Pitot tube blockage affects which instrument?

• A. Altimeter
• B. VSI
• C. ASI
• D. None of the above

Answer: (C)

The airspeed indicator depends on ram pressure from the Pitot tube to determine airspeed. It works by comparing the stagnation (ram) pressure from the Pitot tube with the ambient static pressure from the static ports. If the Pitot tube is blocked, the instrument cannot receive the dynamic pressure it needs, so it cannot accurately compute airspeed (the static pressure alone doesn’t drive the ASI). The static ports continue to feed the altimeter and the vertical speed indicator, so they usually keep functioning as long as their own ports aren’t blocked. In practice, a blocked Pitot tube can cause the airspeed indicator to freeze at the last reading or read zero, while the altimeter and VSI readings remain usable.