During climb-out in IMC, a Cessna 172's pitot tube becomes completely blocked by ice at the ram-air inlet, but the pitot drain hole remains open and unobstructed. The static system is functioning normally. What will the airspeed indicator most likely do?
- ADecrease toward zero regardless of the aircraft's actual airspeed
- BFluctuate rapidly and erratically with every control input
- CRead a steadily increasing airspeed as altitude increases, similar to an altimeter
- DFreeze at the last correct airspeed reading and remain constant
Show answer & explanationAnswer & explanation
Correct answer: A. Decrease toward zero regardless of the aircraft's actual airspeed
With the drain hole open, trapped air in the pitot line can escape and equalize with ambient (static) pressure instead of holding ram-air pressure. Because the airspeed indicator responds to the difference between ram (pitot) and static pressure, this difference collapses toward zero, so the ASI reads a falsely low airspeed approaching zero regardless of the aircraft's true speed. This differs from a fully sealed blockage with no drain hole, which instead causes the ASI to behave like an altimeter.
Why the other options are wrong
- B. Erratic fluctuation is not characteristic of a simple pitot blockage with an open drain.
- C. That result occurs only when the pitot line is completely sealed with no functioning drain hole.
- D. Freezing at a constant reading would require both the tube and drain hole to be sealed with a rigid trapped pressure.
Pitot Blockage with Open Drain Hole
When only the pitot tube inlet is blocked but the drain hole stays open, trapped pressure leaks to ambient static pressure, causing the airspeed indicator to read toward zero regardless of actual speed.
- Fully sealed blockage (no drain) makes ASI act like an altimeter
- Open drain hole allows pressure to bleed to ambient, causing ASI to read low/zero
- Pitot heat prevents ice-induced blockages in flight
Memory trick: Drain open = pressure drains away, needle fades away