CLOSED MaxFacts: What Am I?. @Mats_Edvin_Aaro Got it! Well done "Aviator"

A previous WHAT AM I Question on the Air Start Unit had multiple incorrect answers. It was repeatedly identified as a Ground Auxiliary Power Unit (APU). The Photo shows is an onboard A-320 APU unit, a mini jet engine. APU’s are found on the majority of all commercial Jet & Turboprop aircraft. QUESTION: What is an onboard APU’s purpose, how does it work and where is the A-320’s APU’s exhaust port? (Related Question: Rat’s are standard equipment in some aircraft. In emergencies they generate electrical power to run avionics and major electrical systems necessary for flight. What’s a “Rat”?)

( Ctomplete answers wins: APU Purpose & Function, where’s the 320’s exhausts & what’s a “RAT”)

Oh, sorry! I thought this was another guessing one…

The APU’s purpose is to provide electrical power to the plane when the engines aren’t on. It also provide pneumatical air pressure to start the engines. The exhaust is at the tail.
RAT is Ram Air Turbine, which in event of dual engine failure automatically deploys in the form of a small propeller on the belly of the airplane, which by using the air pushing on the blades, spins and generates just enough electricity to power the vital electrical equipment on board.
basically.

MaxSez: Expanded Ansewer for the record:
Typically Jet & some Gas Turbine engines have an on board Air Start Unit (APU) thus negating the need for GSE & Ground Power. The on board APU provides electrical power and ram air when engines are off. On initial power up, without ground support, an onboard APU will provide electrical power andfacilitate the ignition sequence to one of the engines (usually No. 1) using compressed air provided by hoses to turn the first stage turbine (N1) to facilitate compression and combustion and vented to the pacts running the air conditioning units. . After that the bleed air from the first engine is enough to start the second and the APU is shut down since a power transfer occurs during this sequence. In the A-320 the APU exhausts thru the tail cone. A “RAT” is a Ram Air Turbane, deployed into the slip stream, it generates electricity to major components Typically Jet & some Gas Turbine engines have an on board Air Start Unit (APU) thus negating the need for GSE & Ground Power. The on board APU provides electrical power and ram air when engines are off. On initial power up, without ground support, an onboard ASU will facilitate the ignition sequence to one of the engines (usually No. 1) using compressed air provided by hoses to turn the first stage turbine (N1) to facilitate compression and combustion and vented to the pacts running the air conditioning units. . After that the bleed air from the first engine is enough to start the second and the APU is shut down since a power transfer occurs during this sequence. In the A-320 the APU exhausts thru the tail cone. A “RAT” is a Ram Air Turbine, deployed into the slip stream, it generates electricity to major components necessary for nav & control routinely)
B757 pic

necessary for nav & control routinely in an emergency)
B-757 RAT:

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(“Aviator” a term used here that describes a Pilots exceptional technical expertise)

Actually, that procedures is called “cross-bleed”, and isn’t really Standard Operating Procedure. Some airlines do, some don’t. However, ‘cross-bleeding’ requires a power setting higher than idle on most aircraft. Which means that’s like spending a dollar to save a dime.
Most airlines simply start the engines using the APU. If you attempt a cross bleed with a idle power setting, you will most likely get a hung start, which can be even more expensive.
If you are taxiing with a single engine, then a cross bleed can pay off when you approach the holding point. The APU burns a lot of fuel, but a cross bleed puts a heavier workload on the pilots.
Of course, the third option would be to use an air cart, but that’s a different topic.

MaxSez: Mats, Always technically correct. Thank you Aviator for your sage correction. BZ

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