What is stagnation pressure?

• A. The ambient air pressure sensed by the pitot-static system.
• B. The static pressure at the cockpit crew area.
• C. The total pressure measured by the pitot tube when air is brought to rest.
• D. The pressure difference between ram air and cabin pressure.

Answer: (C)

Stagnation pressure is the total pressure you get when moving air is brought to rest relative to the aircraft. In a pitot tube, air is decelerated to zero velocity, converting its dynamic energy into pressure. The result is the sum of the ambient static pressure and the dynamic pressure from the speed, so the pitot reading is the total pressure only when air is brought to rest. This is the value used in airspeed calculations, since indicated airspeed is derived from the difference between this total (pitot) pressure and the static pressure.

The other descriptions aren’t correct because stagnation pressure is not just the ambient static pressure, nor is it simply a static pressure inside the cockpit, and it isn’t the pressure difference between ram air and cabin pressure.