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Diamond 42 AFM says:

The fuel crossfeed function can be tested simultaneously with both engines. Proper function can be tested by running the engines for approx. 30 seconds with CROSSFEED selected. The operation of both engines with both FUEL SELECTORS in CROSSFEED position, other than for this test, is prohibited.

I looked at the schematics and both fuels crossfeed pipes are completely independent and separate, including the fuel return lines and heat exchangers. Fuel flow looks almost identical (other than longer pipes) when both selectors are in CROSSFEED. So why is the manual prohibiting dual CROSSFEED ?

DA 42 cross feed schematics

Maybe it's not clear from the resolution, but in the original you can see on the left image, that the cross fuel flow passes through all the filters, while the cross return line passes through the heat exchanger (the blue box on the lower right side)

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    $\begingroup$ Going to do some more research, but my first read through would indicate that the method of fueling the engines uses 1/3 of the fuel delivered. The other 2/3's gets sent back to the tank and is heated considerably. The crossfeed system is supposed to be used to balance the right and left main tanks only, and the cooler system is not designed to be used in a cross-feed situation for more than absolutely necessary. This is only supposition at this point, and I'm attempting to find more documentation to back it up. $\endgroup$ – CGCampbell Jun 27 '14 at 0:02
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As I also could find no satisfactory explanation within the diagrams, AFM, POH or Maintenance Manual, I sent an email to the technicians at Diamond Aircraft.

This was their response, so far:

Hi Chris, I can only assume that if this procedure became too routine, eventually a pilot would select the second electric boost pump on with both systems in cross feed, in flight. The high pressure created would damage fuel lines and possibly damage the HPPs for both engines. That would not be a good day. You can see the step prior to going to cross feed, Fuel Pumps check Off. I have asked Austria for an explanation but I think I am close to the mark.

Customer Support,
Technical Services Specialist

Please note, that the customer service technician actually did include his contact information, but that I have removed it for privacy.

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    $\begingroup$ Hmm... I think he might be getting somewhere with the pressure and pumps limitation, but this procedure is also disallowed on DA42's without boost pumps, so there needs to be something more $\endgroup$ – Radu094 Jun 27 '14 at 20:25
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    $\begingroup$ Still no further response from Diamond at this time. $\endgroup$ – CGCampbell Jun 30 '14 at 13:58
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    $\begingroup$ Well, thank you for the bounty. I'm not sure I agree with it, at least not yet. I was really hoping for an answer from Diamond Austria, but nothing yet. $\endgroup$ – CGCampbell Jul 3 '14 at 20:27
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I checked both POH and Maintenance manual: neither explain why.

POH:

CAUTION Do not operate with both fuel selector valves in CROSSFEED position. Do not take-off with a fuel selector valve in CROSSFEED position.

Maintenance Manual:

B. Fuel Transfer

With an engine running and the fuel selector/shut-off valve set to CROSSFEED the fuel supply and return lines to the related fuel tank are shut-off. The engine fuel pump takes the fuel from the opposite engine fuel tank through the related fuel cross feed line. The fuel returned from the engine is returned to the fuel tank which supplied the fuel.

The unwanted fuel flows back from the engine fuel system through a check valve and the fuel selector/shut-off valve. From the fuel selector/shut-off valve the fuel flows through the related fuel return line and a flexible hose to the fuel cooler. Fuel from the fuel cooler the fuel flows through a flexible hose back into the fuel tank that supplied the fuel.

Interestingly, though (italics are mine):

C. Emergency Operation

In an emergency (for example, an engine failure) you can set the related engine fuel selector/shutoff valve to SHUT-OFF. The fuel selector/shut-off valve has a safety gate to prevent accidental selection of the SHUT-OFF position. You must turn and hold the gate in the open position to set a fuel selector/shut-off to SHUT-OFF. When the engine fuel selector/shut-off valve is set to SHUTOFF no fuel can flow to the related engine. The engine can not run.

The pilot can set the other engine fuel selector/shut-off valve to CROSSFEED and use the fuel from both main fuel tanks to supply the engine which is operating. This maintain a similar fuel level in each main fuel tank.

This last paragraph, and the sentence I put in italics in particular, could indicate that if both engines are set to crossfeed they feed on both taks simultaneously, possibly (and this is speculation on my side) leading to a conflict between the two engines over the fuel and/or possible over/misuse of the feed pumps.

I would like to highlight, though, that this sentence does not seem to correspond to the schematics reported (or shown in the manuals).

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    $\begingroup$ Yeah, I think on that last quote they meant "alternate the crossfeed valve and use the fuel from both tanks". The schematics are pretty clear that the crossfeed setting will isolate the other tank and the engine will not be supplied from both. $\endgroup$ – Radu094 Jun 27 '14 at 20:27

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