SU 30 MKM
The procedure for starting a jet engine begins with an Auxiliary Power Unit onboard the plane. It is technically a small jet engine that starts first and then initiates the startup of the main engine (either of the two, in case of twin engine planes). It does so by channeling it’s bleed air to turn the compressor stage. Once the compressor has spun to its self-idling RPM, the bleed air from it can be channeled to start the second engine.