Message text written by "R.C.Harrison"
>I hear your concerns, Nigel, all very very real. I don't have the two pump
situation since my checklist for take off and landing includes switching on
the back up pump to the mechanical engine pump, anyway.<
What your checklist says reflects the design of the system in use. I
believe that your system is very similar in design to the existing 912/912S
system which is what I started from. By changing the system design my
checklists are changed accordingly. Should the main pump not cope with the
engine demands the backup cuts in automatically so there is no need to
switch it on and off for take-off and landing. The automatics get checked
anyway at engine start due to the lack of fuel pressure until the engine is
turning. The only action required (apart from turning on the fuel valves
before flight) is to reset back to the main pump after start. From then on
the system looks after itself until dry tanks both sides.
>However whilst your proposal is well founded I would query how you know
which side of the tank your system is pulling from? (Could it be on reserve
and you be unaware?)<
No. If the reserve side is brought into action, whether it is due to
malfunction of the main side or running out of fuel an amber 'Fuel Pump'
light is illuminated to show the backup pump is running. Also at any time
there is a lack of pressure from the main pump a red 'Fuel Pressure' light
is illuminated. No lights mean that the system is running from the main
side. All very clear and reduced risk of pilot mismanagement. With the
existing system some faults will not be solved by turning on the back up
pump when they would be with the parallel system. Whilst no system is
perfect I think that the parallel setup provides better redundancy and
gives a precise warning of low fuel contents without the risk of
unnecessary fuel starvation.
Nigel Charles
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