grasscarp may well be right for light aircraft. However, air carrier aircraft do not have keys to enter the aircraft. There are specific things that have to be done to open a door or other entrane, however and only those traained or knowledgeable of the systems could do so. Additionally, the door locks on small Cessna, Piper and other such airplanes are notoriously unreliable and usually can be opened with a substitute key, such as a car key or house key. They're more of a deterrent than an actual safe lock.
Here in the U.S., it's a Federal crime, reported to the F.B.I. to break into or steal any aircraft...