If you follow our articles on AvGeek, you already know how aeroplane engines work. Let’s take a more detailed look to understand the starting process fully.

The answer to the question ‘How do you start an aircraft engine?’ is not as simple as turning the key. The ignition of a jet aircraft is a complex process, involving various components, including the turbofan engine, the APU (Auxiliary Power Unit) and the ASU (Air Start Unit).

Starting a jet engine

It all starts with supplying electrical energy to the engine parameter control systems, to avoid overheating and damage during ignition. It’s somewhat similar to starting a car in the depths of winter: it’s good practice to wait before setting off! Next, the fuel and lubricating oil pumps are activated, ensuring the necessary flow for engine ignition.

The turbofan engine has a starting sequence involving simultaneous fuel injection, spark plug ignition and air flow control. The FADEC (Full Authority Digital Engine Control) runs the whole process, regulating the parameters and ensuring safe and efficient ignition.

How to start an aircraft: the APU and ASU

The APU (Auxiliary Power Unit) supplies electrical and pneumatic energy independent from the main engines. This autonomous system is started up before the engines. It provides the required electrical energy to work the onboard systems in the phases before engine ignition (e.g. during passenger boarding). The APU consists of a small turbine engine that receives fuel from the aircraft. Once the engines are running, the APU is turned off, as it’s no longer needed, except on rare occasions.

The ASU (Air Start Unit) provides the compressed air needed to get the engines running and start the combustion cycles when the APU is not available. The unit connects to the aircraft through an access point. It provides the correct initial thrust to start the compressor blades turning. The flow of compressed air makes sure that the engine accelerates rapidly, bringing it up to the speed needed to start the combustion process. Once the engine is running successfully, the ASU is disconnected and the aircraft system switches to autonomous operation of the jet engines.