Check out this video showcasing the Auxiliary Power Unit (APU) starting up on an aircraft. Notice the smoke coming out from the APU exhaust? What’s Happening? I think , It’s primarily due to the combustion process as the APU powers up, especially if the unit hasn't been used recently. Importance of APU ! - Provides electrical power and air conditioning while the aircraft is on the ground. - Supplies bleed air to start the main engines. - Enhances passenger comfort and operational efficiency. APU