Sav,
Compressor seizure - Can be caused by corrosion under the top hat section which holds the compressor stator vanes in place. Have seen it occur in flight where the stator vanes will "lathe" the compressor hub through and basically the hub collapses followed by the complete compressor. Have seen this on a "new" Bell 206. Associated large bang and TOT normally "pegs out" at the max. See
here
N1 lock up - usually on a newly overhauled turbine caused by N1 turbine tip rub. On a hot start just as you are about to light off the N1 section will seize. Not much to worry about, just wait until it cools down and it will free up and you can start it again. Standard procedure until it knocks the high spots off. Best part is the look on the pilot's face! See
N2 lock up - caused by carbon build up on the labyrinth seals on the PT wheels. Can be knocked off by turning rotor backwards. Normally an oil type problem and/or cool down procedure.
Carbon buildup - The drain from the forward N1 turbine bearing is through one of the support struts. If not allowed to cool sufficiently (relative term) the oil will coke in the strut and the bearing cavity will flood and the oil that normally flows through the strut now goes out through the labyrinth seals onto the face of the turbine wheel. Not good. Hot turbine wheels do not like quenching with cold oil. Good chance of wheel failure and exit from engine.
