Aircraft Systems

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