What happens to a turbine engine from the time that a start is initiated until stable idle is achieved?
What can go wrong?
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.
Refer to the image below (Credit: K. Aainsqatsi, Wikipedia):
The portions of the engine I will be referring to below are:
The process of the engine starting follows this basic formula
What can go wrong?
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:
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.
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.
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.