While climbing through 5,000 feet, a pilot's static port becomes completely blocked by ice, but the pitot tube remains clear. Assuming the pilot continues the climb, how will the airspeed indicator most likely behave?
- AIt will read higher than the actual airspeed as altitude increases
- BIt will read lower than the actual airspeed as altitude increases
- CIt will read accurately since the pitot tube is unobstructed
- DIt will freeze at the airspeed indicated at the moment of the blockage
Show answer & explanationAnswer & explanation
Correct answer: B. It will read lower than the actual airspeed as altitude increases
With the static port blocked, the airspeed indicator continues to reference the trapped static pressure from the altitude at which the blockage occurred, which is higher than the actual (lower) static pressure at higher altitudes. Because the differential between pitot pressure and this artificially high static pressure is smaller than it should be, the airspeed indicator will read lower than the true airspeed as the airplane climbs.
Why the other options are wrong
- A. The ASI reads lower, not higher, when climbing with a blocked static source since the trapped static pressure is relatively too high.
- C. The pitot tube being clear does not fix the problem, since the ASI compares pitot and static pressures and static is now trapped and inaccurate.
- D. The altimeter would freeze at the blockage altitude, but the ASI, using differential pressure, does not simply freeze; it changes with altitude.
Blocked Static Port Effects
A blocked static port traps pressure at the altitude of the blockage, causing pitot-static instruments to misbehave predictably during subsequent altitude changes.
- Altimeter freezes at the altitude when static blocked
- ASI reads low during climb, high during descent (with static blocked)
- VSI will read zero after blockage, regardless of actual climb/descent
- Alternate static source or breaking the instrument glass provides emergency correction
Memory trick: Trapped static pressure lies about your climb — ASI reads too slow going up.