of course, as two of my colleagues just hat the same problem today. but they didn't take off, for the reasons given.
firstly, the POH clearly states that unter IFR operations TakeOff is prohibited if the battery is dead or empty and the A/C has been startet with external power. Under VFR you are allowed to do that, but I wouldn't.
The gear is hydraulically operated, but the pump is electric.
The engines MUST have electricity, as the ECU's have to run on Power, as well as the injectors, injection pumps, and actually the complete engine NEEDS electricity, just as it needs JET-A.
The "problem" with the DA42 is, it only hat ONE main battery, opposed to the TWO batteries in the DA40. This is due to the fact that it has two engines and two alternators.
Also die DA42 has one emergency battery which is there soley for the purpose of running the backup horizon and the glowstrip.
Besides that is has 2x2 excitation batteries for the alternators. (i never trust backup batteries that I can't read voltage form because I cant find the gauge or there isn't one.)
Ok so, here's the deal. They started the engine on EX-PO, taxied to the RWY and took off. as they retracted the gear, it seems like the hydr. pump took to much initial current or something, so the alternators quit (this is just speculation from my side) their job. They are both approved to 60 AMPS burst power, but apparently that wasn't enough. Running the Engines plus G100, charging the batteries, running all the lights AND retracting the gear. As the battery couldn't provide any "buffer" as capacitator does in High end car HiFi systems (just an analogy) the whole electrical system failed.
no electricity -> no engine -> no engine -> no thrust -> no thrust -> HTGH (hit the ground hard)
after takeoff you are normally somewhere around 78-82 knots (which is Vy, no Vx published). So imagine, you take off, retract the gear and both engines stop. all instruments dead in an instant. what do you do. right ... down. crap.
realisation usually takes around 1-3 seconds. (meaning that airspeed after that point will be somewhere around 72-70 kts) then there's the 1 second denial phase "naah, can't be" and then you start to take action (medical research they showed us at Airlineschool, differs from person to person, but thats the average)
so there you are around 65 knots. Vmca is 68. you go bye bye.
poor people on that plane.
this is my personal interpretation of the problem. please do not use this in any way.
-- future insane homebuilder --