8. An auxiliary power unit
When boarding the plane, we always hear a noise even though the engines aren’t working yet. It is caused by an APU that provides power for the systems while the engines are out. It also starts the engines. Such a power unit is also helpful in poorly equipped terminals where proper maintenance is unavailable.
7. Air Marshal Service
Air Marshals fly incognito and can quiet down a troublemaker or even disarm a terrorist. The USA, France, Canada, and other countries have their own Air Marshal Services, and there’s even a movie about their work.
6. Crew rest compartments
Some airplanes have small hidden rooms for the cabin crew where they can relax during lengthy flights. These CRCs aren’t especially comfortable: you can only crawl in there, but you can lie down for a while and rest.