During a climb, a pilot's pitot tube becomes blocked by debris, but the drain hole remains open and the static ports remain clear. What will the airspeed indicator most likely do as the airplane continues to climb?
- AIt will continue to read accurately because the drain hole compensates for the blockage
- BIt will freeze and display a constant, unchanging airspeed value
- CIt will behave like an altimeter, showing increasing indicated airspeed during the climb regardless of actual airspeed
- DIt will immediately drop to zero
Show answer & explanationAnswer & explanation
Correct answer: C. It will behave like an altimeter, showing increasing indicated airspeed during the climb regardless of actual airspeed
With the pitot tube blocked but the drain hole open, the pitot pressure becomes trapped at the pressure that existed at the moment of blockage, while the static pressure inside the instrument case continues to change with altitude. As the aircraft climbs, static pressure decreases, increasing the pressure differential and causing the ASI to falsely indicate increasing airspeed, behaving essentially like an altimeter.
Why the other options are wrong
- A. Incorrect — the drain hole does not correct the fundamental error caused by a blocked pitot inlet.
- B. Incorrect — the open drain hole allows static pressure to still affect the reading.
- D. Incorrect — a total pitot and static blockage would cause this, not a pitot-only blockage with open drain.
Blocked Pitot Tube with Open Drain Hole
A pitot-static malfunction where the pitot inlet is blocked but the drain hole remains open, causing the airspeed indicator to behave like an altimeter, increasing in climbs and decreasing in descents.
- Trapped pitot pressure stays constant at the pressure when the blockage occurred
- Static pressure continues to act on the instrument via the open drain hole
- ASI falsely shows increasing airspeed in climbs, decreasing in descents
Memory trick: Blocked pitot with open drain turns your ASI into a fake altimeter