Vacuum failure in solid IMC: recognizing it and deciding what to fly

Cross-check the three gyroscopic sources against the pressure instruments. The attitude indicator shows a climb and a left bank, but the altimeter and VSI are steady, which says the airplane is not climbing; the turn coordinator shows wings level; and the heading indicator is precessing at 3 degrees per minute, which points to the vacuum system rather than the airframe. If a vacuum gauge is fitted, low suction confirms it. Confidence comes from the agreement between the pressure instruments and the turn coordinator, not from the failed gyro. First actions, in order: fly the airplane on the turn coordinator and the pitot-static group; disconnect the autopilot immediately, because an attitude-based system will follow the failed indicator and can roll you into a spiral while you are deciding what to do; turn on pitot heat; cover or disregard the attitude indicator and heading indicator; and notify ATC of the equipment malfunction. That notification is required in IFR conditions in controlled airspace, and it also lets the controller start protecting airspace for your degraded navigation.

The decision hinges on precision, not pride. An ILS to 200-ft minimums demands continuous, fine pitch and bank control that a partial panel makes marginal at best, and a missed approach with no attitude indicator in IMC at -2 C is worse. Reasonable options, in order of preference: divert to the airport reporting clear skies, where the failure stops being an instrument problem entirely; or request vectors for a surveillance approach or an ASR at the destination, with the controller calling your position and a higher-than-normal minimum, using the GPS ground track and the turn coordinator to hold headings. Descending to the 4,000-ft bases or climbing to the 11,000-ft tops is not a reliable plan, because terrain, the minimum vectoring altitude, and the subfreezing temperature all constrain it. Tell ATC what you have lost, what you can still fly, and what you want. Then commit to the lower-precision option while you still have altitude and fuel to spare, rather than demonstrating a partial-panel ILS.

Use the turn coordinator for bank and the altimeter plus VSI for pitch, with power set for the airspeed you want. A standard-rate turn is 3 degrees per second, so the wings should sit on the index mark; make small inputs and wait for the instruments to respond, because the altimeter and VSI lag and the natural instinct is to over-control. Level off by leading the altitude slightly and confirming with the altimeter and VSI together, not with the failed attitude indicator. The traps are specific. The magnetic compass is only reliable in level, unaccelerated flight, so it is useless in a turn. The turn coordinator shows rate, not bank angle, so turbulence can put you past a safe bank before the instrument catches up. The heading indicator is unusable, so heading has to come from the GPS ground track, understanding that it lags and displays track over the ground, not heading. And the failed attitude indicator will keep showing a plausible climb or turn; cover it so you do not unconsciously chase it.