ENGINE START – the technical perspective

By February 24, 2019Simon's facts about

Simon's facts about - ENGINE START - the technical perspective

“How is a jet engine started” – great question by Vince W. from USA

The engine is started by an electric or pneumatic starter motor and fuel combustion. You will find an electric starter motor on the B787 Dreamliner for instance or different turboprop aircrafts. But I’d like to go into more detail of the pneumatic starting system.

Depending on the aircraft type and the engine with its control system the so-called FADEC (Full Authority Digital Engine Control), the engine can be started automatically and/or manually.
The starting system consists of an engine control system, the ECU (Engine Control Unit), one start valve, one air starter, two ignition boxes and igniters. The name of the engine control computer may vary with the engine manufacturer, like EEC (Engine Electrical Control) or ECU (Engine Control Unit) for example.

What conditions must be met for an engine start?
For an engine start you need pneumatic pressure to drive the starter motor which is linked to the gearbox to drive the engine shafts. The pneumatic pressure is delivered by the APU (Auxiliary Power Unit), an external Air Starter Unit (ASU) or by the other aircraft’s engine.
After the engineer or the crew received the startup clearance and air pressure is available, the person in command selects the engine for engine start. The engine start valve opens by an electrical signal and uses pneumatic pressure as muscle pressure to open. The air pressure goes through the open valve and drives the starter motor. The starter motor is attached to the engine’s gearbox which is linked to the (N2) high pressure shaft of the engine. The engine starts turning and if the engine speed exceeds a certain rotation speed, ignition is switched on automatically and fuel is sprayed in the combustion chamber and ignites by one or two igniter plugs. The ignition starts before the fuel is injected, to prevent a hot start.

The difference to a manual start is, if the engine N2 speed has reached a certain rpm, the so-called master lever or start lever is placed in the ON/IDLE position to start the fuel flow and ignition.

The engine rpm of the fan with its N1 (low pressure) shaft, N2 (high pressure) shaft and the oil pressure should increase continuously.
When the engine rpm exceeds a certain speed, the start valve is commanded to close automatically and the engine starter stops driving the gearbox and the engine reaches its idle rotation speed.

Depending on the aircraft’s engine and modeled FADEC, the engine control system may protect the engine by monitoring the temperature and speeds of the shafts. In case the system detects that the figures are out of limit it may abort the start sequence. This could be performed in case of:

Hot start – EGT (Exhaust Gas Temperature) exceedance ; too much fuel -> too high EGT
Wet start – no light up / no flame, detected by no rising EGT
Hung start – no acceleration

Finally some figures of a CFM56 driven A320.
Start valve open – N2 rpm increases
16% N2 speed – ignition ON
22% N2 fuel flow begins
50% N2 start valve closes and ignition cut off
58% N2 ENG at idle power

Thank’s for reading and don’t forget – A good technician is always learning 😉

February 24, 2019