it is usually when full ignition of the fuel in the combustion chamber does not take place, normally attributed to an inop ignitor or HEIU (there are two per engine).
Since when would there be a partial light off?
Combuster interconnectors or Can Annualar arrangements ensure full light off unless a rare blocked fuel nozzle.
Only one ignitor will be used during the start sequence unless a second start is being attempted.
If the first ignitor used is inop then the engine will remain at Max motoring with a lot of fuel mist seen at the exhaust.
As all the fuel is not ignited the turbines are slower to rotate which in order is slow to rotate the compressors (HP & (IP) & LP).
Really. If fuel is lit then fuel is lit..
The engine will take longer than normal to reach self sustaining speed
My experience you would be waiting all day watching a 40% rpm (Hung start).
If you were to let it....
The starter valve will only rotate the HPC to a nominal percent (usually 40-45%)
The starter motor will turn the HPC via the gearbox, start valve opens to allow air to the air driven starter.
once unburnt fuel is ignited it can torch back to the NGVs/ HPT causing damage.
The NGV's and HPT will take a great deal of temp punishment. You are describing late fuel light off and then monitoring the EGT as routine would be the order. If EGT seen approaching the start limit the fuel would be cut-off and engine continued to be motored to clear all unused fuel prior to a second attempt then using both ignitors.
A hung start can also be attributed to late opening of the throttle control valve which would be shown as no initial ignition followed by a sharp rise in EGT/JPT/TET.
Again Max motoring with ignition 'ON' followed by a sudden but controlled EGT rise due late fuel only.
Confusion with Max motoring with no fuel and/or ignition approx 25-30% and Hung start where fuel has been ignited but HPC slows at approx 40% due to insufficient fuel flow or bleed schedule problem.
Sorry just had a bad day
TW