Turbine Engine Operational Considerations

[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 7-23, 7-24]

There are certain operational considerations common to all turbine engines :

  1. engine temperature limits
  2. Air density variation
  3. foreign object damage (FOD)
  4. hot/hung start
  5. compressor stall
  6. flameout (engine failure)

 

Engine temperature limits

Guess what, the highest temperature in any turbine engine occurs at the turbine inlet. TIT is therefore usually the limiting factor in turbine engine operation.

Instrument to monitor the risk: EGT gauge

 

Air Density variation

Air Density ↓ (Hot, Humid, High) ⇒ Thrust ↓

Turbine engine thrust varies directly with air density.

Instrument to monitor the risk:

 

Foreign Object Damage (FOD)

Due to the design of turbine engine’s air inlet, the possibility of ingestion of debris always exists. The debris can be small objects from the ramp, taxiway, or runway, or even bird strikes or ice ingestion.

When ingestion of debris occurs, it is called foreign object damage (FOD) – causes significant damage, particularly to the compressor and turbine sections.

Procedure to reduce the risk: Preflight procedures (visual inspection) for any sign of FOD

 

Hot/Hung start

“hot start” refers to the situation when the EGT exceeds the safe limit of an aircraft. There are (2) two major causes:

  1. too much fuel entering the combustion chamber
  2. “hung start”

 

“hung start” (a.k.a “false start”) occurs if the engine fails to accelerate to the proper speed after ignition – does not accelerate to idle rpm. There are (2) two major causes:

  1. insufficient starting power source
  2. fuel control malfunction

 

Instrument to monitor the risk:

 

Compressor Stalls

Compressor blades are small airfoils – they are subject to the same aerodynamic principles that apply to any airfoil.

A compressor blade’s AOA is a result of inlet air velocity and the compressor’s rotational velocity.

Compressor stalls occur when the compressor blades’ AOA exceeds the critical AOA. At this point, smooth airflow is interrupted and turbulence is created with pressure fluctuations. Compressor stalls cause air flowing in the compressor to slow down and stagnate, sometimes reversing direction.

There are two stages of compressor stalls – 1) transient/intermittent stall and 2) developed stall.

Indications of a transient/intermittent stall are usually an intermittent “bang” as backfire and flow reversal take place.

Often, the flight deck gauges do not show a mild transient stall, but they do indicate a developed stall – Typical instrument indications include fluctuations in rpm and an increase in EGT (exhaust gas temperature). If the stall develops and becomes steady, strong vibration and a loud roar may develop from the continuous flow reversal.

Instrument to monitor the risk:

Notes:

Severe engine damage from a steady state stall is immediate. Recovery must be accomplished by :

  1. quickly reducing power
  2. decreasing the aircraft’s AOA
  3. increasing airspeed

 

Flameout (engine failure)

A flameout occurs when the fire in the engine unintentionally goes out. The causes include but limit to:

  1. over-lean fuel-air mixture
  2. over-rich fuel-air mixture
  3. prolonged unusual attitudes
  4. malfunctioning fuel control system
  5. turbulence
  6. icing
  7. fuel exhaustion (running out of fuel)

In general, “Lean- condition flameout” is more common than “Rich flameout”. Of course, prolonged unusual attitudes, a malfunctioning fuel control system, turbulence, icing, or running out of fuel can lead to flameout.

Lean-condition flameout typically are associated with a descent after a high-altitude flight with engine throttled back (low fuel pressure and low engine speeds) – a weak mixture can easily cause the flame to die out, even with a normal airflow through the engine;

Rich flameout is the situation when the rich limit of fuel-air ratio exceeded in the combustion chamber, the flame will blow out. There are (2) major causes:

  1. very fast engine acceleration where an overly rich mixture causes the fuel temperature to drop below the combustion temperature
  2. insufficient airflow to support combustion

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. 

 

 

 

 

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