Which instrument stops functioning properly due to pitot icing?

• A. Airspeed indicator
• B. Altimeter
• C. Vertical speed indicator
• D. Attitude indicator

Answer: (B)

Pitot icing blocks the pitot tube, which the airspeed indicator needs to sense dynamic pressure. The airspeed indicator shows airspeed by comparing the pressure from the pitot tube (ram air) with the surrounding static pressure. When the pitot tube is iced over and blocked, the instrument can’t detect changes in dynamic pressure and may freeze at the current reading or display an erroneous value, effectively stopping its proper function.

The altimeter relies on static pressure alone to indicate altitude, so it remains functional as long as the static port isn’t blocked. The vertical speed indicator also uses static pressure, so it would be affected by static-port issues, not pitot icing specifically. The attitude indicator is powered by gyros, not the pitot-static system, so it’s not directly affected by pitot icing.

So the instrument affected by pitot icing is the airspeed indicator.