FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
During a simulated instrument approach, a pilot notices the airspeed indicator is indicating significantly lower than expected, while the altimeter and vertical speed indicator appear to be functioning normally. What is the most likely cause of this discrepancy?
- AA malfunctioning vacuum pump.
- BA blocked pitot tube.
- CA clogged drain hole in the pitot tube.
- DA blocked static port.
Show answer & explanationAnswer & explanation
Correct answer: B. A blocked pitot tube.
The airspeed indicator relies on both pitot pressure (ram air) and static pressure. If the pitot tube becomes blocked, ram air pressure cannot enter, causing the airspeed indicator to read zero or significantly low, especially at higher speeds. The altimeter and vertical speed indicator use only static pressure, so they would remain unaffected.
Why the other options are wrong
- A. A malfunctioning vacuum pump affects vacuum-driven gyroscopic instruments (attitude indicator, heading indicator), not pitot-static instruments.
- C. A clogged drain hole in the pitot tube, with the pitot tube itself blocked, would trap pressure and cause the airspeed indicator to act like an altimeter, increasing with climb and decreasing with descent.
- D. A blocked static port would affect the altimeter (freezing) and VSI (zero), and cause the airspeed indicator to read incorrectly (overread in climb, underread in descent).
Blocked Pitot Tube
A malfunction where the pitot tube (which measures ram air pressure) becomes obstructed, primarily affecting the airspeed indicator.
- Airspeed indicator reads low or zero.
- Altimeter and VSI usually unaffected.
- Can be caused by ice, insects, or debris.
Memory trick: Pitot is Speed, Static is Altitude.