Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Flintstone
I've always left mine unlocked while in the hangar. If they're determined enough to break in through industrial grade corrugated steel sheets or cut off a hardened steel padlock they'll make a right mess of an aircraft lock and door.

There's never anything in the aircraft worth stealing apart from a radio and transponder which are replacable. Damage to the door and fuselage would at the very least ground the aircraft for months, if not write it off.
By Lefty
Echoing @Flintstone - A (good) few years ago there was a spate of thefts from aircraft at my home base, very little of value was taken - but the damage to the doors and locks of those a/c that were locked - ran into hundreds of pounds - and several were grounded for weeks whilst awaiting parts / repair.