Once landed and the engage is finished the T/hawk counts as a normal WE.
It can still be broken as normal, when it breaks it would take a point of damage per enemy unit within 15cm
as 3.2.4 states.
Quote:
A broken war engine is assumed to have a number of Blastmarkers equal to its starting damage capacity for all rules purposes. If there are any enemy units within 15cms of the war engine after it makes a withdrawal then it suffers one extra point of damage (no save allowed) for each enemy unit that is within 15cms. Additional hits caused by losing an assault or receiving Blast markers while broken cause one point of damage each. Roll for critical hits from these extra hits as you would normally
If it breaks and there are no enemy units within 15cm then it acts like a normal WE, it takes BM until it breaks (4 in the T/hawks case because of TSKNF) when it flies off in the end phase it would have 4 BM (plus any more it picks up on its disengage move).
losing an assault still auto destroys it wether there are enemy within 15cm are not.