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Why switch on the electric fuel pump when changing tanks?

This pressurises the fuel lines after the pump, while creating a low pressure before the pump where the fuel selector is.

So you are not pressurising the fuel selector, which would seem to be the main point.

And, on Lycoming engine installations, there is normally no bypass back to the fuel tanks, so running the electric pump produces no extra fuel flow anywhere.

Administrator
Shoreham EGKA, United Kingdom

My bet on electric boost pump ON,
- If you switch fuel on empty tank while sucking some air you have it to quickly avoid pressure drop from compressed air bubbles in the fuel lines as you do use it on start-up or takeoff
- The functioning of engine mechanical fuel pump is sensitive to fuel pressure, any sudden drop in fuel pressure during fuel tank switch may mean sudden drop in rpm which could go as another sudden drop in pressure, if you are unlucky engine will stop

If you can start-up and taxi at low rpm with boost pump OFF while fiddling with fuel tank switches then your mechanical pump is working fine, if not then you are better off flying high wings

Paris/Essex, France/UK, United Kingdom

My high wing C177 has a fuel pump also.

Egnm, United Kingdom

Good to know that the Cardinal has a fuel pump, very useful if you fly it inverted !
(I am getting to fly one soon, so useful to learn )

Paris/Essex, France/UK, United Kingdom

I can only assume the reason to turn on the electric pump on (for example) a carburated Lycoming is to refill the float bowl with certainty after moving the tank selector through the off position has taken longer than planned. However the mechanical pump would seem to do it just as well.

Interesting that this has come up, I was thinking about this question only yesterday.

I think that the fuel supply design in the Warrior is marginal, such that you can have unexplained fuel pressure deviations. This happened to me crossing the Skagerak (Brrr!) and was only resolved by changing both Electric and Engine pumps, with no fault found in either. Further evidence is the low pressure permitted (0.5 psi) for what reason? Added to which the fuel changeover valves are notoriously leaky, leading to a smell of fuel when changing tanks. Btw, my variations never went below 2.5 psi, but why any drop at all?

I suspect the ‘pump on’ procedure is to plaster over known design defects when using the engine pump only.

EGBW / KPRC, United Kingdom

On an carbureted engine, very low pressure will deliver enough fuel. Gravity flow is enough when the tanks are above the engine.

Last Edited by Silvaire at 04 Dec 15:16

Interestingly POH checklist for my Grumman tiger (lycoming 0-360)makes no mention of electric pump use for tank change, so I just monitor fuel pressure during change, never seen a fluctuation.

Last Edited by PeteD at 05 Dec 23:19
EGNS, Other

As a supplementary question I wonder how many people change tanks after a specific time interval or for some other predetermined reason regardless of whether it is otherwise a good time to do so?

The chances are the engine will not quit straight away but with a minute or two if it is going to. Waiting to change tanks until such time as you arent over “hostile” terrain or water always seems to me to make sense. I wouldnt change tanks over the middle of the north sea for example if I could help it.

What about changing to the fuller tank before landing?

We have done this before (in fuel management) but I change tanks in the TB10 quite often to keep the aircraft balanced – there is no aileron trim and if you let any significant imbalance develop then it will pull noticeably to one side.

Also because the gauges are next to useless, and hence the above forms part of my fuel management strategy for any non-trivial A to B flight. The notes on my knee board contain ‘paper gauges’ where I draw and cross out blocks of available fuel at timed intervals, usually 15 mins which is approx. 10 litres.

I have never noticed any sort of effect on the engine or any fluctuation of the fuel pressure gauge when changing tank, whether with the pump on or off. Of course the pressure goes up a bit with the pump on, but the change of tank doesn’t seem to do anything. Hence I change per my fuel management schedule without regard for what’s below, but then I have not made a longer water crossing than Weymouth to Guernsey.

I don’t change to the fuller tank in my pre-landing checks, not unless a change is due per the schedule.

Last Edited by Graham at 06 Dec 17:01
EGLM & EGTN
14 Posts
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