How are the turbine engines of large aircraft started?

A gas turbine works by sucking air in at the front, squeezing it through compressors, squirting in fuel and lighting it, which makes the gas expand, then taking energy out of the expanding gas through a turbine at the back. As the turbine is connected to the compressor, it’s self sustaining.

The problem is that unless it’s spinning first, sticking a flame in the middle isn’t going to do anything useful. So, all engines need a way to first get the rotating parts moving fast enough, before igniting fuel in the combustors after which the engine should be self-sustaining.

There are several ways of doing this, including machines off the aircraft which provide a source of pressurised air which is blown though the engine to spin it up. Most modern large aircraft do this on board: they have a very small extra turbine on the aircraft – usually it is buried in the tail section, but often you can see where its exhaust is. The little turbine is lightweight, which makes it easier to start, and probably started electrically from a starter motor. It has 2 main jobs – provide air to start the big engines, and run generators for electrical power without the main engines running or needing a ground power supply. For this reason, it’s called the Auxiliary Power Unit (APU)

So, the APU starts the big engines by sending pressurised air through the turbines, which gets the whole engine steadily increasing in rpm. When it’s high enough, the engine fuel and igniters are allowed to go, and the engine can run itself.

Image – the back end of a 737, showing the APU and the exhaust at the top of the picture.

The very newest aircraft are trying to get more efficient by reducing the number of things done with pressurised air on the aircraft. Aircraft like the Boeing 787 use electrical systems where possible. Here there is still an APU, but it only produces electrical power, and so the engines are started electrically. We will probably see more of this in future.

Other methods have been used to spin up the engine for starting, including having explosive cartridges in the engine to provide a burst of pressurised gas, and using a starter motor to drive the engine mechanically. Some older aircraft used an APU-type start which wasn’t quite so fancy: it could provide the pressurised air for starting, but not the electrical power. This would generally be called an air starter – not nearly so useful as an APU, because on the ground you either need to be plugged in to ground power, or have the main engines running to get electrical power.

It depends on the specifics of the engines & the platforms that they are installed in.

Jet engines have been started electrically, pneumatically, by using an explosive charge to generate a high-volume, high-velocity gas flow and even by blow-starting with the exhaust efflux from an aircraft with its jet-pipe in close proximity to the inlet of the un-started engine.

The usual method for commercial jets is by using a small internal jet engine called an APU (Auxiliary Power Unit) to generate gas flow, or compressed air, to spin the larger engine(s).

It depends on the type of aircraft and specific situation we’re talking about…

Large comercial aircraft (like a Boeing or Airbus model flown by a major airline) have an “Auxiliary Power Unit” (APU). The APU is basically a tiny turbine engine in the tail of the aircraft designed to provide pneumatic and electric power without producing significant thrust.

An APU serves several functions on modern aircraft, and one of those function is to provide the pneumatic (or on the 787 electric) power required to start (at least one of) the main propulsion turbines.

If an aircraft’s APU malfunctions or fails to start the main engines a “start cart” can be used instead. This cart is basically a spare APU in a box, and the APU manufacturer usually charges a hefty premium for it so it’s not a desirable alternative to have sitting around if it’s not needed…

On an Airbus, the pilot turns the engine mode selector (a knob) to IGNITION/START. This single action causes the pack valves to close, power to be supplied to the FADECs, and the ENG page to appear on the lower ECAM display.

After verifying pneumatic pressure on the system display, the pilot places the ENG MASTER switch to ON, and FADEC performs the entire start automatically. First the start valve will open, and air pressure will spin a small turbine which mechanically spins the engine. At the appropriate time, the FADEC will introduce fuel and ignition and control the fuel flow to maintain engine parameters within start limits. As the start completes, FADEC will close the start valve and monitor the engine as it continues to a stable idle.

If there are any malfunctions during start, the FADEC will auto abort the start and perform the necessary actions such as cutting off fuel and continuing to crank the engine (dry cranking) to blow out any residual fuel or vapors and cool the engine.

There is also a “Manual” start option in which the pilot can open the start valve to crank the engine, but delay fuel and ignition by keeping the ENG MASTER off. When the ENG MASTER is turned on, the FADEC performs the start as during an auto start but without the automatic protections. Pilot action is required in the event of a start malfunction.