Gas Turbine Engine Starting

A gas turbine cannot start itself. Until its compressor turns fast enough to deliver a good flow of air, there is nothing for the fuel to burn in and nothing to carry the heat of the flame through the turbine. Engine starting therefore begins with a starter that turns the high-pressure spool, continues with fuel and ignition once the airflow is sufficient, and ends only when the engine can accelerate on its own power to idle.

The start is the most delicate part of an engine's working day. The compressor works far from its design point at low rpm, the turbine is at its most vulnerable while the airflow is small, and a misjudged moment for the fuel can take the exhaust gas temperature (EGT) past its limit in seconds. On most airliners the engine control now runs the start and protects it on the ground, but the crew still has to recognise a hot, hung or wet start and know what the controller will and will not do. Ground procedures around the start are covered in ground handling, pushback and engine start.

Starting requirements

A successful start needs three things together:

• Rotation: the starter turns the HP spool (N2), which also turns the accessory gearbox and with it the fuel and oil pumps.
• Fuel at a pressure the spray nozzles can atomise, introduced only once the airflow can support and cool the flame.
• Ignition: high-energy igniters light the mixture (see gas turbine ignition, flameout and relight).

Fuel introduced too early is the classic cause of a hot start, because there is not yet enough air to carry the heat away. Each type therefore sets a speed for the fuel: on the Boeing 737 NG, N1 rotation seen and 25% N2, or maximum motoring with at least 20% N2; on the A320 in an automatic ground start, ignition at 16% N2 and fuel at 22%; on the E190-E2, ignition and fuel at 18% N2. Maximum motoring is reached when N2 accelerates by less than 1% in about 5 seconds: the starter can turn the engine no faster.

Lubrication must also be established, so oil pressure is watched through the start. On the 737, no oil pressure by the time the engine is stable at idle is an abort condition, and the E190-E2 procedure aborts if there is no oil pressure within 10 seconds of N2 rising. The thrust levers must be at idle: an A320 engine starts whatever the lever position, and if the lever is not at idle the thrust rapidly increases to match it, a hazardous situation on a crowded apron.

Pneumatic starters and start valves

Airliner engines are started by an air turbine starter (ATS), a small turbine spun by compressed air. It is mounted on the accessory gearbox and turns the HP spool through a gear train; on the E190-E2, for example, the drive reaches the N2 spool through the radial drive shaft. Smaller turbines, including many APUs, use electric starters or a starter-generator, one DC machine that starts the engine and then becomes its generator (see DC generators, motors and starter-generators).

The engine start valve, or starter valve, admits duct air to the starter. On the 737 NG the ENGINE START switch in GRD uses battery power to close the engine bleed air valve and open the start valve; the switch is then held by a solenoid until starter cutout, so starter operation needs both pressurised air and electrical power. An amber START VALVE OPEN alert shows the valve open with air going to the starter; if it blinks, the valve has opened without a command. On the A320 the FADEC opens the start valve in an automatic start, and in a manual start the MAN START pushbutton opens it only while N2 is below 20%.

The same 20% figure protects the starter elsewhere. The A320 starter must not run with N2 above 20%, and on the 737 an ENGINE START switch that fails to stay in GRD until cutout can cause a hot start, and must not be re-engaged until N2 is below 20%.

APU, crossbleed and ground air

The starter needs a large flow of air at moderate pressure, and it can come from three places. On the A320 the order of preference is:

1. APU bleed, the usual source, self-contained and independent of ground equipment (see auxiliary power unit). On the A320 it can assist an engine start up to 20,000 ft.
2. Ground air: a ground air start unit, or HP ground air cart, delivers compressed air through a hose to the aircraft's HP ground air connection, which feeds the pneumatic duct. It is used when the APU is unserviceable or to save APU cycles. It is not the ground power unit, which supplies electricity, nor the LP ground air cart, which supplies conditioned air to the cabin. The A320 must not use APU bleed and the HP air start unit at the same time.
3. Crossbleed from an engine that is already running, routed through the crossbleed or isolation valve.

The packs are closed for the start so that the starter receives the full duct pressure; on the A320 they close automatically and reopen 30 seconds after N2 passes 50%, unless the other engine is being started. Where the ground connection joins the duct can matter: on the E190-E2 it enters on the right-hand side of the crossbleed valve, so with a cart the right engine, number 2, is started first.

A crossbleed start is the least preferred method on the ground, because it needs power above idle on the running engine to give enough duct pressure, with the jet blast that implies; the area behind the aircraft must be checked clear. It is also a normal in-flight technique: the 737 shows a magenta X-BLD indication, based on altitude, airspeed and N2, when crossbleed air is recommended because the airspeed is too low for a windmilling start (see bleed air and pneumatic systems).

Start sequence and light-up

The A320's automatic ground start shows the sequence. With the ENG MODE selector at IGN/START and the ENG MASTER switch ON, the FADEC opens the start valve and N2 rises. At 16% N2 one igniter is energised, the FADEC alternating igniters from start to start. At 22% N2 the HP fuel valve opens, and light-up follows, seen as a rise in EGT and N1. At 50% N2 the start valve begins to close, fully closed by 56%, and the igniter goes off. The engine then accelerates to idle.

On the 737 NG the crew judge the moment for the fuel: with the start switch at GRD, the start lever goes to IDLE at 25% N2, opening the spar and engine fuel valves and energising the ignition. The EGT must rise within 15 seconds, the starter cuts out at 56% N2, and the engine is stable at idle, about 59% N2, when the EGT start limit redline disappears. The ATPL texts give a typical light-up time of 20 seconds.

A start has a fixed rhythm, and the crew watch each parameter against it: N2 and N1 rotation, fuel flow, EGT, oil pressure. On the E190-E2, no N1 rotation by 45% N2 is an abort criterion. In cold weather, 737 crews may select both igniters for the first flight of the day at aerodromes at or above 2,000 ft with a temperature below 5 °C.

After start, the engine is warmed up before high thrust is used; the E190-E2, for example, requires an oil temperature of at least 24 °C for take-off thrust (see thrust levers, idle modes and engine handling).

Self-sustaining speed and starter cut-out

Self-sustaining speed is the speed at which the turbine produces enough power to drive the compressor and the accessories without help. Light-up happens below it, so the starter keeps driving after light-up, which shortens the start and keeps the EGT down. It is disengaged above self-sustaining speed, at the starter cutout speed: 50% N2 on the A320 and the E190 E1, about 56% N2 on the 737 NG. The igniters are switched off at about the same point, by a speed switch in the ATPL texts' description.

The cut-out must be confirmed. At 56% N2 the 737 pilot verifies that the ENGINE START switch returns to OFF, moving it there by hand if necessary, and that the START VALVE OPEN alert goes out, before calling "starter cutout". On the E190 E1 a starter control valve that stays open beyond 50% N2 gives an ENG START VLV OPEN caution.

Starter duty cycles

An air turbine starter is designed for short periods of use and overheats if run for too long, so each type publishes a starter duty cycle:

Hot, hung and wet starts

Three abnormal starts appear in every syllabus.

A hot start has too much fuel for the air passing through the engine. The EGT races towards the start limit, which is 725 °C on the CFM56-5B, and the only protection is to cut off the fuel and ignition at once. The textbook causes are a tailwind reducing the intake airflow, residual heat from a previous run, a thrust lever not at idle, slow rotation, or fuel introduced too early. An exceedance must be reported, because turbine blades may have been damaged.

A hung start lights up but stops accelerating below self-sustaining speed. The EGT is high for the low rpm, because there is only just enough air for combustion and none to spare for cooling, while the fuel flow sits at about its idle value. The start is aborted before the temperature climbs further.

A wet start, or no light-off, has fuel flowing but no combustion: the fuel flow indication rises, the EGT does not. The 737's EEC detects it and shuts off the fuel and ignition 15 seconds after the start lever reaches IDLE.

On the ground, the engine control protects most starts. The A320 FADEC aborts an automatic ground start by itself for a hot start, an over-temperature, a stalled start or no light-off; it then closes the HP fuel valve and the start valve, cuts the ignition and dry-cranks the engine, as long as the ENG MASTER stays ON. In a manual start it only aborts if the EGT limit is exceeded before 50% N2. The 737 EEC flashes a white box around the EGT readout for an impending hot start or stall and, with current software, shuts off fuel and ignition itself. None of this protection works in flight (see gas turbine ignition, flameout and relight). On the 737 the aborted engine start recall is the start lever to CUTOFF, and a start lever closed by mistake during a start must not be reopened to catch the engine.

Dry motoring and blow-out cycles

Dry motoring, also called dry cranking, turns the engine on the starter with no fuel and no ignition. It clears fuel and fuel vapour after a failed start and cools a hot engine. After a wet start, the ATPL texts call it the blow-out cycle: the engine is motored with the HP fuel cock shut and the igniters off, clearing the unburnt fuel from the combustion chamber and jet pipe, which could otherwise torch when the next start lights it.

On the A320, a dry crank is set up with the ENG MASTER OFF, the ENG MODE selector at CRANK and the MAN START pushbutton ON; setting MAN START OFF stops it. A manual start interrupted after fuel has been introduced is followed by a dry crank. The 737 allows extended engine motoring within the limits in the table above.

Some engines motor themselves as part of a normal start. The E190-E2's PW1900G first turns at 8 to 11% N2 to counter rotor bow, the slight bending of a rotor that has cooled unevenly after shutdown, so a normal start takes between 30 and 78 seconds.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.