Vacuum Gyro Drift After Filter Replacement
After an annual inspection on a PA-34-220T Seneca V, the vacuum filter and both flexible lines to the attitude indicator were replaced. The aircraft has two engine-driven vacuum pumps, a vacuum gauge, an electric turn coordinator, and a standby vacuum switch. On the first flight, at 2,000 ft and 24 inHg manifold pressure, the pilot reports: vacuum gauge 4.2 inHg (POH minimum 4.5 inHg), attitude indicator erects slowly and shows a 5° right bank in straight-and-level cruise, directional gyro drifts 12° in 20 minutes, and the turn coordinator is normal. At idle on the ground the gauge reads 3.0 inHg and recovers to 4.3 inHg at 1,500 rpm. The AME must decide whether to release the aircraft, adjust the regulator, or remove a gyro. The maintenance manual gives a suction test at cruise power and a gyro run-down check.
1. Interpret the indications. Is the fault most likely in the vacuum supply, a single gyro, or the gauge? Justify your answer.
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Both the attitude indicator and the directional gyro are affected, and the vacuum gauge is below the POH minimum at cruise and at idle. That pattern points to a common supply problem rather than one failed gyro. A single gyro problem would normally affect the AI or the DG, not both, and the electric turn coordinator is normal. However, the gauge itself could be reading low, so it must be checked against a calibrated test gauge before condemning the pump or regulator. The recent work on the filter and AI lines makes an installation error likely: a filter fitted backwards, a restricted or collapsing hose, a loose connection, or a regulator misadjusted during the annual. Low suction at idle that improves with rpm also fits a supply or regulator issue. Do not release the aircraft at 4.2 inHg; the POH minimum is 4.5 inHg and both gyros are showing abnormal behaviour.
2. Describe the troubleshooting sequence and acceptance criteria before return to service.
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First verify the cockpit gauge with a calibrated suction test set at the engine speeds specified in the AMM. Inspect the new filter for correct part number and flow direction, check the flexible lines for kinks, internal collapse, or incorrect routing, and check all connections. Test each pump separately if the system allows, inspect the regulator and setting, and check the standby vacuum valve for leakage. Adjust the regulator only per the AMM. After correction, the suction must be within the POH range at cruise and must meet the ground/idle check. Then check gyro erection and run-down: the AI should erect within the specified time without a persistent bank, and the DG should not exceed the allowed drift. If suction is correct but the DG still drifts 12° in 20 minutes, remove the DG for bench check; if the AI also remains tilted, remove the AI. Return to service only when suction and gyro performance meet the approved data.
3. What would you conclude if the suction is correct at 4.9 inHg but the DG still drifts 12° in 20 minutes?
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If a calibrated gauge confirms 4.9 inHg at cruise and the filter, lines, regulator, and pumps are serviceable, the supply is no longer the likely cause. The DG is then suspect: worn bearings, a contaminated or failing erection mechanism, or a gyro that is not reaching speed. The AI should also be checked because it showed a 5° bank, but if the AI is now normal and only the DG drifts, the DG is the defective unit. A 12° drift in 20 minutes is excessive for a serviceable DG under the AMM limits. Remove the DG and bench check it; do not attempt to compensate by adjusting the vacuum regulator above the POH range. The aircraft remains unserviceable until the DG meets the drift and run-down limits.