Static Blockage in a Descent
You are hand-flying an unpressurized single-engine aircraft in IMC. You are level at 8,000 ft, then cleared to descend to 5,000 ft. You reduce power and establish a 500 ft/min descent. Two minutes later you notice the airspeed indicator has risen from 120 kt to 145 kt, although pitch attitude and power are unchanged. The altimeter still reads 8,000 ft, and the vertical speed indicator is exactly zero. The outside air temperature is −2°C, and there is light rime icing on the windshield and wing strut. Pitot heat is on. The turn coordinator and heading indicator are normal. ATC asks for your altitude. You have a GPS moving-map with terrain, but no radio altimeter. The aircraft has a selectable alternate static source. The destination is 20 min away in IMC; the minimum safe altitude in the area is 6,000 ft.
1. Diagnose the most likely instrument failure and explain why the airspeed, altimeter and VSI indications are consistent with it.
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The most likely cause is a blocked static port, probably ice. The altimeter and VSI are connected only to static pressure. If static pressure is trapped, the altimeter freezes at the trapped altitude and the VSI reads zero.
The ASI uses pitot pressure and static pressure. In a descent, ambient static pressure rises, but the trapped static pressure remains near the 8,000 ft value. The pitot pressure rises as the aircraft descends, while the reference static pressure is too high, so the ASI overreads. Pitot heat is on, and a pitot blockage would not freeze the altimeter and VSI; it would make the ASI behave like an altimeter. The combination of a frozen altimeter, zero VSI and an overreading ASI in a descent points to the static source.
2. What immediate actions do you take, and what changes should you expect when you select the alternate static source?
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Maintain pitch and power, do not chase the ASI, and tell ATC that your altitude is unreliable. Select the alternate static source. Expect a transient jump in the altimeter, VSI and ASI because the cabin static pressure is not identical to the blocked outside static pressure. Treat the first readings as unreliable and allow them to settle.
Cross-check with GPS altitude for situational awareness, not for separation. If the alternate source gives a sensible VSI and ASI, continue with increased caution. If not, climb to the minimum safe altitude of 6,000 ft and divert to VMC or the nearest suitable airport. Do not continue a 20 min IMC segment with an unreliable altimeter when a safer diversion is available.
3. Why is the Mode C altitude ATC sees also suspect, and how should you phrase the call?
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The Mode C encoder usually uses the same static source. If the static port is blocked, the encoder may continue transmitting the trapped 8,000 ft even while you descend, so ATC may not see your actual altitude. That is a traffic-separation hazard.
Use a clear call such as: 'Mayday, static blockage, altitude unreliable, Mode C unreliable, request vectors and nearest suitable airport.' Do not enter cloud below the minimum safe altitude without a reliable altimeter. Use GPS terrain awareness with a wide margin, and accept ATC altitude verification only as an aid, not as a replacement for a working altimeter.