Three Instruments Disagree in IMC
You are flying a non-pressurised light twin in IMC, climbing to 11,000 ft. Pitot heat is ON. At 9,000 ft you notice the airspeed indicator has been slowly unwinding - it now reads 78 kt against a normal 105 kt for this power and attitude - while the altimeter has stopped at 9,020 ft and the VSI reads exactly zero. The attitude indicator, turn coordinator, engine instruments and the GPS ground speed (which has fallen from 118 kt to 96 kt as you climb, consistent with the climb) all look normal. The aircraft has an unheated pair of flush static ports on the aft fuselage, and a selectable alternate static source in the cabin. Outside air temperature is −8 °C with visible moisture. ATC has cleared you to FL110 and asks for your altitude.
1. Diagnose the failure and explain the mechanism behind each of the three readings.
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All three symptoms come from the static source. Trapped static pressure in the line is fixed at the value it had when the ports blocked, so the altimeter - which is only a pressure gauge - keeps showing the altitude corresponding to that pressure: 9,020 ft, frozen. The VSI measures rate of change of static pressure, hence exactly zero. The ASI measures pitot pressure minus static pressure; with static frozen at a lower-altitude value, the differential falls as the aircraft climbs and the airspeed indication unwinds. Pitot heat is on, so pitot icing is not the explanation; the unheated flush ports in visible moisture at −8 °C are. GPS ground speed, attitude and power confirm the aircraft is still flying normally, and that is the important part: the aeroplane is fine, the instruments are lying.
2. What happens when you select the alternate static source? Predict the direction of change in the ASI and altimeter and give your correction strategy.
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Selecting the alternate static source connects the instruments to cabin air, which is at a slightly lower pressure than true static because of airflow over the cabin and any extractor. Lower static pressure means the altimeter reads higher than the true altitude (typically a few tens of feet), and the ASI reads higher because the differential is larger. The VSI will show a brief climb and then settle. So the alternate source gives usable but biased information - do not simply accept the numbers. Fly on attitude, power and GPS, use the alternate source for trend, and apply the AFM correction if one is published. Tell ATC the altitude is unreliable and ask for a radar-verified altitude. Then leave the icing conditions and descend to VMC or a warmer layer.
3. Contrast this with a blocked pitot and state how you would tell them apart in flight.
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A blocked pitot is the opposite signature. If the pitot tube blocks (drain hole also blocked) and static is fine, the ASI sees a trapped pitot pressure against a falling static as you climb, so the ASI behaves like an altimeter: it climbs as you climb and unwinds in a descent, while the altimeter and VSI stay perfectly normal. With a blocked static, the ASI falls in the climb and both the altimeter and VSI freeze. The discriminator is therefore the altimeter and VSI: normal means pitot, frozen means static. In IMC the response is the same in principle - stabilise on attitude and power, cross-check with GPS and engine instruments, tell ATC, and get to conditions where you can see.