Fuel pump question

[Paul12 2]

How can it be that the PMDG-737 engines continue to run after all 6 fuel pump switches are turned off ?

How does the fuel get into the engines with pumps switched off ?

 

Can anybody enlighten me ?

 

Hubert Werni

 

 

[KriVa 307]

The wing fuel tanks are above the engines. The engines are gravity fed with fuel.

So yes, this is realistic behaviour.

[atrdriver 7]

There is an engine driven pump that draws fuel from the respective wing tank with the electric pumps off. 

[Paul12 2]

There is an engine driven pump that draws fuel from the respective wing tank with the electric pumps off. 

Thanks.However,if this is the case why are the fuel pumps needed ? I am sure they must have other

purposes/uses but what ? I study the manuals and the Chris Palmer videos but could not fine a

satisfactory answer.

Hubert Werni

[atrdriver 7]

Hubert,

 

The simple answer is redundancy. If you lose that engine-driven pump, you have electrical backups. Also, you need those electric pumps for engine start since the engine-driven pump isn't operating until engine spool up. Multiple pumps, more reliable fuel delivery!

[teopereira 25]

I heard about a pilot who forgot to turn on these pumps in a A320 and had a dual flame out in flight. He was fired because of this mistake. In the 737NG can you have a flame out too?

[tsp 2]

atrdriver, don't mix fuel pumps with hydraulic pumps, please

All fuel pumps are AC electrically powered.

 

 

And for the question about all pumps off and flame outs, the FCOM is your friend:

 

"When main tank fuel pump pressure is low, each engine can draw fuel from its
corresponding main tank through a suction feed line that bypasses the pumps. As
the airplane climbs, dissolved air is released from the fuel in the tank due to the
decrease in air pressure. This air may collect in the suction feed line and restrict
fuel flow. At high altitude, thrust deterioration or engine flameout may occur as a
result of the fuel flow reduction."

[Fabo 213]

Also for the center tank, there is no other way to use the fuel but pumps, the same is true for crossfeed.

[kevinh 409]

Hubert,

 

The simple answer is redundancy. If you lose that engine-driven pump, you have electrical backups. Also, you need those electric pumps for engine start since the engine-driven pump isn't operating until engine spool up. Multiple pumps, more reliable fuel delivery!

It's not for redundancy because if the engine driven pump fails the engine will flame out, regardless of whether the boost pumps are running. Also you need boost pressure at altitude.

 

Nor is it for starting. The engine driven pump begins to operate as soon as N2 rises so by the time you open the fuel valve it's fully operational.

 

Even without gravity feed the engine pump can draw fuel from the tank by suction as long as there was no vapour or air in the line. But the suction reduces inlet fuel pressure towards the vapour pressure of the fuel. The boost pumps are there to provide positive fuel pressure to the engine. Hence the name. Boost pressure prevents the fuel vaporising at low ambient pressures.

[Spin737 102]

I believe you can get poor engine performance above FL250 without the electric pumps. If the engine driven pumps die, I think the engine fails.

[atrdriver 7]

Thanks for the correction guys. Sorry to put out obviously wrong information! I went back and looked at the FCOM and re-educated myself. I am currently going thru a 737-800 type rating course and am still mixing up information from previous airplanes I've flown. Time for a brain dump!

 

Thanks again!

[77west 139]

Just a bit of FYI, on the 777, the system will automatically start the Left FWD fuel pump when you select APU on, irrespective of switch position. Obviously because the APU is well above the fuel tank and AFAIK has no suction ability. Does the same happen on the 737?

[FLEX1978 373]

I heard about a pilot who forgot to turn on these pumps in a A320 and had a dual flame out in flight. He was fired because of this mistake. In the 737NG can you have a flame out too?

Do you have a reference for that? Sounds pretty bizarre! The engines on the A320 would still run with the pumps off, not to mention you would have ECAM messages and a big red master caution staring both pilots in the face if the pumps where left off.

 

Cheers

[pete_auau 1,157]

I imagine there would be  a audio  warning  as well

[77west 139]

I am also hesitant to believe that any modern Airbus crew would not notice the glaring warnings long enough to have a dual flameout.