OK, so if the engines were without defect, if there was no ingestion of foreign objects coming in to damage the engines, and the fuel was not contaminated, what do we have left?

Electrical malfunction (either in the computer software or the control hardware, such as an electrical short), pilot error, and . . . ?

Confused,
Darrin

[Papa9571]

You are forgetting the main part of the story.

So far they have uncovered the following.

Engines running at touchdown.
Fuel crossfeed open.
Water in the fuel well within limits.
No computer or software problems.
Throttles fully functional.
Autopilot fully functional.
Pilot error has been ruled out.
There was no collision with birds.

So what is the problem then?

They have found significant damage  the fuel pumps.
They think this was caused by eithr low pressure or air mixing with the fuel.
They have also found small pieces of plastic and paper in the fuel tank.

[ozzy72]

Could be like the Qantas Boeing problem and some water got into the electrics and knackered things before evaporating....

[expat]

OK, so if the engines were without defect, if there was no ingestion of foreign objects coming in to damage the engines, and the fuel was not contaminated, what do we have left?

Electrical malfunction (either in the computer software or the control hardware, such as an electrical short), pilot error, and . . . ?

Confused,
Darrin

I did write you a long and detailed answer, but Firefox froze and I lost everything, so here is the short version.

It is being reported that the direction of the investigation is now on the engine fuel pumps. It would appear that they have evidence of cavitation. This is when the pump either pumps a mixture of fuel and air or a restriction in the system flow causes the pump to try and pump a fuel vacuum (instead of air to nothing, fuel is pumped to a negative pressure. Fill a straw with drink, put your finger over the end to stop the liquid falling out and then try to suck the drink out with your mouth.) Cavition can cause the pump to erode itself away before your very eyes in extreme situations, thus destroying the pump. However the cavitation that has been seen on G-YMMM, the pump manufacturers say would not have effected pump operation. The AAIB are now trying to replicate this pump damage.

AAIB latest report

Matt

[Xyn_Air]

OK, so if the engines were without defect, if there was no ingestion of foreign objects coming in to damage the engines, and the fuel was not contaminated, what do we have left?

Electrical malfunction (either in the computer software or the control hardware, such as an electrical short), pilot error, and . . . ?

Confused,
Darrin

I did write you a long and detailed answer, but Firefox froze and I lost everything, so here is the short version.

It is being reported that the direction of the invetigation is now on the engine fuel pumps. It would appear that they have evidence of cavitation. This is when the pump either pumps a mixture of fuel and air or a restriction in the system flow causes the pump to try and pump a fuel vacuum (instead of air to nothing, fuel is pumped to a negitive pressure. Fill a straw with drink, put your finger over the end to stop the liquid falling out and then try to suck the drink out with your mouth.) Caviation can cause the pump to erode itself away before your very eyes in extreme situations, thus destroying the pump. However the cavitation that has been seen on G-YMMM, the pump manufacturers say would not have effected pump operation. The AAIB are now trying to replicate this pump damage.

AAIB latest report

Matt

Thank you very much for the explanation!  This I can understand!  Car fuel pumps can have the very same problem, especially older models of fuel pumps.  Run your tank dry, or get some air in your fuel pump for whatever reason and your fuel pump becomes useless.  Well, I am not sure if aircraft fuel pumps work exactly like that, but it sounds liek a similar possible problem.  Anyway, thanks for catching me up to date.

Darrin

[expat]

OK, so if the engines were without defect, if there was no ingestion of foreign objects coming in to damage the engines, and the fuel was not contaminated, what do we have left?

Electrical malfunction (either in the computer software or the control hardware, such as an electrical short), pilot error, and . . . ?

Confused,
Darrin

I did write you a long and detailed answer, but Firefox froze and I lost everything, so here is the short version.

It is being reported that the direction of the invetigation is now on the engine fuel pumps. It would appear that they have evidence of cavitation. This is when the pump either pumps a mixture of fuel and air or a restriction in the system flow causes the pump to try and pump a fuel vacuum (instead of air to nothing, fuel is pumped to a negitive pressure. Fill a straw with drink, put your finger over the end to stop the liquid falling out and then try to suck the drink out with your mouth.) Caviation can cause the pump to erode itself away before your very eyes in extreme situations, thus destroying the pump. However the cavitation that has been seen on G-YMMM, the pump manufacturers say would not have effected pump operation. The AAIB are now trying to replicate this pump damage.

AAIB latest report

Matt

Thank you very much for the explanation!  This I can understand!  Car fuel pumps can have the very same problem, especially older models of fuel pumps.  Run your tank dry, or get some air in your fuel pump for whatever reason and your fuel pump becomes useless.  Well, I am not sure if aircraft fuel pumps work exactly like that, but it sounds liek a similar possible problem.  Anyway, thanks for catching me up to date.

Darrin

Although cars and aircraft can suffer the same problem, they used different pumps to pump fuel. The aircraft industry use spur geared pumps for fuel. As far as I am aware the auto industry only uses them for oil pumping and then only as a single turning in an off set port to produce fluid movement.

Basic spur gear pump principle as found on aircraft.



The other type of fuel pump found on aircraft engines is the Swash Plate Pump. The fuel is sucked in on the low pressure side as the pistons are drawn back. Due to the swash plate the pistons are rotated and the fuel is pushed out under high pressure. As with both pumps the higher the rotation speed, the higher the pressure. The swash plate pump has the advantage of not suffering from cavitation, but the disadvantage of a hundred moving parts and the need for lubrication. This his type of pump however you will not find on a modern engine. Most likely on early turbojet engines, but just for interest, shown here.

Swash Plate pump principle



Matt

[Xyn_Air]

Wow!  Thanks for that very detailed, yet easy to understand, explanation, Matt!

So, any news on the aircraft fuel pumps and what happened?

Darrin

[a1]

I think this is a fuel pump problem. But what about the AP demanding for more thrust and the engines not responding? is that because of the fuel pump problem?

[expat]

I think this is a fuel pump problem. But what about the AP demanding for more thrust and the engines not responding? is that because of the fuel pump problem?

As the report says, the AP, the EEC and the stepper motor on the engine all acted as they should have. All computer inputs did what was required of the situation and the fuel metering unit would have demanded a higher fuel flow. The fuel pumps are driven from the accessory gearbox which is in turn driven by the HP compressor. You now have a vicious circle. RPM drops, fuel requirement increased, fuel pumps damaged and can only deliver sightly above idle fuel load, engine cannot turn faster due to the restricted fuel delivery. System will demand more and more fuel, but the delivery is not there. RPM will stay at what ever the delivery rate of the pumps in a damaged condition can produce.
One possibility is that the pumps became damaged during the flight, but due to the engines turning at cruise RPM, the pumps would have been around max delivery. In a damaged condition or a developing damaged condition they may have been able to produce enough delivery that it would not have been noticed. One the engines where pulled back for the approach phase there would not have been enough RPM to overcome the pump damage and through RPM force the pumps to produce fuel flow.................if that makes sense.

Matt

[C]

Now I understand why people were talking about all the things that had been found left in fuel tanks at work today...

[expat]

Now I understand why people were talking about all the things that had been found left in fuel tanks at work today...

During a Major/C check you will be amazed at what you will find in fuel tanks..............tanks that would have been inspected before the lids are bolted down.  :o

Matt

[C]

Now I understand why people were talking about all the things that had been found left in fuel tanks at work today...

During a Major/C check you will be amazed at what you will find in fuel tanks..............tanks that would have been inspected before the lids are bolted down.  :o

Matt

Seat cushions seemed to occur more than once!

[expat]

Now I understand why people were talking about all the things that had been found left in fuel tanks at work today...

During a Major/C check you will be amazed at what you will find in fuel tanks..............tanks that would have been inspected before the lids are bolted down.  :o

Matt

Seat cushions seemed to occur more than once!

Good for soaking up those last pools of fuel before you vent the takn to work in it.

Matt

[Slotback]

I had always understood that fuel COULD if needed be supplied by suction?