What is the effect on jet engine thrust when humidity increases?

• A. Increases
• B. Slightly Decreases
• C. No Change
• D. Dramatically Increases

Answer: (B)

Humidity changes affect jet engine thrust mainly through the properties of the intake air. Thrust comes from how much air mass the engine moves and how much energy is released when that air is burned with fuel. When humidity increases, more water vapor is present in the air, which lowers the air’s density and reduces the amount of oxygen available per unit volume. With less dense air and slightly less oxygen, the engine ingests less air mass per second and burns a bit less fuel effectively, so the momentum change in the exhaust is reduced. The result is a small, but measurable, decrease in thrust rather than an increase. The effect is modest, so it’s not a dramatic change, nor is there no effect at all.