Which instrument uses dual pressures?

• A. Airspeed Indicator
• B. Altimeter
• C. Vertical Speed Indicator
• D. Attitude indicator

Answer: (A)

The instrument being tested uses two pressure inputs to determine what it displays. It relies on both Pitot (total) pressure and ambient static pressure to measure dynamic pressure, which is what airspeed is based on. The Pitot tube provides the impact pressure, while the static port supplies the ambient static pressure. The airspeed indicator responds to the difference between these two pressures (dynamic pressure = Pt − Ps) and is calibrated so that this dynamic pressure translates into indicated airspeed.

Other devices use only static pressure (like the altimeter, which senses altitude from static pressure) or rely on a gyroscope or a different mechanism (like the attitude indicator) rather than pressure differences. The vertical speed indicator uses static pressure changes over time, not the dual-pressure input used for airspeed.