PA34 Elec Fuel Pump

[clunk1001]

Hi,

Hopefully someone can shed some light on this relatively trivial query.

The PA34 Seneca II has electric fuel pumps (with HI and LO setting), the POH makes reference to the auxiliary pumps as a backup to the engine driven pumps, also in relation to vapour suppression and as part of the cold weather start procedure.

Other Piper aircraft POHs (at least the ones I've flown e.g. PA28 Arrow II) are much more specific and state that the Auxiliary pump should be ON during takeoff and landing and changing tanks.

I've noticed that some schools teaching IR/MEP do not use Electric Pumps during takeoff, and some do.

I cannot understand why you wouldn't have the Pumps on LO for takeoff. A failure of an engine driven pump right after rotation on a short strip (below vyse, no more runway) is going to be very messy. And I can't understand why the POH doesn't include any mention to this practice specifically.

Can anyone shed any light on :

(a) why you would not have the electric pumps on LO during take-off,

(b) why the PA34 POH is far less specific about the operation of the Electric Pumps than other Piper aircraft POHs?


Thanks

Clunk

[A and C]

It depends on the engine fitted, if the PA28 is NOT a turbo it will be fitted with a Lycoming, this fuel injection system is a derivative of the pressure carburetor and requires the electric pump to be on during critical phases of flight as fuel metering is unaffected by extra pressure delivered by the electric fuel pump.

Aircraft fitted with the Continental engine have an engine driven pump that provides a basic metered flow ( trimmed by other means) if you use the electric pump in flight under normal operation it will result in the delivery of too much fuel and the engine is likely to stop.

On these engines the electric pump is used only for priming or if the engine driven pump fails, even under under an EDP failure situation the pump provides a very rough and ready fuel flow, the engine runs but not very well.

[clunk1001]

Thanks - that explains it!