Using the below on a helicopter (haven't tested with other aircraft types) while the helicopter is in flight does not negate the non-scripted damage that should be negated when the Event Handler's code returns "0". It does however negate the damage if the helicopter is stationary on the ground.
_unit addEventHandler ["HandleDamage", {0}];
More details can be found here:
https://forums.bohemia.net/forums/topic/205515-handledamage-event-handler-explained/?page=2&tab=comments#comment-3333776
https://forums.bohemia.net/forums/topic/205515-handledamage-event-handler-explained/?page=2&tab=comments#comment-3346171