I've been looking at the FDR traces that have been released, and would like to share my analysis.
My credentials: I'm an amateur at this, but I have been debugging computer systems for over 40 years, so I'm used to looking at what evidence there is, and trying to work out what happened.
So: "Correct me if I'm wrong" applies to all of the below! When I say "the system" I mean some part of the automation - I don't know enough to say which particular box is involved, and it doesn't really matter at this point. All times are approximate, as it's a bit tricky to interpolate between the marked times.
It looks like everything was OK until 52:30. Then (at about the same time):
Objective TQ1 rose to 100%
No.1 Bleed Valve was closed
Prop.2 started to feather
I believe this is "Autofeather and Uptrim" by the system, because it believed No.2 engine had a problem.
Engine 2 Flame Out Warning was given
Fuel Flow 2 started to reduce, followed by No.2's ITT and RPMs (NL, NH, NP) and torque.
This seems to be the system bringing No.2 back to Idle, but without moving the Power Lever (can it do that on this aircraft?)
I'm going to call the small amount of time over which the above happened: "Point 1".
52:40 As a result of the above, No.2 spooled down, and by now it was at idle, feathered. (Its torque trace starts a strange triangle-wave, with about a 4-second period, spiking from zero up to about 150%.)
Power Lever 1 is retarded a bit, I'm assuming by the crew. CVR would be useful at this point to find out who/why.
They are still climbing, albeit slowly, until:
53:04 PL1 is retarded a lot further, then further still, then PL2 is firewalled, then PL1 pulled back further still, with consequent reduction of TQ1 to practically zero. Prop.1 is still in normal pitch and NP1 is about 80%, so for the next five seconds or so it must be windmilling. I'm calling this "Point 2".
53:20 Prop.1 starts to feather and Fuel Shutoff 1 is closed, shutting down that engine, (and with its torque trace taking on the same triangular wave pattern as No.2, and then its oil pressure trace doing the same thing).
53:28 they have both props feathered, No.1 shut down, No.2 idling but with its PL still firewalled, and they're at about 1200', descending.
Without the CVR we can only imagine what happens in the flight deck for the next three-quarters of a minute, but then:
54:14 Fuel Shutoff 1 is reopened, PL1 advanced slightly, so this is a relight attempt. They are at about 700'. "Point 3"
54:19 No.1 is spooling up, but Prop.1 is still feathered and not really turning.
54:27 Power Lever 2 is retarded most of the way, the Flame Out 2 warning stops, and Prop.2 starts to unfeather, so the system seems to think No.2 is OK now, and starts to spool it up, but it never contributes any thrust - unfeather never completes and the RPMs and torque barely get off the stops before the end of the trace, so I don't see the "sudden thrust from No.2 raised the right wing" happening, as some have said.
As I see it these are the questions:
1. What caused the system to "think" there was a No.2 flameout at Point 1? Before that the engine indications should still have looked normal to the crew, but it seems they had no chance to override the spool-down that resulted, and perhaps no reason to think they needed to.
2. Why was PL1 retarded 10 seconds after Point 1? It wasn't to shut down the engine as it was only a 10degree change.
3. At Point 2 they seem to have ignored the normal readings of No.1 and the "Idle" readings of No.2, and shut down No.1 Is this a "confirmation" problem because they were expecting No.1 to be a problem before they set off? Also, would the windmilling No.1 "confirm" they were shutting down the bad engine?
4. Why did retarding PL2 clear the erroneous Flame Out condition (not just cancelling the warning, but also bringing up the fuel flow and unfeathering)?
5. If they had retarded PL2 at Point 3 instead of trying to restart No.1, would they have got out of the situation in time?
6. (Unimportant) What's with the three triangle-wave traces?
Finally, why are the FDR traces all engine related, except the Main Gear, VHF1 (what is this?) and height readings? Where is the airspeed, vertical speed, control positions, etc?
Cheers,
Howard