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.

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.

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

Correct

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:

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.

 

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. 

 

 

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.

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

 

Always nice to hear them, keep em coming.