For a constant RPM in a gas turbine engine, a decrease in air density will cause which of the following?

• A. Increased Fuel flow and Increased Thrust
• B. Decreased Fuel flow and Decreased Thrust
• C. No change in Fuel flow or Thrust
• D. Decreased Fuel flow and Increased Thrust

Answer: (B)

At constant compressor speed, the engine’s thrust depends on how much air mass it can ingest and how much energy is added by fuel. When air density falls, the mass of air entering the engine for the same volume of intake decreases. With less air available, there’s less oxygen to burn, so the fuel flow is reduced to keep turbine temperatures within limits. The combination of less air mass and less fuel burned means less energy is added to the exhaust, resulting in lower thrust.