Which factor has the greatest effect on thrust?

• A. Temperature
• B. Humidity
• C. Altitude
• D. Air density

Answer: (A)

Thrust depends mainly on how much air mass the engine can push and how fast the exhaust leaves. The mass flow of air through the engine is tied to air density: for the same engine speed and intake size, denser air means more air mass can be accelerated per second, giving more thrust.

Ambient temperature has the strongest direct effect on air density among the options. Warmer air is less dense, so the engine ingests less air mass and thrust falls; cooler air is denser, increasing mass flow and thrust. Humidity changes density slightly—water vapor is lighter than dry air, so humid air is a touch less dense—but the change is relatively small, so its impact on thrust is minor in comparison to temperature.

Altitude affects density as well because higher altitude means lower pressure, which reduces density and thus thrust; however, for the typical day-to-day variations and the way the question is framed, the direct impact of temperature on density—and therefore on thrust—is the dominant factor among the choices.