Static Port Blockage During Climb
You are flying an unpressurised twin turboprop climbing through 12,000 ft in icing conditions. The altimeter has stopped at 9,800 ft, the VSI reads zero, and the airspeed indicator has dropped from 160 to 145 KIAS while pitch and power remain unchanged. The outside air temperature is −15°C. The aircraft has a heated pitot head but no heated static port. The autopilot is engaged in altitude hold, and ATC has cleared you to climb to FL180. The standby altimeter is fed from the same static source. You suspect a static port blockage and must decide what to do before entering controlled airspace at FL180.
1. Explain the three instrument indications and confirm whether they are consistent with a blocked static port.
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Yes. A blocked static port traps the pressure corresponding to 9,800 ft. As the aircraft climbs, ambient pressure falls, but the trapped pressure remains, so the altimeter cannot sense the change and stays at 9,800 ft. The VSI measures rate of change of static pressure; with trapped pressure it sees no change and reads zero. The ASI measures pitot pressure minus static pressure; because the trapped static pressure is higher than ambient, the differential is reduced, so indicated airspeed underreads. The 15 kt loss is consistent with a static blockage in a climb, not with a power loss. The standby altimeter is also unreliable because it shares the same source.
2. What actions would you take, and what errors would you expect after selecting the alternate static source?
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Disengage the autopilot altitude hold, maintain a safe pitch and power setting, and advise ATC of the unreliable altitude indication. Select the alternate static source. In an unpressurised aircraft the cabin static pressure is usually slightly lower than ambient, so expect a momentary VSI climb, an altimeter overread of a few hundred feet, and a small ASI overread. Apply the AFM correction and cross-check with GPS altitude and ATC radar. Do not use the frozen altimeter for terrain separation. If icing is the cause, descend to warmer air if terrain and airspace allow, and consider a diversion.
3. How would you manage altitude and altimeter setting for the rest of the flight?
Read the worked answer
Use the alternate static source with the known correction as the primary pressure altitude, and cross-check against GPS and ATC. Keep the correct QNH/QNE setting; do not set 1013 until in the standard pressure region. If no reliable pressure altitude remains, declare an emergency and request vectors to a terrain-free altitude or VMC. Do not enter FL180 without a reliable altitude reference. After landing, have the static system inspected and leak-checked before further flight.