Originally Posted by
procede
So it's a B77W?
Assuming that the operator has been correctly reported as Thai, it's either a Trent-powered B773 or a GE-powered B77W. The photo doesn't look like it's a Trent.
I think the main reason for a different code is that the 300ER has an increased wingspan, not due to the different type of engines.
ICAO designators are assigned primarily on the basis of performance differences that are relevant to ATC, for example the extra 30,000 lbf of thrust on the B77W.