Have you ever wondered how an aircraft is started? While the idea of pilots passing keys between each other at an airport is great – it does not happen! Modern jets can be accessed and started, without any keys at all.
Only in general aviation
We will start with where aircraft do have keys. With smaller general aviation aircraft and executive jets, they may be used. This could be to start the engines (more common with smaller aircraft) and also to lock the aircraft doors. Throttles, or other key controls, can also be fitted with locking devices to prevent use.
Many smaller aircraft (including Cessna and Piper aircraft) have a key ignition system to start the propeller, much like you would find in a car.
These types of aircraft are often left at smaller airports or remote landing strips, sometimes for long periods of time. It obviously makes sense to be able to secure access at such locations. At larger airports, you would hope this is less necessary.
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No keys for commercial jets
No modern commercial jet aircraft have keys. They also don’t have door locks. Ground staff need to be able to access the aircraft in the event of an emergency, and locks could complicate that. The fact that they operate from secure airports is enough to mean that they have not been designed this way. When left at a gate, doors may be marked or taped so that crew will know if someone attempted to access the cabin, but they will not be locked.
Starting a jet aircraft
So, given that you don’t turn a key, how do you start up an aircraft? A combination of battery power and bleed air from the auxiliary power unit (APU) is used, The APU is essentially a small turbine engine, usually located in the tail of the aircraft, that provides power via a generator and air pressure for starting the main engines, This can also be achieved with ground power, but having an APU allows aircraft to operate without this.
To start the aircraft, battery power is used to start the internal electrical systems, lighting, communication systems, and the APU. Once started, the APU can power a generator to supply electrical power.
The main engines require airflow before they are started. Without this, they could be damaged by overheating. The second function of the APU is to provide high-pressure bleed air to spin the main engine turbine blades. Engines are started one at a time, with fuel sent to the engine and ignited once it is spinning sufficiently.
The second engine (and third and fourth, of course) can then be started in the same way, using either APU bleed air or air from the engine already spinning. This same method can be used to restart an engine during flight if needed.
It may seem strange that large aircraft don’t use keys. Feel free to discuss this in the comments.