Depends on the aircraft. In the small airplane it does work with a key. In the old days you had to get somebody to spin the propeller (Just like in all those old WWI flying movies). On the Piper PA-32 you have to turn the magnetos on (unground them via a switch) and then there is a rocker switch that engages the starter on each engine. Just push it until the engine catchs
DC-10 does do a pretty good of explaining how most jets are started today. I do want to add a couple of things.
Some smaller jet engines use and electric starter that is about the same as your car. Most large engines have a compressed air starter. In this type of starter there is a impeller in the starter that air is forced through to cause it to spin and in turn it spins the engine. The compressed air either comes from the APU(auxillary power unit), any running engine or a air start cart. I have started a lot of airplanes useing the airstart carts. The theory of starting the airplane is about the same in any case. Get the moter spinning fast enough and have the ignightors heat the engine hot enough so that if fuel is introduced it starts to burn and the engine is running.
There are two other ways that can be used to start an airplane. On the F-16 there is a fuel burn system where a seperate burners burns fuel producing expanding gasses that are passed through the starter much like the air start method. This doesn't require external connections. Also you can generate the pressurized gasses to turn the starter by firing a special cartridge that looks like an overgrown 12 gauge shell into the system. This isn't as popular now but was used on both piston and jet engines. If you want to see this system in action rent the movie 'Flight of the Phoenix'. It is a classic with James Stewart, Richard Attenbrough, George Kennedy and I can't think of his name, had the helicopter company off the TV show Airwolf. Sat in the back and ran the chopper.
Anyway it is a good classic movie. It was the one that Paul Mantz was killed during filming of it. I hope this explains how to do it and I didn't confuse you.
I have one more story to tell.
One day one of our electras landed and shut down. Our Electra's weren't equipted with APU's so you had to airstart them every time when they landed. Found out after it shut down that out Airstart was out of service and we had no other way of starting the airplane. About a half hour later one of our 727's landed. We went ahead and switched the loads on the two planes which was the original plane. They crew and passengers boarded. We started the 727 first since it had it's APU working and then the pilot of the 727 turned so his jet blast was blowing right over the Electras wing. The pilot of the Electra then edjusted the pitch of the prop blades on his aircraft so they started to spin in the blast on the 727. Once he got the prop spinning fast enough he started the engine and one he had that one started he used the bleed air off that engine to spin the other three. Schoolyard tactics, but just another day on the job.
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