It is pretty simple to do this if you use a fuel pump and relay coil
that require the same voltage. Use a pressure sensor to control the
relay. Connect the relay output to the fuel pump AND to the relay
coil (input). That way when the fuel pump comes on (due to a low
pressure signal), the relay locks itself on, regardless of the state
of the pressure sensor. To turn the pump off, you would have a
momentary switch in the circuit that would break the connection to the
relay coil. This used to be called a "seal-in" circuit for industrial
controls.
Steve Ruse
> Jeff Page wrote:
>
>
> I just came across a notation I made quite a while ago, after
> noticing a neat idea, probably on this list.
>
> It utilizes a low pressure switch to provide an "automatic" mode to
> the electric fuel boost pump, so that if the engine driven pump
> failed, the electric one would be powered up.
>
> This seemed like a great idea at the time. It would likely mean an
> engine hiccup, followed by the fuel pump on LED illuminating - much
> better than the pilot conjecturing the engine failure is due to fuel
> starvation and manually turning on the pump (would be a checklist
> item).
>
> However, looking at the schematic as I drew it, as soon as the
> electric pump provided sufficient pressure, the low pressure switch
> would open and the pump would shut off, and then back on, and then
> off. Ooops :-(
>
> Is it worth fabricating a little latching circuit to provide this
> automatic operation, or better to keep things simple ?
>
> I don't do hard IFR, and manually turning on the fuel pump switch in
> most circumstances would be sufficient to avoid an unpleasant
> landing.
>
> Thoughts ?
>
> Jeff Page
> Dream Aircraft Tundra #10
>
>
