Figure 7-28.  Comparison of normal and distorted airflow into the  compressor section.  Compressor stalls can be transient and intermittent or steady  and severe. Indications of a transient/intermittent stall are  usually an intermittent “bang” as backfire and flow reversal  take place. If the stall develops and becomes steady, strong  vibration and a loud roar may develop from the continuous  flow reversal. Often, the flight deck gauges do not show  a mild or transient stall, but they do indicate a developed  stall. Typical instrument indications include fluctuations  in rpm and an increase in exhaust gas temperature. Most  transient stalls are not harmful to the engine and often correct  themselves after one or two pulsations. The possibility of  severe engine damage from a steady state stall is immediate.  Recovery must be accomplished by quickly reducing power,  decreasing the aircraft’s AOA, and increasing airspeed.  Although all gas turbine engines are subject to compressor  stalls, most models have systems that inhibit them. One  system uses a variable inlet guide vane (VIGV) and variable  stator vanes that direct the incoming air into the rotor blades  at an appropriate angle. To prevent air pressure stalls,  operate the aircraft within the parameters established by the  manufacturer. If a compressor stall does develop, follow the  procedures recommended in the AFM/POH.  A flameout occurs in the operation of a gas turbine engine in  which the fire in the engine unintentionally goes out. If the  rich limit of the fuel-air ratio is exceeded in the combustion  chamber, the flame will blow out. This condition is often  referred to as a rich flameout. It generally results from  very fast engine acceleration where an overly rich mixture  causes the fuel temperature to drop below the combustion  temperature. It may also be caused by insufficient airflow  to support combustion.  Figure 7-29  shows how four types of engines compare in net  thrust as airspeed is increased. This figure is for explanatory  Equivalent shaft horsepower (ESHP)—with respect  to turboprop engines, the sum of the shaft horsepower  (SHP) delivered to the propeller and THP produced by the  exhaust gases.  When comparing performance, the following definitions  are useful:  It is possible to compare the performance of a reciprocating  powerplant and different types of turbine engines. For  the comparison to be accurate, thrust horsepower (usable  horsepower) for the reciprocating powerplant must be used  rather than brake horsepower, and net thrust must be used  for the turbine-powered engines. In addition, aircraft design  configuration and size must be approximately the same.  Symptoms of a flameout normally are the same as those  following an engine failure. If the flameout is due to a  transitory condition, such as an imbalance between fuel  flow and engine speed, an airstart may be attempted once  the condition is corrected. In any case, pilots must follow  the applicable emergency procedures outlined in the AFM/  POH. Generally these procedures contain recommendations  concerning altitude and airspeed where the airstart is most  likely to be successful.  Any interruption of the fuel supply can result in a  flameout. This may be due to prolonged unusual attitudes,  a malfunctioning fuel control system, turbulence, icing, or  running out of fuel.  A more common flameout occurrence is due to low fuel  pressure and low engine speeds, which typically are  associated with high-altitude flight. This situation may also  occur with the engine throttled back during a descent, which  can set up the lean-condition flameout. A weak mixture can  easily cause the flame to die out, even with a normal airflow  through the engine.  Performance Comparison  Flameout  7-24  • Brake horsepower (BHP)—the horsepower actually  delivered to the output shaft. Brake horsepower is the  actual usable horsepower.  • Net thrust—the thrust produced by a turbojet or  turbofan engine.  • Thrust horsepower (THP)—the horsepower equivalent  of the thrust produced by a turbojet or turbofan engine.  Normal inlet airflow  Distorted inlet airflow 