The helicopter crash physics (and arguably all the vehicle collision physics in general) in this game are quite poor in comparison to many other aspects of the game. Among the issues I've noticed are:
-Structural damage due to the rotor colliding with a pole or tree will sometimes inexplicably result in an abrupt explosion before contact with the ground.
-Even at low horizontal velocities, helicopters explode/combust immediately upon contact with water, which is unrealistic and results in choppy animations.
-When the engine has been shot out, even relatively soft falls from low altitude tend to result in abrupt explosions and death.
-Insta-death results from helicopters sinking into/becoming partially submerged in water, rather than gradual drowning.
-The damage radius of destroyed vehicle fires often extends beyond any visual fire effects, and lag spikes prevent players from reacting once taking damage, which effectively means insta-death for those who go within a certain arbitrary radius of burnt out vehicles.
-The overall damage model generally feels very inconsistent and ground vehicles wheels seem to take an excessively disproportionate amount of damage from rocks.
-Occasionally when an APC/tank turret/cannon comes into contact with a tree or pole the entire vehicle will be sent flying into the air, flipping rapidly.
-Helicopters in contact with the ground abruptly explode when tilted too far, especially when the helicopter is drifting laterally. This makes it difficult to taxi, and helicopters also tend to explode unrealistically during sliding landings.
-Landing on certain irregular/convex objects, however slowly, will randomly damage the engine until the helicopter explodes, even if the skids are the only part in contact with the surface.
-Passengers often repeatedly glitch out of helicopters (especially the co-pilot seat), and can damage the rotor if they happen to collide with it.
-Collisions between larger ground vehicles such as APCs or MRAPS and quadbikes have unrealistic physics and result in choppy animations.
-When helicopters take damage, their trajectory changes very unrealistically and abruptly, as if entering some kind of immersion-breaking pre-made cutscene. Instead, the game's physics should be allowed to simulate a crash realistically.
-Helicopter rotor damage is too binary (on/off); either the rotor is perfectly functional or the rotor is completely still/has magically disappeared.
-Moving against the remnants of destroyed structures (such as the lighthouse on the south of Stratis) can cause instant and abrupt death for no apparent reason.
-Helicopter collisions with even the outermost branches of a tree results in instant catastrophic destruction. Realistically, the smaller branches on the outside of a tree have a lot more give (and would even sway in the wind), so even if they disrupt flight they shouldn't suddenly wreck a helicopter. Perhaps the collision box for trees should be made to be smaller.