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3 Answers

Auxiliary fuel pump

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Aircraft Systems

Guys, I have a doubt about the fuel system. One of my doubts already was answered at this post asked by Drew at the link http://www.askacfi.com/27340/gravity-fed-fuel-system.htm but, follow the next one:   As we know, generally we have two schematic possibilities in a fuel system such as by gravity either by fuel pump system. However, if the auxiliary fuel pump has the mainly objective to provide added reliability to the fuel system, why we just use this boost pump only at landings and takeoffs as also crossfeed? Why we cannot already let this auxiliary fuel pump “always” on to avoid any problem during flight?   And is it normal when activate the auxiliary fuel pump, my fuel flow gauge increase?   Thanks in advance.

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3 Answers



  1. jeff on Apr 09, 2015

    Cadu,

    There are too many variations of fuel systems to give a one size fits all answer, however, many systems work as follows: fuel in the wing tanks are pumped through the system by a system of scavenger pumps. Often these work by a venturi effect and become self sustaining once fuel is introduced through them. These however, are a low pressure pump system. Depending on the engine, fuel needs to be delivered at a much higher pressure and the main boost pump delivers that high pressure fuel to the engine (s). Often, these pumps turn on automatically when fuel pressure drops below a certain pressure (even in the off position). Turning the boost pumps on during times of critical operation such as take off or landing makes the pumps run continously and ensures that the engine is recieving the higher pressure fuel it needs as that would be a critical time for the engine to be starved pf fuel, even transiently and shut down. The big “got ya” And the reason not all aircrafts checklists require the boost pumps at the same time, is that there are many engines and situations where you can shut the engine down by providing too much fuel/too rich of a mixture. Even certain model aircraft may hAve different cheklists with regard to when to turn the boost pumps on depending in the model year. For example, in the C421, there was a fuel pump SB which if the aircraft had it incorporated required the pumps off for take off and landing where as the non modified serial numbers required the pumps on. There have been several NTSB reports of pilots loosing an engine because of the wrong boost pump operation!! Moral of the story is to always follow the manufacturers checklist with regard to when to turn the pumps on and keep in mind when transitioning to another aircraft, even the same model with a different model year, to read the checklist for normal procedures and understand the system in the aircraft you are flying. Hope that helps. I am not an engineer nor an A&P. but when i do type rating instruction in the Citation or Citationjet, we spend a good portion of the ground school going over systems and the students are required to have a working knowledge of all the aircrafts systems.

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  2. Mark Kolber on Apr 09, 2015

    As jeff said, it depends on the aircraft. Keep in mind that where there is an electric “boost pump” there is also an engine-drive fuel pump. So, it’s supplemental and, in come cases, using it will cause too much fuel to enter the cylinders. In some of the Pipers, So there are big variations in its use. Some examples of the variation:

    The Warrior manual tell us to only use the pump for takeoff and landing and switching tanks – events they apparently feel require the addition of a boost to the engine-driven fuel pump. As to why not all the time, Piper tells us, “The electric fuel pump should be normally OFF so that any malfunction of the engine driven fuel pump is immediately apparent.” So it’s a safety feature to confirm the engine-driven pump is operating properly.

    One of the common (owner, not POH) recommendations for Cirrus aircraft is to keep the electric fuel pump on for all ops. Apparently, it’s not strong enough to cause any problems and, as I recall, it helps in the case of fuel imbalances to endure continuous flow.

    In the Bonanza, the fuel pump is for priming start and emergency use only – not even for takeoff and landing.

    In the old Comanches, leaving the fuel pump on too long can lead to vapor lock, so continued operation with it one would be a very bad thing.

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  3. Cadu on Apr 09, 2015

    Thanks a lot.You clarified a lot my thoughts about it. The best way is understand your aircraft system and follow manufactures’ procedures.

    Best Regards,

    Cadu

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