Static Port Blockage After Repainting
During a 400-hour inspection at a NZ maintenance organisation, both static ports on a PA-31P Pressurized Navajo were masked for repainting. The aircraft was returned to service after a maintenance release. Three days later, on climb through 6,000 ft from Wellington (QNH 1012 hPa), the pilot reports: altimeter frozen at 1,200 ft, VSI zero, ASI falling from 140 KIAS to 90 KIAS with constant power, and cabin altitude rising normally. The pilot selects the alternate static source; the ASI recovers but reads 8 KIAS high and the altimeter moves. The operator asks whether the instruments are defective and whether the aircraft can be ferried to the base 25 nm away under VFR. The AME has the maintenance records, a pitot-static test set, and the aircraft maintenance manual. No storm or icing was reported; pitot heat was on. Both static ports are found taped.
1. Identify the most likely fault and explain why the airspeed, altimeter, and VSI behaved as reported. Distinguish this from a blocked pitot tube.
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The symptoms point to a blocked static system, not three independent instrument failures. With both static ports taped, the static pressure trapped in the system stays at the value present when the ports were covered, about 1,200 ft. The altimeter therefore freezes, and the VSI sees no changing static pressure, so it reads zero. The ASI uses pitot total pressure minus static pressure. As the aircraft climbs, pitot pressure falls with ambient pressure, but the trapped static pressure remains relatively high, so the pressure difference falls and the ASI underreads. That matches the fall from 140 to 90 KIAS. A blocked pitot tube produces the opposite ASI behaviour: the ASI tends to climb like an altimeter because total pressure is trapped while static pressure falls. In that case the altimeter and VSI remain normal. Selecting alternate static changes the source to cabin pressure, which is lower than ambient in this installation, so the ASI overreads and the altimeter moves. The instruments themselves are probably serviceable; the static system is not.
2. State the maintenance actions required before further flight and explain the leak-test and return-to-service logic.
Read the worked answer
Remove the tape, inspect and clean both static ports, and confirm there is no paint or sealant blocking the passages. Inspect the static lines, alternate static valve, and drain traps. Because the aircraft is pressurised, follow the AMM for the static system and cabin pressure system. Perform a pitot-static leak check with an approved test set, then verify the ASI, altimeter, and VSI at the specified test points, including the alternate static source. Check the transponder encoder if fitted. A ferry flight with a known static blockage should not be released as normal; it needs the defect rectified or a specific approval. Return to service only when the leak check is within the AMM limits and the instruments respond correctly. Record the taping error and consider a maintenance human-factors review, because the mask was left on through the release.