Originally Posted by AllanJ
The APU on many jet planes contains a little jet engine. So a 747 has five engines and a 737, three. The exhaust comes through the hole in the rear which in turn accumulates discolored stains.
Travel tips:
http://members.aol.com/ajaynejr/travel.htm
Also, don't forget the RAM Air Turbine that some aircraft have in the event that both engines lose power. Its a little prop that drops from the fuselage that is supposed to help power vital systems.