Which statement best describes the impact of humidity on jet engine thrust?

• A. It increases thrust
• B. It decreases thrust slightly
• C. It causes thrust to become unstable
• D. It has no effect on thrust

Answer: (B)

Humidity introduces water vapor into the intake air. This moist air is less dense than dry air at the same pressure and temperature, so the mass of air entering the engine per unit time decreases. Since jet thrust mainly comes from accelerating that mass of air to a higher speed, a smaller mass flow means a smaller thrust, and the change is typically small. In short, humidity lowers thrust slightly; it’s not a large effect, nor does it cause instability, and it’s not truly nonexistent.