Night approach with a vacuum failure and an unreliable attitude indicator
You are on a night IFR approach in a light twin. During the initial approach, the attitude indicator begins to show a slow, steady roll to the right that does not match the turn coordinator or the heading indicator. The turn coordinator shows a steady wings-level indication, the altimeter is stable, and the airspeed is steady. The vacuum gauge reads low. You have 8 minutes to the final approach fix and the weather at the aerodrome is 600 ft ceiling and 2 km visibility. The missed approach requires a climb to 3000 ft.
1. Identify the failure and explain how you would confirm which instrument is reliable.
Read the worked answer
The low vacuum gauge plus an attitude indicator that disagrees with the other instruments points to a vacuum or gyro failure affecting the attitude indicator. The turn coordinator is electrically driven in many light twins, so it is likely unaffected; the heading indicator may share the vacuum source and should be treated as suspect until cross-checked against the magnetic compass.
Confirm reliability by cross-checking: the altimeter and airspeed are pitot-static instruments and are independent of the vacuum system, so a steady altitude and airspeed support the turn coordinator's wings-level indication. The heading indicator can be checked against the magnetic compass in straight flight. The attitude indicator is the outlier and should be covered or ignored. The principle is to trust the majority of independent instruments and to avoid chasing a single failing one.
2. What are your options and what would you do?
Read the worked answer
You have partial-panel conditions at night in instrument meteorological conditions with a 600 ft ceiling. Options are to continue the approach using partial-panel techniques, to request a diversion to an aerodrome with better weather, or to declare an emergency and obtain priority.
Continuing the approach is high risk: the final approach and missed approach demand precise attitude control, and the missed approach to 3000 ft at night leaves little margin for error. A better option is to advise ATC of the partial panel, request vectors to an aerodrome with a higher ceiling and longer runway, and use the autopilot if it is independent of the failed gyro and is approved for the conditions. If the autopilot depends on the same vacuum source, disconnect it.
Declare the condition so ATC can provide separation and priority. If no better aerodrome is within range, fly a stabilised partial-panel approach to a long runway with a generous minima buffer, and brief the missed approach carefully before the final approach fix.
3. How would workload and instrument cross-check change during the missed approach?
Read the worked answer
With one attitude source lost, the cross-check becomes a scan of airspeed, altimeter, turn coordinator and heading, with small control inputs and patience. The temptation is to over-control, which produces oscillations; instead, establish a known power and pitch attitude, then trim and verify with the airspeed and vertical speed.
Before the final approach fix, brief the missed approach in detail: the initial climb attitude, the power setting, the target heading and the altitude at which to level off. Assign the pilot flying and pilot monitoring roles explicitly, and have the monitoring pilot call airspeed, altitude and heading trends. Reduce workload by asking ATC for vectors and by delaying configuration changes until the aircraft is stabilised. If the workload becomes unmanageable, level off, declare and request delay vectors rather than pressing the approach.