All turbine engines have common operational considerations including engine temperature limits, hot starts, foreign object damage, compressor stall and flameout. These potential problems are important to understand, especially in the turbine engine overhaul industry.
Engine Temperature Limitations
Turbine inlet temperature is where the highest temperature occurs in any turbine engine. This makes the turbine inlet temperature the limiting factor in turbine engine operation.
Hot and Hung Starts
A hot start is when the exhaust gas temperature (EGT) exceeds the safe limit in a turbine engine. This occurs when too much fuel vs. air enters the combustion chamber or when there is insufficient turbine r.p.m. A hung start is when an engine fails to accelerate to proper speed or does not accelerate to a sufficient idle r.p.m. This may be caused by insufficient battery power or fuel system component malfunction.
Foreign Object Damage
Debris ingestion is common among turbine engines due to the design and function of the air inlet. Foreign Object Damage (FOD) normally consists of small nicks and dents when small objects on runways or taxiways are drawn into the engine by the air inlet. This debris typically causes nicks and dents, but usually do not cause major damage. However, bird strikes or ice ingestion can cause enough damage to a turbine engine that the entire engine can be destroyed. For this reason, prevention of FOD is a high priority.
The simple definition of a compressor stall is an imbalance between inlet velocity and compressor rotational speed. When the compressor blade angle of attack exceeds the critical angle of attack, smooth airflow is interrupted and turbulence is created. Compressor stalls can be intermittent or steady. Recovery from a compressor stall must be accomplished quickly by reducing power, decreasing the airplane’s angle of attack and increasing airspeed. Very subtle throttle changes are the order of the day!
When the fire in a turbine engine goes out unintentionally, it is known as a flameout. When an overly rich mixture causes the fuel temperature to drop below the combustion temperature, known as a rich flameout, it is normally caused by very fast engine acceleration. Sometimes, flameout occurs due to low fuel pressure and low engine speeds especially in wet, cold weather coupled with air turbulence.
Engine thrust varies with air density. Turbine engines are affected by high relative humidity and as air density decreases so does thrust. Turbine engines can experience a loss of thrust in high relative humidity.
Operational considerations must be understood when flying or working on turbine engines. Knowing why an engine may have a hot start or what to do during a compressor stall is necessary for determining what needs to happen during aircraft maintenance. Covington Aircraft, a leader in turbine engine overhaul, invites you to learn more about operational considerations at www.covingtonaircraft.com.