How do you start a plane?
Technically anyone could get in and start a big jet aircraft, but it would have to be pointing in the right direction. Getting a large transport jet going is allot more complicated than just starting the engines. The whole aircraft has a multitude of systems that have to be brought on line even before the start is selected. Failure to do so would have the computers on board sulk until you said good morning to them. The first operation, thus, is to 'power-up' the aircraft, this means to establish electrical power. This can be done in two ways, one being establishing a ground power unit and secondly, the aircraft's own auxiliary power unit, which is normally a little jet engine in the tail of the aircraft. The APU as it's called (auxiliary power unit) will also supply air pressure for cabin air conditioning and to the air starter units used to spin the main engine turbines at high speed to start the main engines, when commanded to do so. This is why the air-conditioning temporarily goes off when the engines are starting, because the APU cannot supply both engine start air and conditioning air at the same time.
After electrical power has been established, the on-board computers will come to life. Some computers are in their own little world and control things that are fully automatic, you only hear from them if they detect a defect in their own self test. Other computers are in pairs and threes, they monitor each other and alert the pilots and engineers via other computers if any detects faults in the others. There are two or three inertial navigation systems that are driven by lazar light. In each unit there are at least three lazars in different orientation. These units can detect the slightest movement and calculate the position of the aircraft by adding that movement to the aircraft's current position. The IRS units are very - very clever since they measure the movement by measuring the shift in the light spectrum when the aircraft moves. Since they measure movement, they have to know the speed the world spins at and the speed of the earth through space, to deduct that from the small movement on the aircraft in order to calculate the position. Allowing the IRS units to come on line is essential and must be done before main engine start. The aircraft must be completely stationary for this to happen.
After all that has been done the main navigation computers are programed for the route to be flown and with other information. The Flight management computers (as they are called on some aircraft) can also acquire their own information, like how much fuel is on board, and in flight the airspeed of the aircraft the angle at which the aircraft is at and from that it will calculate the weight of the aircraft.
So when all the systems are up and running and the computers have done their thing and have been programmed where required we can carry out all the other checks, switch on the galley power and call for a cup of tea and when we have drunk that we are nearly ready to start the main engines.
When ready to start the main engines the air system has to be re-configured to enable the starters to operate, following that the normal procedure is to place the engine start switch in the auto start position, where all the starting functions are carried out automatically. Both pilots monitor the engine start as sometimes things can go wrong like a hung start or a hot start. These sort of malfunctions can seriously damage a multi-multi million dollar engine in a flash and no pilot would want to be responsible for that. As the engine spins up to speed, at a predetermined percentage RPM the captain will select the fuel switch to the run position which will allow the fuel to be injected into the engine for light-up. Following a normal start the air system is then reconfigured once more as part of the after start check list, along with other post start items.
All in all what I'm trying to say is that it is a complicated procedure and not just about turning a key like a car. So if you were to board a dead aircraft on the tarmac and turn the start switch nothing would happen.
So what if the engineer wanted to run an engine for an engine test, would he have to follow the same procedure? Well, apart from loading the flight data into the flight computer, he would have to do everything else, if only to prevent all sorts of warnings that would occur trying to short cut the procedure, since the only warnings he would wish to see is anything that is relevant to the engine start, otherwise engine starting and running warnings may well be disguised by other more insignificant malfunctions. Hope this has been helpful.
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