The attitude and flight profile of theTransAsia ATR 72 that crashed after take-off from Taipei's Sung Shan airport - shown in high quality video of the aircraft's last few seconds - indicates it was fully stalled.
An aircraft stalls when it’s flying too slowly to generate sufficient lift from its wings, and it starts to fall.
In the video footage the aircraft first appears just above the top of a line of high buildings, the pilots holding the nose up to try to clear them. When clear, the left wing drops dramatically to a 90deg bank, the aircraft loses height rapidly, and the wingtip strikes a road before the aircraft tumbles into the river next to it.
If an aeroplane is flying too slowly in level or descending flight, it is normally because there is insufficient power to keep the aircraft’s speed up. The question for the investigators to answer is why there was insufficient power.
The pilots of flight GE235 had made a Mayday call to Sung Shan air traffic control tower declaring an engine flame-out. Initially, having taken off from runway 10 for a domestic flight to Kinmen, the aircraft carried out a climbing turn right, but then started losing height and banking left.
As seen in the video, both propellers were clearly turning, but that does not necessarily mean they were being supplied with sufficient power to fly safely.
If engine power is lost, the un-powered propeller can cause a lot of drag by windmilling, making the aircraft difficult to handle. Under those circumstances the crew would normally “feather” the propeller to cut the drag.
An ATR 72 should be able to fly safely with only one engine producing power, so it seems there were secondary factors in this case.