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Engine and aux fuel pump in series


Rich

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Good afternoon,

 

My aircraft maintenance manual shows the main and aux. fuel pumps hooked up in parallel. I actuality, threy're in series.

 

I don't have a problem with this, as long as a failed main pump cannot render the aux. fuel pump inop due to broken parts restricting the fuel flow when the aux. pump is turned on.

 

Comments appreciated,

Thanks,

 

Rich

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Hi Rich,

 

When the mechanical pumps fails it fails open so even gravity feed from a high wing will bypass it, but you wouldn't get full throttle. You should be fine. If it was in parallel you would need a check valve to prevent back flow, but series won't be an issue. What typically starts to fail in the mechanical pump is the diaphragm, but the internal check valve(s) stay open. Out of curiosity do you know what your fuel pressure is with both pumps on?

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Thanks Roger,

 

No, I've never run both together in flight.

 

Rich, I think that you must check this, the engine must working properly with at least one pump, but also with both - is not desirable to use it in flight (electric one) and don't know what is happened and what parameters to expect.

 

Ref. serial vs parallel: I have a friend with a non-CT low wing airplane but Rotax, that have had some problems with serial configuration because of...electrical pump failure (in off-line status, the cvasi-broken electrical pump in serial with mechanical one had restricted the fuel flow for full throttle). I don't know your exact configuration for engine+pumps and if could be applicable a similar above situation...

 

Regards,

Alec

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My set-up is in a low wing, Sting, S4. From the main tank>electric Facet pump>main engine fuel pump>split to both carbs.

 

I use the aux. Facet pump to fill the carb bowls until I see a pressure read-out on my EFIS. At that time the aux. is shut off and the engine is started using only the main engine fuel pump.

 

My POH is very vague on the use of both pumps for take off. It says "aux pump as desired." Now, what does that mean?

 

I have not used both together as it was mentioned that both together may over power the floats and flood the engine, possibly causing a stoppage.

 

I may try both together and see what the pressure reading is when flying over the airport.

 

Rich

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Also an inline Facet, and I start in the same manner you do.

 

My experience is that there's not a huge difference in pressure with the boost pump on once the engine is running.

 

Certainly not enough in my installation to "overpower the floats".

 

 

Best viewed in HD and full screen.

 

This video should start at an approach to a landing. The fuel pressure gauge is right above my left knee and the needle is firmly in the green with the boost pump on the whole time - about where it is with the boost pump off.

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Rotax says .5 is normal and doesn't want more than a 1 psi increase. The old mechanical pumps had problems with this, but the newer AC pumps and the brand new pumps don't seem to have an issue. Personally I see no reason to run both pumps all the time. The mechanical pump should have more than enough draft even for a low.

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