A crash is usually defined as any unwanted aircraft / obstacle interaction.
Depending on the helicopter, and the expectations of the pilot, a crash may be anything from a heavy landing, tipping over, controlled flight into terrain (such as a tree moving into your flight path), or an out and out funky chicken crash, usually caused by losing orientation, a dumb thumb or brain fade.
Quite often pilots know they are going to crash a second or so before it actually happens (for example, if they have lost orientation, the helicopter will inexplicably refuse to obey the pilot's commands properly). When this happens, the best course of action is to engage throttle hold; with any luck this will at least save your main gear, and hopefully the main rotor will have slowed down a bit by the time the impact occurs, perhaps reducing the number of parts mangled.
After a crash you will want to check, at a minimum, that:
- the main mast is straight;
- the feathering spindle is straight;
- the flybar is straight, and the paddles haven't twisted.
- the blades have no damage;
- the main gear hasn't lost any teeth.
- the tail shaft is straight, especially in a tail-first crash;
- the tail boom is straight, and the tail belt tension is correct;
- that no ball links or other control linkages have come off (try to twist the main blades in opposite directions by hand---if they move, then something has come loose);
- that the servos have no damaged gears.
Failure to do this can lead to excessive amounts of vibration at best, to loss of control or the helicopter disintegrating (due to extreme vibration) at worst.
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