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Flickering electric fuel pump light

This light has always flickered. It is implemented as a pressure switch, downstream of the electric (backup) pump.

Lately, it has been worse, being almost off if the pump is turned on during the flight, but it comes on after a bit.

On the ground, it sometimes goes off for a second or so, but listening to the electric pump shows no irregularity.

Has anyone come across this? My plan is to change the pressure switch.

Administrator
Shoreham EGKA, United Kingdom

Quite a common problem, the pressure switch is usually the problem.

What does the light signals ? That the pump really works ? Or just that fuel pressure is above a certain threshold ?

LFOU, France

The light comes off a simple pressure switch which AFAIK switches at about 4psi.

Why the light flickers (it is normal) I don’t know, given that the electric pump is a gear pump which should be delivering a constant volume. It pumps against the RSA fuel servo flow controller which doesn’t need that much volume so most of the time the pump’s output flows back around it through a string loaded bypass.

Some engine installations, notably I think Conti, take the bypass back to the fuel tank, and I have no idea how one does a fuel totaliser installation in that case…

Administrator
Shoreham EGKA, United Kingdom

Fuel flow systems that are in a system that returns fuel to the tank have a flow transducer in the return line and the system does the sums.

As to the flickering of the light I can’t give a definitive answer but changing the pressure switch is almost always the answer to this problem.

I am ordering a replacement switch, and I will order an overhauled pump to keep on the shelf ($1k).

Re the 2nd flow transducer mentioned, I am surprised this produces an accurate result, because the return flow is going to have an intermittent flow rate and thus inherently difficult to measure with a turbine transducer. But maybe it works ok.

Administrator
Shoreham EGKA, United Kingdom

Generally the return flow on these systems is quite high so the Fuel flow measurement system is quite accurate.

As an aside some turbines have so much unmetered fuel flow they use it as muscle power for some FADEC controlled valves .

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