Despite all the previous posts about following the POH and SOP (which are technically correct, mind you), I still tend to lean towards blaming the aircraft, not the crew.
Sure, if you have flown big iron for all of your life, you know that there's a checklist or SOP for every possible contingency. And since most of these aircraft are multi-crew anyway, one pilot can fly while the other thumbs through the POH to see if a certain checklist matches the abnormal situation that has arisen.
But if you come from a smaill airplane background, single pilot, the situation is a bit different. Sure, there might be a checklist in the POH, but we all know that nobody uses the checklist from the actual POH itself. Instead, you use an abbreviated checklist because the actual POH is too big to have on your lap at all times. First question: did this abbreviated checklist contain a separate item "starting with ground power" or at least a warning that the "starting with ground power" procedure is significantly different from "starting with battery power", and that reference needed to be made to the actual POH? I just checked the three POHs I have here, and the corresponding (owner/operator-issued) abbreviated checklists. I found that both the DA-40 (!!!) and the Robin do not have "GPU-assisted start" checklists in the actual POH (let alone in the abbreviated checklists I use), and while the Piper Warrior has such a procedure in the POH, that procedure gets no mention into the two abbreviated checklists I have here (from two different operators).
The second factor is that if the crew (or single pilot, let's not rule that out) even knew that such a procedure existed in the POH, did he remember at the correct time? We don't know what kind of flight it is, whether he had (potentially nervous) passengers around and such. But we can assume that he was under a little more stress than usual. After all, if you find yourself preflighting an aircraft and discovering the battery is flat, you may have to do all sorts of things that you've never had to do before: find a GPU and somebody knowledgeable and willing to assist you with it (and will that person have specific experience with the DA-42, or only with more traditional twins & singles?). Find the GPU receptacle, possibly discovering that the standard plugs do not fit. The stress of having somebody walking very close to the fuselage with the engines running. And the stress of all your mates looking at you at the clubhouse, because this is something out of the ordinary and that attracts attention by default. Now you can argue that one of the things a pilot needs to do is manage stress, and that all this should not impact your performance, but you have to admit that this may just be a factor in not starting to read the POH, looking for the appropriate checklist.
And the third factor is that the POH may not be of much help anyway. I just read through the Warrior POH with regards to the external power startup (admittedly, first time I ever read that part, didn't even know it was in), and basically it comes down to this:
- All electrics off
- Connect GPU
- Normal start according to normal start checklist
- Lowest revs as possible
- Disconnect GPU
And that's exactly what I would have expected it to say. The GPU is connected, replaces the function of the main battery so you can do a normal start, and you've got to be careful while somebody removes the GPU. I even can understand that with a twin the procedure would be to connect the GPU on the left hand side, then start the right hand side, disconnect, start the left hand side. But I would read that as a safety issue with regards to the person disconnecting the GPU, not something that would assure a sufficiently charged battery to raise the gear (far later into the flight). So if I were to find out that after starting the first engine and disconnecting the GPU, the second engine wouldn't start? I would hook up the GPU again and start the second engine.
As I said before, yes, formally we've got to use the POH and SOP with everything we do. But in reality, things are not that simple, for starters because the POH itself is way too bulky to keep on your lap throughout the flight. So we use abbreviated checklists, improvise when necessary, do things from memory and even forget things from time to time. The aircraft should be designed so that this does not lead to a potentially deadly incident, which EFATO clearly is. So if it is possible to get into a situation where you have both engines running but a depleted main battery (either though an improperly executed GPU procedure or in the situation where the battery just had enough juice to start two engines but then gave up the ghost) then the aircraft should be designed so that at the very least, when you raise the gear, the engines keep on running.
The DA-40 has ECU backup batteries which cannot be depleted except in case they run the ECU. As far as I'm concerned, that's a good start. Without something like this, it's an accident which can happen again, despite the procedure in the POH.