Vacuum Failure in IMC near Cebu
You are flying a Cessna 172S IFR from Clark to Cebu at 8,000 ft in IMC. The aircraft has a vacuum-driven attitude indicator and heading indicator, an electric turn coordinator, and a Garmin 430 with a moving map. About 40 nm from Cebu, you notice the attitude indicator showing a slow right bank and the heading indicator drifting about 20°; the turn coordinator shows wings level, the airspeed is steady at 105 kt, the VSI is 0, and the altimeter is unchanged. The vacuum gauge reads 2 inHg; normal is 5. You are cleared to descend via the RNAV arrival, and Cebu is 1,000 ft overcast with 4 km visibility. Dumaguete, your alternate, is 3,000 ft scattered with 10 km. Fuel remaining is 1+30.
1. Diagnose the situation and identify which instruments you can still trust.
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Two vacuum instruments are failing together, the vacuum gauge is low, and the turn coordinator agrees with the pitot-static instruments-this is a vacuum pump failure, not an attitude problem.
Stop using the attitude indicator and heading indicator; they are unreliable and the AI's false bank can lead you into a spiral. The reliable instruments are the turn coordinator, airspeed, altimeter, VSI, and the GPS track. The pitot-static system is independent of the vacuum system.
Cover or ignore the failed AI and DG; use the turn coordinator for bank, the altimeter and VSI for pitch, and the airspeed for energy. The GPS gives you ground track and distance, but not attitude. Do not try to "verify" the AI by following it-cross-check the turn coordinator and pitot-static instruments instead.
2. What should you tell ATC, and how do you fly the aircraft and approach with partial panel?
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Advise ATC clearly: "Cebu Approach, Cessna 1234, vacuum failure, partial panel, request no-gyro vectors and a lower altitude if VMC is available." A no-gyro approach means the controller tells you when to start and stop standard-rate turns; you use the turn coordinator and time the turns (3°/s).
Keep wings level on the turn coordinator, control pitch with power, airspeed, VSI, and altimeter, and cross-check the GPS track. Request a descent only if you can remain above the MEA/MVA; Cebu's 1,000 ft ceiling may allow you to reach VMC over the sea, but do not descend below a safe altitude without clearance.
If you cannot get VMC, fly the RNAV approach using the GPS and partial panel. Brief the missed approach before starting the approach and set a hard decision altitude.
3. Do you continue to Cebu or divert to Dumaguete? Explain the judgement.
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Cebu's 1,000 ft overcast is above the LNAV minima but leaves little margin for a partial-panel approach, and a missed approach would be demanding. Dumaguete is VMC at 3,000 ft scattered, 10 km, and you have 1+30 of fuel-enough to divert with reserve.
The safer decision is to divert to Dumaguete, where you can cancel IFR and land visually, unless Cebu is clearly VMC at your arrival or you have recent partial-panel proficiency. Cebu's weather may be below your personal minimums for partial panel.
Advise ATC of the diversion, request a clearance, and monitor fuel. If you stay with Cebu, brief the missed approach and set a hard decision altitude; do not press a partial-panel approach to minimums just because the destination is closer.