FAA Instrument Rating Airplane (IRA)Flight Instruments and SystemsMedium
During a climb, a pilot notices the altimeter frozen and the VSI reading zero. Troubleshooting reveals a blocked static port with no alternate static source available. Under this condition, as altitude increases, the airspeed indicator will most likely:
- AContinue to read accurately
- BRead higher than actual airspeed
- CRead lower than actual airspeed
- DRead zero immediately
Show answer & explanationAnswer & explanation
Correct answer: C. Read lower than actual airspeed
With static pressure trapped at the blockage altitude, the reference static pressure stays higher than the true (lower) ambient static pressure encountered at higher altitude. Since ASI reads pitot pressure minus static reference, this smaller-than-true differential causes the ASI to under-read in a climb (and over-read in a descent).
Why the other options are wrong
- A. The airspeed indicator becomes inaccurate once static pressure is trapped.
- B. Overreading occurs during a descent, not a climb, with this failure.
- D. The reading degrades gradually with altitude change, not immediately to zero.
Blocked Static Port Effects
A blocked static port freezes the altimeter and VSI at the blockage altitude/zero, and causes ASI to under-read in a climb and over-read in a descent because the static reference no longer matches actual ambient pressure.
- Altimeter freezes at blockage altitude
- VSI reads zero regardless of actual climb/descent
- Alternate static source restores accuracy if available
Memory trick: Trapped static lags reality—climb makes ASI look slow.