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What happens to a turbine engine from the time that a start is initiated until stable idle is achieved?

What can go wrong?

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2 Answers

The means of starting most turbine engines is high pressure bleed air. This bleed source typically comes from an APU but may also come from an already running engine (crossbleed start) or an external huffer cart.

In some airplanes the start is completely FADEC controlled and need only be initiated and monitored by the flight crew, and in other airplanes certain steps must be manually initiated.

The process below best describes a turbofan engine. Read @Falk s answer for information on differences with turboprop engines.

Turbofan basics

Refer to the image below (Credit: K. Aainsqatsi, Wikipedia): Turbofan diagram

The portions of the engine I will be referring to below are:

  • The N1 fan, N1 shaft and N1 turbine which are displayed in green and labelled "Fan/Low-pressure compressor", "Low pressure shaft" and "Low pressure turbine". These units are connected and move as one piece.
  • The N2 compressor, N2 shaft and N2 turbine, displayed in purple and labelled "High pressure compressor", "High Pressure shaft" and "High pressure turbine". These are connected and move as one, but independently of the N1 shaft. Un-pictured is an accessory drive that is geared to the N2 shaft to drive engine accessories.
  • The hot section, pictured yellow and labelled "Combustion chamber". This is home to a constantly burning fire in a jet engine.

Engine Start

The process of the engine starting follows this basic formula

  • Through the opening of bleed air valves, bleed air is sent to an air turbine starter. These devices typically use the high pressure bleed air to spin and engage a centrifugal clutch connected to the engines accessory drive. This in turn causes the N2 shaft within the engine to spin.

  • With the N2 shaft spinning, the N2 compressor and the N2 turbines are spinning. This begins to force air through the engine from front to back.

  • With the accessory shaft and N2 shafts spinning, accessories should start working and this can be verified by oil pressure indications on the EICAS.

  • With increased N2 rotation, ignition will be turned on. These igniters are located in the hot section of the engine and produce small sparks. There should be an indication on the EICAS that the ignition is active.

  • With further increase in N2 rotation, fuel flow will be introduced. This will be verified on the EICAS. Once fuel flow is noted, it is important that the next stop happen fairly soon.

  • Light off! The fuel is lit by the ignition and now the fire burning in the hot section supplied by air from the compressor is producing thrust across the N2 and N1 turbines.

  • As the engine is producing thrust across on the N1 turbine, the N1 shaft is spinning the N1 fan and the EICAS will note this increase in N1 rotation. N1 and N2 rotation speeds increase.

  • Above a N2 threshold, bleed air valves supplying the the air turbine starter will close and the starter disengage. The igniters will turn off at some N2 threshold.

  • The engine will settle into a stable idle thrust setting.

What can go wrong?

  • Hung start

    • N2 fails to spin up sufficiently for fuel flow to be introduced.
    • Each start has a time limit and the start will be aborted.
  • No ignitors

    • Abort the start and switch igniters.
    • Give MX a call.
  • Failure to ignite

    • Fuel flow is introduced but light off has not occurred in a proper timeframe
    • Fuel is building up in the engine and if light off does occur, it can be damaging.
    • The start is aborted and dry motoring is performed to clear the engine.
  • ITT exceedence

    • There is usually a limitation in the inter-turbine temperature during engine start.
    • If this is exceeded the start must be aborted
    • This may require coordination with MX before another start is attempted.
  • The air turbine starter fails to disengage

    • This must be corrected before departing
    • In some engines, MX can correct this with the engine running.
  • The bleed air shutoff valve supplying the air turbine starter fails to close

    • This may require an engine shutdown to remedy
    • However, in some airplanes, this valve can be manually shut by MX with the engine running
    • Coordination with MX required.
  • Runaway N1 or N2

    • If the FADEC isn't managing fuel flow properly, the engine may not settle but continue to spin up.
    • Abort the start.
    • Coordinate with MX before attempting a restart.

What about starting the engine in the air?

If the engine suffers a flameout, an airborne restart may be attempted. These starts typically happen one of a few ways:

  • Crossbleed start
  • APU start
  • Windmilling start

The APU start is essentially the same process as above. The crossbleed start, which can also be done on the ground, merely substitutes a running engine at a high power setting to provide the bleed air for starting and is otherwise the same as above.

The interesting start is the windmilling start. The necessity for this means something bad has happened. To need a windmilling start, this means that there are no bleed air sources to supply the air turbine starter. This can mean that all engines are out and the APU is unavailable (BAD!), or merely that bleed valves to a shutdown engine have failed closed and are unable to be opened.

For the EMB-145 that I am familiar with, a windmilling start required descending at an airspeed between 260 KIAS and 320 KIAS and could not be attempted above FL250. In short, you hope that the mass flow through the engine is enough to spin the N2 compressor as the ATS would. With an N2 indication within the engines airstart envelope, you introduce spark and fuel and hope that the engine lights. In the worst case, if you too slow and unable to provide enough airflow prior to light off, the engine can quickly overtemp and be damaged. For this reason it is especially important to abort this kind of start as soon as an abnormality is detected.

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There's also the failure of the bleed air start valves, which has happened before (as a pax). –  Qantas 94 Heavy Feb 27 at 6:46
can you also add the differences with mid-flight restarts (with and without another engine to provide power to provide bleed air) –  ratchet freak Feb 27 at 9:41
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If you like to know about jet engines, please read @casey s answer. It's very detailed and good.

There are some turbine engines which are started a different way. We are talking about turbo prop or turbo shaft engines. A famous example is the PT6 turbine, delivering power to many small turbine powered propeller airplanes like Beachcrafts King Air series and Pipers Cheyenne and JetProp.

This engines consist of two parts, the gas generator and the free power turbine. The gas generator is pretty much a jet engine. It consists of an inlet, a compressor, an combustion chamber and a turbine. I don't know any turbo prop engine with a nozzle to create thrust, like a jet engine has one, but if one of you knows about it, I would be keen to read your comment. On a turbo shaft engine the exhaust isn't at all used to generate thrust so some of them are even inverted (compressor at the rear part of the engine, but of cause still feeded with air from an inlet directed foreward). You maybe noticed that at some turbo shaft driven aircraft, the exhaust is very close to the prop - here's the reson.

The second component is a free power turbine, driven by the exhaust of the gas generator, driving the propeller via a reduction gear box (free power turbine speed > 30.000 RPM , prop speed < 3.000 RPM). In most engines these components are only hold together by the engine case, that's why it's called free power turbine.

Now we all have a basic idea of this kind of engine and can talk about the start sequence. A so called starter generator is connected to the gas genetator. Basically it's an electro motor powered by the battery or any other DC power source used to crank the gas generator shaft. Once the engine is started this 'motor' can be used as a generator delivering DC power. If you look at the principles of an electro motor and a DC geberator you'll see that they are technically the same. You only need to change sine connections. This is usually done by operatin a three position rocker switch 'start-off-generator'.

The start sequence is pretty much the same as it is at a jet engine.

  • Crank the gas generators shaft with the starter
  • At a specific rotation speed (usually 10-15%) add fuel
  • Monitor the engine parameters
  • Switch to the generator

Usually the free power turbine starts turning the propeller very soon after fuel is added, anyway some engines are equipped with a propeller brake preventing the free piwer turbine section from spinning. This is useful to operate the gas generator section only to obtain electric or hydraulic, sometimes pneumatic power - some kind of APU. Aerospitals ATR series is a famous example. The called it hotel mode.

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Thanks for the compliment and broadening the answers to cover more than the turbofan in my answer. –  casey Feb 27 at 3:30
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