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On an aircraft like a 4 engine jet passenger plane why does

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On an aircraft like a 4 engine jet passenger plane why does the airport crew start only one engine with the air starter. Once one engine is started it looks as if the pilot turns the others. Why is that?
Submitted: 5 years ago.
Category: General
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Expert:  replied 5 years ago.
Hello. I can assist you with your question. In order for any turbine engine to start, the blades have to spin - which is sort of a catch 22, yes? If enough air can be forced through the fan blades to charge the compressor, the fuel can be ignited in the combustion chamber.

This starting process normally uses an electric motor to spin the main turbine shaft. The motor is bolted to the outside of the engine and uses a shaft and gears to connect to the main shaft. The electric motor spins the main shaft until there is enough air blowing through the compressor and the combustion chamber to light the engine. Fuel starts flowing and an igniter similar to a spark plug ignites the fuel. Then fuel flow is increased to spin the engine up to its operating speed. If you have ever been at the airport and watched a big jet engine start up, you know that the blades start rotating slowly. The electric starter motor does that. Then you (sometimes) hear a pop and see smoke come out of the back of the engine. Then the engine spins up and starts producing thrust.

Then engines on jet aircraft are also used for the main power - so with the electric motor, yet another catch-22. In some large aircraft one dedicated engine is for just that (usually aircraft with a turbine in the tail). This dedicated turbine is called An auxiliary power unit (APU), with it's main function being to provide power to start the main engines. The APU is started with a battery or with air.

The APU exhaust at the tail end of an Airbus A380

In many aircraft, the main and auxiliary power generation is distributed among the engines used for thrust. In some cases, the first turbine must be started using air to provide enough power for the electric motors to start the turning of the shaft in each of the other engines. Of course, this is not an issue once the aircraft is in flight.

Does that answer your question?

Michael Hannigan, Consultant
Category: General
Satisfied Customers: 11819
Experience: 25+ Years in the field
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