Heathrow Triple 7 crash update

[expat]

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

Not with a jet engine. The tanks will gravity feed, but the engine will not run on suction. The fuel spray nozzles in the combustion chamber are working from anything between a couple of hundred psi at idle to a couple of thousand psi at max chat. Also the spray pattern and shape of it is very, very important to the well being of the "hot end".

Matt

[Splinter562]

Not with a jet engine. The tanks will gravity feed, but the engine will not run on suction.

For transport category aircraft, this is usually not correct. Most that I know of have engine-driven fuel pumps to provide pressure to the fuel injector. The boost pumps are there to get the fuel from the tanks to the engine (and take some of the load off the engine-mounted pumps so they last longer). With boost pumps inoperative, the engines are usually able to suction-feed themselves.

A real world example: with all booster pumps off, the 737-200 using the PW JT8D

[expat]

Not with a jet engine. The tanks will gravity feed, but the engine will not run on suction.

For transport category aircraft, this is usually not correct. Most that I know of have engine-driven fuel pumps to provide pressure to the fuel injector. The boost pumps are there to get the fuel from the tanks to the engine (and take some of the load off the engine-mounted pumps so they last longer). With boost pumps inoperative, the engines are usually able to suction-feed themselves.

A real world example: with all booster pumps off, the 737-200 using the PW JT8D

[Splinter562]

Yup, crossed-wires it is. I thought I had read the news saying the area of interest was the booster pumps. I just went to the actual AAIB report and it is, as you say, the engine-driven high-pressure pumps that they are interested in. There is most defiantly an important distinction between the two. LP booster pump failure is manageable, HP pump failure is a bad day.

I still have a hard time swallowing that pill. Given that they are two separate pumps, on two separate engines, feeding from two separate tanks, the chances of them both failing in the same way within seconds of each other seems a bit coincidental. I think there's another factor involved here.

[Chris_F]

I still have a hard time swallowing that pill. Given that they are two separate pumps, on two separate engines, feeding from two separate tanks, the chances of them both failing in the same way within seconds of each other seems a bit coincidental. I think there's another factor involved here.

Unless they partially failed during cruise and this was the first time they were put under significant load during the decent.

[expat]

Yup, crossed-wires it is. I thought I had read the news saying the area of interest was the booster pumps. I just went to the actual AAIB report and it is, as you say, the engine-driven high-pressure pumps that they are interested in. There is most defiantly an important distinction between the two. LP booster pump failure is manageable, HP pump failure is a bad day.

I still have a hard time swallowing that pill. Given that they are two separate pumps, on two separate engines, feeding from two separate tanks, the chances of them both failing in the same way within seconds of each other seems a bit coincidental. I think there's another factor involved here.

Better crossed wires than pages of, your are wrong I am right
Just a bit of information to add to what I posted last night, I had a look in the 737-800 AMM this morning. The booster pumps produce:
Centre tank 27 psi and low pressure warning at 22psi
Wing tanks is about 15psi and low pressure at 10psi.

As for a hard pill to swallow, I agree, but it is not impossible. It looks as if the pumps suffered cavitation damage. How much is the question at the moment. The pump manufacturers say not enough to reduce fuel delivery. The biggest question of course is what caused the cavitation.  As I have already said, fuel air mix, or pipe blockage. We will have to wait and see.

I still have a hard time swallowing that pill. Given that they are two separate pumps, on two separate engines, feeding from two separate tanks, the chances of them both failing in the same way within seconds of each other seems a bit coincidental. I think there's another factor involved here.

Unless they partially failed during cruise and this was the first time they were put under significant load during the decent.

Not so much as a significant load in decent, at cruise they would have been under high load, but more in the direction of what I put earlier.

"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 decent/approach phase there would not have been enough RPM to overcome the pump damage and through RPM force the pumps to produce fuel flow"


Matt