Static Port Blockage in IMC
You are climbing an unpressurised single-engine aircraft in IMC at 700 ft/min. At 7,000 ft the static port ices over. Pitot heat is on. The altimeter remains at 7,000 ft, the VSI reads zero, and the ASI slowly falls from 110 kt to 85 kt although attitude and power are unchanged. GPS groundspeed is 105 kt. Terrain ahead rises to 8,200 ft, ATC has cleared you to 9,000 ft, and you have 30 minutes of fuel to your alternate. The aircraft has an alternate static source that vents to the cabin, and a GPS with terrain warning. Cabin altitude is approximately ambient. You are in daylight and visual conditions are improving above 9,000 ft. There is no icing report from other traffic and the outside air temperature is −4 °C.
1. Diagnose the instrument problem and explain the ASI behaviour.
Read the worked answer
The trapped static pressure explains all three symptoms. The altimeter is fed by static pressure and freezes at 7,000 ft; the VSI sees no pressure change and reads zero. The ASI uses pitot pressure minus static pressure. As the aircraft climbs, true static pressure falls, but the blocked static line keeps the higher pressure from 7,000 ft. Pitot pressure falls with altitude, so the pitot-minus-static differential becomes smaller than it should be; the ASI underreads. At 85 kt indicated you may be closer to 110 kt true, but the indication is not trustworthy. In a descent the same blockage would make the ASI overread. Pitot heat being on does not clear a blocked static port.
2. State your immediate actions and how you would manage terrain clearance and ATC.
Read the worked answer
Maintain climb attitude and power; do not chase the falling ASI. Select the alternate static source, expecting a transient: the altimeter will jump toward the cabin-derived pressure, the VSI will swing, and the ASI will move. Tell ATC immediately, state that altitude and speed information is unreliable, request a block altitude and time to troubleshoot, and use the GPS terrain warning for awareness. Keep pitot heat on. Maintain the cleared 9,000 ft and do not descend until pressure altitude is reliable; GPS altitude is not a substitute for terrain clearance in cloud. If the alternate source does not restore plausible indications, divert to VMC and a suitable airport. Use attitude, power, and GPS groundspeed as primary references, and treat any approach in IMC as unavailable.