An aircraft's pitot tube becomes completely blocked by ice while the drain hole remains open, and the aircraft then climbs to a higher altitude. What will happen to the airspeed indicator?
- AIt will freeze at the last correct reading before the blockage occurred
- BIt will indicate an increase in airspeed as altitude increases
- CIt will read accurately throughout the climb
- DIt will read zero regardless of altitude change
Show answer & explanationAnswer & explanation
Correct answer: B. It will indicate an increase in airspeed as altitude increases
With the pitot tube blocked but the drain hole open, the pitot line traps the pressure that existed at the moment of blockage while the static line pressure continues to decrease normally as altitude increases. Since airspeed indication depends on the differential between pitot and static pressure, the falling static pressure during the climb causes the airspeed indicator to falsely show an increasing airspeed.
Why the other options are wrong
- A. It does not freeze at the last reading; the differential still changes as static pressure drops.
- C. The system is no longer receiving updated ram air pressure, so it cannot read accurately.
- D. Zero indication would occur only if both pitot and static pressures equalized through an open drain to ambient.
Blocked Pitot Tube Effect (Drain Open)
When the pitot tube is blocked but its drain hole remains open, airspeed indication behaves like an altimeter, rising during climb and falling during descent due to changing static pressure.
- Trapped pitot pressure stays constant
- Static pressure changes normally with altitude
- Airspeed indicator falsely mimics altitude changes
Memory trick: Blocked pitot with open drain turns your airspeed dial into a fake altimeter