You may have noticed the hole in the tail of most aircraft. It's probably no surprise to know that it's an exhaust outlet. But this is nothing to do with the main engines. It is for a second, much smaller, turbine engine that all commercial jets have. This auxiliary power unit (APU) provides important electric power for aircraft systems and bleed air to start the main engines.
Read More Here: What Is An APU? | SimpleFlying.com
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