On a glass cockpit PFD, a comparator monitor caution message such as 'ATT MISCOMPARE' appears during an instrument cross-check. What does this indicate, and what should the pilot do?
- AThe autopilot has disconnected; re-engage it immediately
- BGPS position has been lost; switch to VOR navigation immediately
- CA discrepancy exists between the primary and standby attitude systems beyond a set threshold; cross-check backup instruments to identify the failed source
- DThe static system has failed; activate pitot heat
Show answer & explanationAnswer & explanation
Correct answer: C. A discrepancy exists between the primary and standby attitude systems beyond a set threshold; cross-check backup instruments to identify the failed source
Comparator monitors in glass cockpit systems continuously compare outputs from independent attitude sources (e.g., primary AHRS and standby instruments). An 'ATT MISCOMPARE' or similar caution means the values disagree beyond an allowable tolerance, and the pilot must cross-check the standby attitude indicator and other cues to determine which system is providing valid data.
Why the other options are wrong
- A. Comparator alerts relate to attitude source disagreement, not autopilot status.
- B. GPS loss triggers different annunciations, not an attitude comparator caution.
- D. This alert is unrelated to the pitot-static system; pitot heat would not resolve it.
Comparator Monitor (Glass Cockpit)
A comparator monitor compares data from redundant attitude, heading, or air data sources and alerts the pilot when values disagree beyond a set tolerance, prompting a cross-check to identify the failed system.
- Common alerts include ATT, HDG, or AIRSPEED miscompare
- Requires cross-check with standby instruments
- Does not by itself identify which source is faulty
Memory trick: Miscompare means 'check both, trust neither yet.'