Gas Turbine Performance Aspects: Flat Rating, Flex Temperature, and EPR

021-11-05Pablo Asensio Martínez2026-03-272 min

Thrust and Pressure

  • Air Acceleration: As the engine accelerates the air, dynamic pressure increases and static pressure decreases (Bernoulli's principle).
  • Specific Thrust: Defined as Net Thrust per unit of Air Mass Flow.
    • If intake airflow increases (due to higher speed/density) while net thrust remains constant, specific thrust decreases.

Flat-Rated Engines

Turbine engines are often "flat-rated" to provide consistent performance across a range of temperatures and to protect the engine components.

  • Flat-Rated Range (Cold/Standard OAT):
    • Occurs when Outside Air Temperature (OAT) is below the "flat-rated temperature" (typically around ISA +15°C).
    • Thrust is limited by internal pressure limits (Maximum Mass Flow / Structural limit).
    • The engine produces a constant maximum thrust regardless of the OAT dropping further.
  • Temperature Limited Range (Hot OAT):
    • Occurs when OAT is above the flat-rated temperature.
    • Thrust is limited by EGT/TGT (Exhaust/Turbine Gas Temperature) to prevent melting or creep of turbine blades.
    • As OAT increases, maximum available thrust decreases.

Reduced Thrust Take-off (Flex / Assumed Temp)

Used to extend engine life by using less than maximum thrust when takeoff performance allows (light weight, long runway).

  • Principle: The FADEC/Computer is "tricked" into thinking the outside air is hotter than it actually is.
  • Method:
    • The pilot enters an Assumed Temperature (or Flex Temp) into the FMS.
    • This assumed temperature is higher than the actual OAT.
    • Since the engine is "temperature limited" at high OATs, the computer calculates a lower N1/EPR limit.
    • Result: The engine produces less thrust, running cooler (lower EGT), reducing wear.

Thrust Indication: EPR

  • EPR (Engine Pressure Ratio): A primary thrust parameter on many jet engines (others use N1).
  • Definition: The ratio of Low-Pressure Turbine Exhaust Pressure ($P_{exit}$) to Compressor Inlet Pressure ($P_{inlet}$).
    • $EPR = \frac{\text{Total Pressure at Turbine Exit}}{\text{Total Pressure at Compressor Inlet}}$
  • Sensors: Can be electromechanical bellows or digital transducers.