DA62 road landing in Dallas, Texas

[Chris]

I agree and am really curious about what the rest of the story is. Were both battery packs wired up incorrectly? What other things, if any, lined up to result in complete electrical system failure? If the mis-wired batteries were indeed the sole cause, then it quite ironic given that they exist solely as an emergency backup power source.

[ememic99]

P.S. the 42NG and the 42-TDI batteries are identical to the 62.

Why didn't DAI issue same SIL for DA42NG and DA42TDI?

[tjmoody]

The NSTB final just got published. Thanks to my friend Ian for pointing this out to me.

Unfortunately its still a head scratcher. There are two issues here:
1. Something happened with the electrical system to require the ECU backup batteries to support the ECUs. While there is some speculation in the report, they weren't able to get to a root cause
2. The ECU backup batteries (which would normally provide 30 min when all else fails) were installed incorrectly. Apparently in parallel instead of in series so only 12v was available instead of 24v .. so no backup

If the backup batteries had been correctly installed, this should have been a non-event and the pilot could have easily gotten to the airport. After we learned about this, I've asked my shop to take a photo of the battery wiring after installation to double check all is correct. Apparently there are tests in the TDI manual to confirm correct installation but not in the NG manual.

As far as the primary issue that lead to this, its frustrating that there's no answer. They reference that the battery had slightly low "capacitance" but the alternators should have been providing power as well. My understanding was that both the alternators and the main battery need to die (incredibly unlikely) before we need the backup batteries to come into play? There is reference to "Additionally, the alternator relays were wired such that the alternator would not disconnect the alternator power from the main electrical system." I'm not sure if this means there was a problem in the wiring for this particular plane or a design issue?

[rdrobson]

My understanding was that both the alternators and the main battery need to die (incredibly unlikely) before we need the backup batteries to come into play? There is reference to "Additionally, the alternator relays were wired such that the alternator would not disconnect the alternator power from the main electrical system." I'm not sure if this means there was a problem in the wiring for this particular plane or a design issue?

From my reading of the report, it seems that the alternator relays were wired incorrectly at some point. Either during build or some later event. I think if the design were that way, they would say something different on page 2 instead of 'wired incorrectly'.

This is a bit scary as this used to be my airplane. Glad it never dropped me into Lake Michigan as I crossed that body of water a number of times during my ownership.

[jast]

If I remember correctly there is a diode from the main 24V system to the ECU bus which is supposed to be 24V as well and keep the ECU batteries charged. If it’s 12V it overcharges the batteries which draws too much power, blows fuses and causes other issues. Every signal line between the now 12V system and than 24V system creates leakage currents because it was not designed to be 12V. It’s likely a real bad mistake causing an avalanche of issues.

However I’m wondering if there is a possibility to check the ECU batteries and their voltage in a pre flight check. Especially after a ship visit and before over water or over high terrain flights you want to be sure…

[tjmoody]

If I remember correctly there is a diode from the main 24V system to the ECU bus which is supposed to be 24V as well and keep the ECU batteries charged. If it’s 12V it overcharges the batteries which draws too much power, blows fuses and causes other issues. Every signal line between the now 12V system and than 24V system creates leakage currents because it was not designed to be 12V. It’s likely a real bad mistake causing an avalanche of issues.

However I’m wondering if there is a possibility to check the ECU batteries and their voltage in a pre flight check. Especially after a ship visit and before over water or over high terrain flights you want to be sure…

Yes that's correct but there is also a 32A fuse which I assume blew and could have been the "pop" the pilot heard when the 24v system was connected to the 12v ecu batteries. The engines went quiet at this point but at least the fuse protected against an electrical fire etc.

For those with an electrical interest, its worth reading the attached Systems Report which was in the NTSB docket. It gives more detail on the mis-wiring of the alternator relays. Basically they were bypassed. I may be missing something but I don't see how this would have contributed to the engine failures. By accidentally bypassing the alternator relays, I think this would have just prevented the alternators from being disconnected from the LH/RH Main Buses. There is no mention of the alternator switches being moved. So if I am interpreting this correctly, the mis-wiring of the alternator relays (while clearly a bad thing) didn't contribute to the engine failures.

Regarding a preflight check of the ECU backup batteries, that would be great but there doesn't appear to be anyway to do this.

[jast]

Thank you! That's very interesting!

What bugs me is that there is no check as pilot of the ECU batteries are working or not according to a checklist. Based on the electrical wiring diagram I thought of a simple idea how to proof that the ECU batteries are working without pulling fuses or just switching off Master in ground. If you get into the aircraft and activate the either engine switch and just listen for the fuel pump sound (which is clearly audible), I think you can be sure that this engine would run on the ECU battery if the main battery would fail. Obviously you would have to do it one engine after another and every time you do this you would create an additional pre-heating cycle (which likely is good for the engine anyway if it's cold). I asked this question also to our Diamond support if there are any negative side effects if you do this tests e.g. before a flight over water or at night. Answer is pending, but I'm curious to hear thoughts from experts here?!

[tjmoody]

That's interesting and simple. I'd be interested to hear what response you you get. One question I would have is would you still hear the pumps activate if there was only 12v available ... as in this scenario with the DA62 in Dallas? I don't know the answer. If the pumps require the ECU to be running then it might be a valid test.

[mfdutra]

What bugs me is that there is no check as pilot of the ECU batteries are working or not according to a checklist.

The NTSB doc above explains perfectly how they tested it. Pages 22 and 23.

It's quite simple actually.

[jast]

Yeah, but they do it with the engine running and this could a) potentially create hiccups/ecu faults and b) needs more time after a startup clearance because of the master SW off/on and c) doesn’t bring you to a clean state of a checklist start so it’s more a modification rather than a pre-check. But maybe it is the right way…