Airspeed indicators derive airspeed from the difference between which pressures?

• A. Static pressure and total (ram) pressure.
• B. Instruments: static pressure and engine oil pressure.
• C. Total pressure and cabin pressure.
• D. Temperature difference.

Answer: (A)

Airspeed is determined from dynamic pressure, the part of the air pressure caused by motion. The instrument uses two pressures: the stagnation (total or ram) pressure from a forward-facing pitot tube and the ambient static pressure from a static port. The airspeed indicator responds to the difference between these two pressures, which equals the dynamic pressure (q = 1/2 ρ v^2). This dynamic pressure is what changes as speed changes, so the indicator is calibrated to display airspeed based on that pressure difference. The other pressures listed aren’t what the ASI uses to measure speed—for example, engine oil pressure, cabin pressure, or just total pressure without subtracting static pressure don’t relate to how fast the aircraft is moving.