Engine wet start
This happened when the the fuel start to inject into the combustion chamber and ignition system
has an abnormal delay(ignition did not start to work at the right time),this can be noticed
by a fire coming out the tail-pipe by ground crew and by the high EGT indication in the cockpit.
In P&W4000, during engine shut down two fuel nozzels will keep injecting fuel and stop after that for the the next engine run, in the next engine run you will notice
smoke coming out the tail-pipe, but as per thire standards this is not considered as wet start
Compressor stall
When the air flow going through the compressor is not
enough or inadequate
air flow passing through it,in this case stall will start to happen and it will lead to somthing called (surge). when the surge exists
, a bang sound coming from the engine can be heard plus back fire
see this vedio for more info about surge
[youtube]https://www.youtube.com/watch?v=pE2LeVQ2DtM[/youtube]