soay, that's good to know... HOWEVER. I have here an article written for my clubs newsletter which deals with exactly the same problem. I just remembered reading about that, so I looked it up. It's a bit long (and in Dutch), so I'll try to summarize it here.
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If you switch off the Electric Master while the engine is running idle (on the ground) the engine will halt, despite the fact that it should continue running on the ECU B Backup Battery. The articles analysis is that the switch to ECU B does happen as designed, however, the Thielert is a high-compression engine and at IDLE power only runs at 890 RPM or so. With this rotation speed there is insufficient impulse-moment to maintain rotation while ECU B is taking over. Because of that, the engine halts. Apparently, the engine needs at least 1300 RPM to maintain its momentum throughtout the process of ECU B taking over.
If you switch on the Electric Master again while the engine is running idle, the reboot of ECU A plus its takeover from ECU B takes even more time. Again, experimenting showed that at least 1300 RPM is required for a smooth takeover.
Conclusions:
- In the air, switching off, or switching off/on of the Electric Master, the engine will continue running due to the windmilling effect, even at idle power.
- On the ground, at least 1300 RPM is required for the engine to keep on running when the Electric Master is switched off, or off/on.
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This analysis was published in July 2006, and although its conclusions may be right, with what I've read on this forum, the whole issue might not be sufficient impulse-moment, but ECUs that are too sensitive to power spikes.
How all this applies to the DA-42 that this thread is about, I don't know, since I assume that its engines were running at full speed, since they were just after take-off, raising the gear. But I do have that nasty feeling that there is a link somewhere.
Glad the DA-40 has "gear down and welded" though.