FAA Instrument Rating Airplane (IRA)Flight Instruments and SystemsEasy
A pilot's pitot tube ram-air inlet becomes obstructed by ice, but the drain hole remains open and the static system is unobstructed. If the pilot enters a climb, the airspeed indicator will most likely:
- ARemain frozen at the airspeed indicated at the moment of blockage
- BIndicate a higher airspeed than actual
- CIndicate a decreasing airspeed, approaching zero
- DIndicate airspeed accurately because the drain hole compensates for the blockage
Show answer & explanationAnswer & explanation
Correct answer: C. Indicate a decreasing airspeed, approaching zero
With the ram-air inlet blocked but the drain hole open, trapped pitot pressure slowly vents and equalizes with the falling static pressure as altitude increases. This causes the differential pressure driving the ASI to decrease, so indicated airspeed decreases and can approach zero even though true airspeed is constant.
Why the other options are wrong
- A. A frozen reading occurs only if both the inlet and drain are blocked, trapping pressure.
- B. Airspeed decreases, not increases, because trapped pressure equalizes toward the lower static pressure at altitude.
- D. The drain hole does not compensate; it actually causes the erroneous behavior described.
Partial Pitot Blockage (Drain Open)
When the ram-air inlet is iced over but the drain hole stays open, the pitot system slowly equalizes with static pressure, making the ASI behave like an altimeter.
- ASI decreases in a climb, increases in a descent
- Opposite result occurs if both inlet and drain are blocked (frozen reading)
- Pitot heat prevents this scenario
Memory trick: Drain open = ASI 'drains' toward altimeter behavior.