The jet engines like any other engine requires air, fuel and a source of combustion to start. If you have these three conditions the engine can be started.
In flight, if the engine fails, a restart can be attempted. A jet engine sucks in air through the inlet due to the rotation of the compressor. This compressor needs to be turned at a correct RPM to get the engine started. If it is turned too slow, the engine may fail to start (hung start) or it can also lead to a hot start due to the lack of air flow in the combustion chamber. On ground, a starter motor is used to turn the compressor. This motor is mechanically geared to the high pressure compressor of the engine. High pressure air is passed through this motor either from an APU (Auxiliary power unit) or a ground air supply unit. As the motor runs, it turns the compressor. Once the the compressor is run up to the correct RPM, fuel is introduced and the fuel/ air mixture ignited to light up the engine. As the engine self stabilize and achieve the self sustaining speed, the motor declutches from the compressor.
What we understood from the above explanation is that we need to run the compressor to get the engine started. When you are flying, and if an engine flames out, the compressor can still be turning or windmilling due to forward speed of the aircraft. If this speed is sufficient enough, the pilot can simply run the ignitors and introduce the fuel to get the engine restarted (air starter is not required). Every engine has a stabilized windmilling speed, and this speed can be found in the aircraft flight manual. For the aircraft I fly, the speed for stabilized windmilling is about 300 knots/ 0.78 Mach. At this speed, you are guaranteed a start without any assistance from the starter motor as windmilling is more than enough to get the engine fired up. If this speed cannot be achieved, you can always start the APU and get the bleed from it to run the starter motor which can then mechanically windup the compressor to the required RPM.
The aircraft manuals and emergency procedures provide the pilots with an engine relight envelope (see below) to check whether they need the starter motor assistance or not. We then use this envelope to adjust our relight procedures.
Author – Anas Maaz ( Airline Pilot )
