Frozen Altimeter and Rising Airspeed in IMC
During an IMC descent in a non-pressurized single, the pilot reduces power and establishes a 500 ft/min descent from 8,500 ft to 6,000 ft. The attitude indicator shows the correct descent attitude, but the altimeter remains frozen at 8,500 ft and the VSI reads zero. The airspeed indicator steadily rises from 95 kt to 115 kt although pitch and power are unchanged. OAT is -2°C and the aircraft has been in visible moisture; pitot heat is on. The static system has an alternate static source. GPS altitude shows 8,100 ft and decreasing; the minimum safe altitude ahead is 9,000 ft and terrain rises to 7,000 ft. ATC has been advised of the descent but not of any instrument problem. The pilot is current and the autopilot is off.
1. Identify the most likely system failure and explain why the ASI increases in the descent while the altimeter and VSI are frozen.
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The likely failure is a blocked static port, probably ice, because pitot heat protects the pitot tube but not necessarily the static port. The altimeter is frozen at 8,500 ft because it is still seeing the trapped static pressure from that altitude. The VSI reads zero because the trapped static pressure is not changing.
In a descent, the trapped static pressure is lower than the actual outside static pressure at 8,100 ft. The ASI measures pitot pressure minus static pressure. With the static side trapped at a lower pressure, the differential is larger than it should be, so the ASI overreads and rises even though pitch and power are unchanged. This is consistent with a static blockage, not a pitot blockage.
2. State immediate actions, including use of the alternate static source, and the likely errors after selecting it. What altitude reference should be used?
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Level off and climb immediately toward the 9,000 ft minimum safe altitude; the GPS shows 8,100 ft and decreasing while terrain rises to 7,000 ft. Select the alternate static source, advise ATC of the instrument problem and request a climb. Cross-check with GPS altitude and the attitude indicator; do not continue the descent to 6,000 ft.
After selecting the alternate static source, expect the altimeter to read somewhat high in a non-pressurized cabin because cabin pressure is slightly lower than outside static pressure. The ASI may also read high, and the VSI will show a transient change. Treat these indications as approximate, use GPS altitude and ATC terrain separation, and fly a known safe altitude rather than chasing the faulty altimeter.
3. How would the indications differ if only the pitot tube were blocked?
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If only the pitot tube were blocked and the static port remained open, the altimeter and VSI would continue to work normally because they use static pressure only. The ASI would become unreliable: with the pitot line blocked and its drain open, it would act like an altimeter, increasing in a climb and decreasing in a descent. If the drain were also blocked, the ASI could simply freeze. In this scenario the altimeter and VSI are frozen, so the fault is on the static side, not the pitot side.