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Paul12

Fuel pump question

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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

 

 

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

So yes, this is realistic behaviour.

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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

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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!

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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?

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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."

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

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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.

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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.

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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!

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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?

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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

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

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Obviously because the APU is well above the fuel tank and AFAIK has no suction ability. Does the same happen on the 737?

 

IIRC the 737 APU has a DC pump, but it is prefferable that tank pump should be on to ensure positive pressure.

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IIRC the 737 APU has a DC pump, but it is prefferable that tank pump should be on to ensure positive pressure.

DC fuel pump is an option, I believe, not standard equipment.

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DC fuel pump is an option, I believe, not standard equipment.

Correct

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Well that at least explains why I was pretty sure it was there but on the other hand had a nagging feeling that might not be (always) the case  :smile:

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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

It could happen to any aircraft, gravity feed or not, if ambient pressure is lower than the vapour pressure of fuel which it probably would be at that sort of altitude.

 

And it would be a big amber master caution light for the fuel pumps of course.

Obviously because the APU is well above the fuel tank and AFAIK has no suction ability. Does the same happen on the 737?

 

The APU is able to suck fuel from the tank so presumably it has a mechanical fuel pump, like the engine fuel system. However, as with the engines, a positive boost pressure is always desirable.

 

FCOM Vol 2, 7.30.2 says this:

 

Fuel to start and operate the APU comes from the left side of the fuel manifold when the AC fuel pumps are operating. A DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. During APU start and operation, the pump operates automatically when the APU fuel control unit senses low fuel pressure. The pump shuts off automatically when an AC fuel pump pressurizes the fuel manifold. If the AC and DC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. During APU operation, fuel is automatically heated to prevent icing.

 

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Also keep in mind that jet engines need positive fuel pressure/supply to operate effectively. Without a surplus of fuel, at high demands, you can have sluggish operation and line cavitation. The other main reason you need the boost pumps is for the fuel ejectors. In many aircraft,these ejectors use boost pump pressure/flow to collect fuel from outboard sections/low points and deposit it into the surge/main fuel box through venturi effect. This is also where the main boost pumps and supply reside. This process keeps the fuel box full through out the fuel level range. This also mixes cold soaked fuel with warmer fuel to aid in temperature control. With out the boost pumps, its possible to deplete the surge box at low fuel states and starve the engines. Here's an example.

 

At the end of a flight in a DC-10, we decided to shoot some approaches and Go arounds in order to knock out some required events. After a few touch and goes in the VFR pattern, we went back to RADAR to get the other guy an auto-G/A. During the go-around, as the pitch hit about 22 degrees, the number 3 engine rolled back along with a  fuel que master caution. I tapped his throttle hand as i told him to lower the nose. At 15 degrees, the engine spooled back to GA power. I took the aircraft and the FE told me we had 20,000 pounds of fuel and things appeared normal. I brought her back around for a full stop and gave her to the mechanics. Later that night the guys told me they found a large crack in the #3 main tank's surge box. During the G/A, fuel leaked from the surge box faster than the ejectors could fill it. This fuel starved the engines until the ejectors could over come the leak. I know this post is long, but i like to give alot of info. 

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I know this post is long, but i like to give alot of info.

 

Definitely a good story illustrating the point.  Thanks for sharing that!

Plus, I'm not one who can judge for long posts.

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