Which facility is separate from the terminal and primarily used for servicing aircraft?

The facility that is separate from the terminal and primarily used for servicing aircraft is a hangar. Hangars are large buildings primarily used for the storage, maintenance, and servicing of aircraft when they are not in flight. They provide a controlled environment to protect aircraft from the elements and ensure that maintenance can be performed safely and efficiently.

In contrast, gates are the designated points within terminals where passengers board and deplane from the aircraft. The tarmac refers to the area where aircraft are parked, loaded, and prepared for departure, but it is not enclosed like a hangar, and aircraft servicing can occur there as well. Runways are specifically designed for aircraft takeoff and landing, not for servicing, as they must remain unobstructed for those operations. Thus, the hangar is the correct choice as it is specifically designed for comprehensive aircraft servicing away from public areas.