Turbine Engines
Core concepts
Turbine engines are described by sections: inlet, compressor, diffuser, combustion section, turbine, and exhaust. Learn what each does and what happens when it fails. Compressors are centrifugal (impeller plus diffuser) or axial (alternating rotor and stator stages). Axial compressors use variable inlet guide vanes and bleed valves to avoid stall at low RPM.
A key fact: turbine engines run far leaner than stoichiometric. At high power the overall air/fuel ratio may be 60:1 or leaner, and much of the air bypasses the combustion zone for cooling. Fuel controls schedule fuel using N1/N2 and EGT and limit acceleration to prevent over-temperature or surge.
Expect questions on compressor stall and surge - symptoms include loud bangs, rising EGT, RPM fluctuation, and possible flameout; causes include distorted inlet airflow, bleed valve malfunction, and abrupt throttle movement. Distinguish a hot start (EGT exceeds limits during start) from a hung start (RPM stabilizes below idle). Know N1 (gas producer) versus N2 (power or free turbine); on a free-turbine engine N2 can be stationary while N1 turns.
Trap
Do not assume N1 always means fan. In a turbofan N1 is usually the fan/low-pressure spool; in a turboprop N1 is the gas producer and N2 is the propeller shaft.
Rule of thumb
Turbine engines are temperature-limited, not detonation-limited. If EGT approaches the limit before idle during start, abort. A start that has not reached idle within the starter time limit (often 30-60 seconds; check the AFM) is treated as a hung start.