Aircraft Performance and Altitude

Gas Turbine Engine Performance

• At sea level, the ISA value is 14.7 psi, and pressure decreases with altitude, affecting engine performance.
• Temperature also decreases with altitude, but at the tropopause (36,089 ft), it remains at -56°C.
• Both pressure and temperature affect density, which impacts engine performance, with pressure having a greater effect.
• Density reduction is more significant due to pressure than temperature.

The Reactive Force of Thrust

• Thrust is not created by pushing outside air, but rather by the reacting force within the engine.
• Continuous flow cycle is utilized to create the acting force within a turbine engine.
• Gas turbine engines operate on a principle of continuous combustion, with one unit of mass airflow in and one unit out.
• Thrust is transmitted to the aircraft through the engine mounts.

Newton's Second Law

• Force is proportional to the product of mass times acceleration (F = ma).
• Mass and weight are different quantities, but in Earth's gravity, they can be treated as similar.
• Weight is mass with gravity applied (W = mg).

Turbojet and Turbofan Engines

• These engines burn fuel to accelerate a mass of gas rearwards, creating a forward reaction called thrust.
• Bypass air flow and fan duct design can affect engine performance.

Engine Types and Bypass Ratios

• Low-bypass engines have a bypass ratio of approximately 1:1.
• Medium-bypass engines have a bypass ratio of 2:1 to 3:1.
• High-bypass turbofan engines have a bypass ratio of 4:1 to <9:1 and offer higher propulsive efficiencies and better fuel economy.

Engine Ratings and Limitations

• International Standard Atmosphere (ISA) is used to standardize engine performance comparisons.
• ISA defines a temperature lapse rate of 1.98 °C per 1000 ft, resulting in a fall from 15 °C at sea level to -56.5°C at 36,089 ft.
• ISA standard pressure at sea level is 1013.2 mbar, 29.92 inHg, or 14.69 psi.

Engine Thrust in Flight

• Net thrust is determined by in-flight variables, including mass airflow, altitude, air temperature, humidity, airspeed, ram effect, ram drag, engine rpm, and engine conditions.

Engine Operating Limitations

• Engine operating limitations are determined by turbine and nozzle materials used.
• If limits are exceeded, borescope inspections may need to be performed.
• Engine operating limitations include maximum thrust settings, fuel and oil pressures, engine oil temperatures, engine operating temperature limitations, and engine maximum operating rpm.