View Full Version : Fuel Suction Feed question???
AeroTech
8th Feb 2006, 19:32
Hi,
-In the event of all AC power loss, it is possible to restart engines after flame-out. What is the maximum suction feed ceiling (mostly for airliners)?
-Can airplane flying in adverse conditions be affected by engine(s) operating for an extended period of time on suction feed?
-I found this statement: apparently "new" fuel contains a good deal of entrained air that makes suction feed less reliable (this is called "fuel weathering"). Is this true?
Feedback appreciated. Thank you.
Best regards
Swedish Steve
9th Feb 2006, 08:22
When I used to do installation ground runs on the Tristar, we used to run the engine to take off power, and then turn off all the wing tank fuel pumps. Nothing should happen. Fuel tanks are pressurised slightly in flight, so should work better when flying.
Don't know about 3), but for the 737 Boeing state that gravity feed should suffice up to 30,000ft.
Clarence Oveur
9th Feb 2006, 08:44
You are probably looking at something like FL200-250 as maximum for gravity feed. Of course it depends on type, but also on which tanks you are feeding from. Outboard tanks will generally have a higher max level for gravity feed than inboard and center tanks.
CaptainSandL
9th Feb 2006, 14:43
Interesting, if not highly unlikely, question. You are talking about failure of all generators and all engines.
On the 737 each engine has an engine driven fuel pump, but if the engine has failed it may not be turning enough to deliver fuel for restart. If your windmill relight envelope started at say FL240 (depends upon type) then you would certainly have enough altitude from cruise levels to dive and reach Vno by this level. This may be enough to wind-up the N2 sufficiently to operate the EDP to give you fuel for a relight.
The wing tanks have a small positive head of pressure from the surge tank vents but I doubt it is enough to feed a stopped engine.
A good one for the end of a sim trip.
The main purpose of fuel booster pumps is to prevent cavitation at the high pressure pumps feeding the fuel injectors.
Cavitation soon turns a high pressure pump into a piece of junk so avoid suction feed like the plague.
AT,
I believe you are correct in that 'new' fuel does indeed contain a large amout of entrained gas that 'boils' off as outside air pressure reduces during climb. If you loose your booster pumps (due to electrics loss for instance) then the risk of cavitation is high for the first 30 minutes or so when the a/c is above FL170. I think this is the height above which the phenomenon starts to occur.
It is my understanding that once the gas has boiled off then this chance of cavitation ceases to be a problem. In this instance if you have tankered fuel into a port and not refuelled (as currently is the case I understand for some carriers at LHR) then you don't have the gas in the fuel to start with.
Gravity feeding (in the Airbus) for a relight or continued engine operation, would only be assured once the fuel has been 'degassed' for 30 minutes. I would assume this is the same for all types.
Swedish Steve, I have not heard before that the tanks are slightly pressurised in flight. would the pressure be coming from the vent tank's NACA duct ? Interesting.
CaptainSandL
10th Feb 2006, 10:23
IDG,
From Boeing Airliner Sep-Oct 1966 page 12, in an article on the 737 fuel system:
“A small positive pressure is maintained in the tank during flight by ram air entering the wing tip vent scoop. This positive pressure serves to reduce fuel evaporation at high altitude and to maintain pressure at the boost pump and by-pass inlets.”
S&L