An article earlier this year in Sport Aviation shows a diagram with the electric
and mechanical fuel pumps in parallel, with the return line T'ing off the off
the line just before the carb. They mentioned that it was a better way, but
that the industry had gravitated to the pumps 'in-series' installation.
Cleve Lee
A198 Mono XS Jabiru 3300
>
> With reference to all this interesting discussion on fuel systems, someone
> told
> me just recently that if the mechanical fuel pump on the 912 fails it will not
> allow fuel to flow past/through it. Can this be correct? If it is the case
> that
> a broken mech pump blocks the fuel flow, then whatever configuration of
electric
> fuel pumps you may have wouldn't appear to make any difference in this
'disaster
> scenario'?!
>
> I hope this is not the case, but does anyone know?
>
> David
>
> -----Original Message-----
> Subject: Re :- Fuel Levels was RE:- Gates fuel lines
>
>
> 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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