I was curious if the ECU’s could distinguish between a failed fuel pressure sensor and failed fuel pump, so thanks for commenting on that too.

This clears it all up for me. Thanks again for sharing your knowledge.

[quote=thommy152 post_id=94147 time=1666833436 user_id=3859]
Thanks for the thoughtful response, Dave. Your comment has spurred me on to read more and this is what I found. If you don’t mind, I’d like your input.[/quote]
The sentence you highlighted seems to imply that the ECUs will switch from a failed pump to a good (inactive) pump automatically, but doesn't say anything about ECU switchover. I'm guessing that the only way the pumps can be swapped is to also swap which ECU is active.

Loss of fuel pressure due to electric fuel pump failure vs. a bad fuel pressure sensor (erroneously indicating low fuel pressure) is likely indistinguishable to the ECUs. As shown by my DA40NG in-flight experience above, this will result in dual "ECU Fail" annunciations. I find it surprising that the engine would automatically swap from one "failed ECU" to another "failed ECU," but maybe that's the way the ECUs are programmed.

Thanks for the thoughtful response, Dave. Your comment has spurred me on to read more and this is what I found. If you don’t mind, I’d like your input. Trying to get these details worked out in my mind…

I downloaded the Austro engine manual to read more. The image I’ve uploaded from the manual seems to say that although both ECU A and ECU B share a fuel pressure sensor, the logic is set up so that the inactive fuel pump becomes the active fuel pump in the event of a fuel pressure loss. I suppose this could be an actual pressure loss or a bad sensor indicating a pressure loss in error. But either way, do you understand this to mean that the active fuel pump will swap?

[attachment=0]D4623FCD-ED2A-4BC6-AEAE-179ECD37624A.jpeg[/attachment]

If ECU B failed (most likely due to one of its sensors producing a value that was out of range) and was in use, the engine would automatically switch to ECU A. This would also switch fuel pumps.

But if ECU B's associated fuel pump failed, I don't think there's any logic that would automatically switch to ECU A just to use ECU A's fuel pump. You might get a low FUEL PRESS annunciation, but there's only a single fuel pressure sensor used by both ECUs -- so that would likely generate dual "ECU A Fail" and "ECU B Fail" messages without a switchover.

[attachment=0]IMG_3074.jpg[/attachment]
(Note: this occurrence in flight was traced to a bad fuel pressure sensor -- fortunately the engine continued operating just fine.)

Bottom line: You really should turn on the fuel pump switch(es) for takeoffs and landings for parallel fuel pump operation.

[quote=CFIDave post_id=81974 time=1602124079 user_id=1092]
The DA40NG has a single mechanically-driven high-pressure fuel pump on its Austro engine -- used to pressurize the common rail that feeds fuel to the injectors. The pilot has no control of this pump, which operates whenever the engine is running.

It also has a PAIR of electrically-driven fuel pumps -- one associated with ECU A, and another associated with ECU B -- that are used to transfer fuel from the wing tanks to the engine. When ECU A is in control of the engine, its pump is ON (the other is OFF). Similarly when ECU B is in control of the engine its pump is ON (and the other is off). Since ECU A and B are in control on alternate flights, this equalizes wear across both electric fuel pumps.

During takeoff and landing, the pilot turns the panel-mounted fuel pump switch ON -- which causes BOTH the ECU A and ECU B-controlled pumps to turn on and operate simultaneously in parallel. That way if either pump fails, fuel delivery from the wing tanks to the engine will continue.

Diamond DA42/DA62 twins with Austro engines have 2X everything described above. (DA42 TDI and DA40D aircraft with Thielert/Continental engines lack panel-mounted fuel pump switches.)
[/quote]

@CFIDave

I’m trying to get a better understanding of the fuel system in preparation for my DA62 scheduled to be delivered March 2023. My question is: if the pilot failed to turn on the fuel pumps for takeoff, would the EECU automatically switch, for example, to ECU A if the fuel pump on ECU B failed? Is turning on the fuel pump switches just an extra layer of protection in the event the ECU switching does not do its job?

Thanks, now it makes sense!

In the DA40NG, when the fuel selector is in the EMERGENCY position, the fuel pumps draw from the right tank. The excess fuel from the mechanical high-pressure fuel pump is always returned to the left tank. So if you don't switch back to the NORMAL position (so the fuel again is drawn out of the left tank), you can empty the right tank and have an engine stoppage - even though you have fuel in the left tank.

Note that this is only necessary should the fuel transfer pump stop working. In normal operation, fuel is always drawn from the left tank and the transfer switch/pump is used by the pilot to move fuel from the right tank to the left tank. See page 7-28 in the DA40NG AFM.

Sorry if this is a stupid question, but if the pumps supply fuel to the engine from the left tank, and the emergency transfer can lead to all fuel to end up in the left tank as well, how is fuel starvation an issue?

With all due respect to Dave's notoriously complete mastery of Diamond's systems, I offer the following additions to Dave's description of specifically the DA40NG.

The two electrically driven pumps, associated as Dave writes with their respective EECU's, pump only from the Left tank of a DA40NG to the mechanically driven high pressure pump.

A fourth electrically driven pump, manually controlled by a panel mounted switch, transfers fuel from the Right tank to the Left tank at a rate of about 1gpm.

And finally, in the event of a failure of the transfer pump, a mechanical valve in the cockpit can be rotated to cause the electric fuel pumps associated with the EECUs to draw from the Right tank. Since about 85% of the fuel pumped is returned to the Left tank (a characteristic of modern diesel engines), the effect is to turn the engine into a fuel transfer pump. This valve must only be in the "Emergency" position for 10-15 minutes since it will, at a rate of roughly .85gpm, be transferring fuel from the Right tank to the Left tank. If left in the "Emergency" position it would lead to fuel starvation as all the fuel will end up in the Left tank. So once sufficient fuel has been transferred, the valve needs to be returned to the Normal position so that the engine can access the fuel in the Left tank.

The DA40NG has a single mechanically-driven high-pressure fuel pump on its Austro engine -- used to pressurize the common rail that feeds fuel to the injectors. The pilot has no control of this pump, which operates whenever the engine is running.

It also has a PAIR of electrically-driven fuel pumps -- one associated with ECU A, and another associated with ECU B -- that are used to transfer fuel from the wing tanks to the engine. When ECU A is in control of the engine, its pump is ON (the other is OFF). Similarly when ECU B is in control of the engine its pump is ON (and the other is off). Since ECU A and B are in control on alternate flights, this equalizes wear across both electric fuel pumps.

During takeoff and landing, the pilot turns the panel-mounted fuel pump switch ON -- which causes BOTH the ECU A and ECU B-controlled pumps to turn on and operate simultaneously in parallel. That way if either pump fails, fuel delivery from the wing tanks to the engine will continue.

Diamond DA42/DA62 twins with Austro engines have 2X everything described above. (DA42 TDI and DA40D aircraft with Thielert/Continental engines lack panel-mounted fuel pump switches.)