FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
During a preflight briefing, a client asks what happens to the airspeed indicator if the static port becomes completely blocked while the pitot tube remains clear during a climb. Which of the following will occur?
- AThe airspeed indicator will read lower than actual airspeed as altitude increases
- BThe airspeed indicator will freeze at the airspeed present when the blockage occurred
- CThe airspeed indicator will read higher than actual airspeed as altitude increases
- DThe airspeed indicator will immediately read zero
Show answer & explanationAnswer & explanation
Correct answer: C. The airspeed indicator will read higher than actual airspeed as altitude increases
With a blocked static port but clear pitot tube, the static pressure in the airspeed indicator case remains trapped at the pressure present at the altitude where the blockage occurred. As the aircraft climbs, ambient static pressure decreases, but the pitot (ram air) pressure changes normally, causing the differential to increase and the airspeed indicator to over-read as altitude increases.
Why the other options are wrong
- A. This is backwards; the indicator over-reads, not under-reads, during a climb with a blocked static source.
- B. The reading changes with altitude because pitot pressure still varies; only static pressure is frozen.
- D. A blocked static port does not cause a zero reading; that would happen with a blocked pitot tube and open drain hole.
Blocked Static Port Effects
If the static port is blocked, the airspeed indicator acts like an altimeter, reading high in a climb and low in a descent relative to the trapped reference pressure.
- Trapped static pressure stays constant
- Climb causes ASI to over-read
- Descent causes ASI to under-read
- Altimeter and VSI also freeze
Memory trick: Blocked static, ASI goes ballistic on climb-out