What effect does high atmospheric humidity have on the operation of a jet engine?

• A. Decreases compressor and turbine RPM.
• B. Has little or no effect.
• C. Decreases engine pressure ratio.
• D. Increases fuel consumption.

Answer: (B)

Moisture in the air changes the composition of the intake air, but for a jet engine this effect is negligible under normal operating conditions. The water vapor makes the air slightly less dense, so the mass flow entering the compressor is a bit lower for the same volume. Yet this density change is small enough that the engine control system and the engine maps compensate and keep the compressor operating at its intended conditions. So rpm, the engine pressure ratio, and overall fuel flow aren’t meaningfully altered by humidity alone. Real performance variations come from temperature and pressure conditions rather than humidity.

That’s why humidity has little or no effect. The other options imply noticeable changes in compressor speed, pressure ratio, or fuel consumption, which don’t occur with typical humidity levels.