Aircrafts are not fully safe even when they don’t fly. Even huge buildings get damage time to time, for various reasons.
Smaller aircraft can be packed into the shelter of sturdy buildings, but bigger ones relatively often stay outside waiting for the season to start. Main risks are mostly related to ground maintenance of aircrafts. Professional aviation naturally has more functions and also liabilities on the ground. As examples, some sources of hazards can be listed as follows: hangars, stevedores, airports, air traffic control.