Aux fuel pumps

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Boatguy
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Aux fuel pumps

Post by Boatguy »

Normally when I switch on the aux fuel pumps for TO/LDG I hear the pump whine. Yesterday I made three flights and switching on the pump I heard nothing. Since I took multiple flights, the "aux" pump should have flipped between the ECU A and ECU B pump. But I heard nothing. I also noted no increase in amperage when turning on the pump. All three flights (5.2hrs) were entirely normal.

There were no ECU failures or other messages in the CAS, but I also don't see anything in the documentation to suggest there would be.

Losing my hearing? Pumps got quieter? ANR is working better? General engine paranoia?

My next step is going to be pull the breaker and seeing if I get a CAS. Suggestions?
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Re: Aux fuel pumps

Post by VickersPilot »

Perhaps download the G1000 logs from the SD card and check them for a amp change at the time the pumps are turned on. The logs are likely more granular than the gauge.

You can upload the logs to Flight Data to analyse more easily:
https://www.flightdata.com/
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Re: Aux fuel pumps

Post by ememic99 »

Boatguy wrote:
Thu Nov 25, 2021 5:59 pm
My next step is going to be pull the breaker and seeing if I get a CAS.
I would do this as a first step in diagnostics - in other words "Can I expect any feedback if the pump doesn't work at all?"

If there is some feedback than I guess it's based on load which can be confirmed at ammeter. If there's no feedback on pulled breaker then I would assume pump dead.
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Re: Aux fuel pumps

Post by CFIDave »

Switching the backup fuel pumps switch turns on *both* pumps (in parallel, in case one were to fail during takeoff or landing).

A single pump is running whenever the Engine Master is ON: Pump A if ECU A is active; Pump B if ECU B is active. If you haven't started the engine yet, one pump running (after turning on Engine Master, but not the backup pump fuel switch) is quite loud and always audible on all the DA40NGs I've flown. This should be the case regardless of whether the plane picks ECU A or B.

I'm not sure if you can hear both pumps running more loudly than one (with or without the engine running) after turning the backup fuel pumps switch ON. But you might try this with the Engine Master ON, but the engine not yet started.
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Re: Aux fuel pumps

Post by Boatguy »

Dave: There was always an audible whine when the second pump came on. Odd because I never hear the primary pump running, but turning on the second pump always produced an audible whine which my wife (who's hearing is better than mine) confirmed was missing on Wednesday's flights.

Today I did some testing and I'm pretty sure it's a bad switch. All of this was on the ground, in the hangar, using a GPU.

With ECU A selected, engine master ON produced the expected pump noise. Voltage also dropped from 28.7 to about 24.4, but then eventually recovered to 28.4. I think the big voltage drop was from the glow plugs (enunciated in the CAS). Pulling the A Pump circuit breaker stopped the pump noise and there was a small voltage increase. There was no CAS warning. Resetting the circuit breaker restarted the pump noise. Switching the Fuel Pumps ON produced no change in noise or voltage.

I repeated this with ECU B selected and got the same results.

That confirms (as did the previous flights) that the pumps are both working fine and suggests the switch has failed.

It also unfortunately confirms that the CAS will not inform us when the aux pump is not running.
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Re: Aux fuel pumps

Post by ememic99 »

Boatguy wrote:
Fri Nov 26, 2021 10:17 pm
It also unfortunately confirms that the CAS will not inform us when the aux pump is not running.
That’s not something I’d expect. Even old designs (in other aircraft types) have positive feedback on systems like this working e.g. simple bulb in circuit adjusted to appropriate load is sufficient.
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Re: Aux fuel pumps

Post by CFIDave »

FYI: Take a look at the DA40NG AMM from Diamond's website:
http://support.diamond-air.at/fileadmin ... mplete.pdf

The 2nd to last page of the manual shows the electrical schematic for the fuel pump system.

It seems to show that the ECUs can individually trigger the the A or B relays that each operate the A and B fuel pumps -- with power supplied by the ECU busses.

But it also shows the backup electric fuel pump switch (shown at the bottom of the schematic) will trigger both fuel pump relays in parallel. -- again with pump power supplied by the ECU busses.
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Re: Aux fuel pumps

Post by Boatguy »

CFIDave wrote:
Fri Nov 26, 2021 11:20 pm
FYI: Take a look at the DA40NG AMM from Diamond's website:
http://support.diamond-air.at/fileadmin ... mplete.pdf

The 2nd to last page of the manual shows the electrical schematic for the fuel pump system.

It seems to show that the ECUs can individually trigger the the A or B relays that each operate the A and B fuel pumps -- with power supplied by the ECU busses.

But it also shows the backup electric fuel pump switch (shown at the bottom of the schematic) will trigger both fuel pump relays in parallel. -- again with pump power supplied by the ECU busses.
I definitely see the backup electric fuel pump switch triggers both relays in parallel. I don't see the either ECU can trigger either pump in the drawing you reference, but I do infer it from the attached PDF, though I don't know where I got this drawing.

Are you, and the PDF, suggesting that if A pump failed the B pump could be automatically triggered by ECU A? If that was the case, why would we need a manual fuel pump switch?
Attachments
ECU block diagram.pdf
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Re: Aux fuel pumps

Post by Boatguy »

It turned out to be a bad switch. The only way to know that the switch is bad is if you don't hear the pump "whine" when you flip on the switch. If an actual pump was bad, I think you'd get an ECU fault on the bad pump and the engine would switch to the other ECU and it's associated pump.
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