The instrument comparator monitors for instrument differences. If an instrument difference occurs, the appropriate warning flag will appear in the Captain and F/O's instrument comparator panel.
A difference of 6 knots to 10 knots between the IAS indicators will cause the AIRSPEED instrument comparator flag to appear. An IAS fail flag might also appear in the IAS indicator, but not necessarily. The higher the indicated speed, the grater the error must be to get a comparison flag. At 200 knots, a 6 knot difference will trip the comparator. A 10 knot difference required at 450 knots.
After an initial VG/ISS failure, a difference in attitude indications exceeding 5º will cause an ATTITUDE comparison flag to appear on both instrument comparator panels. an ATT flag may appear on the ADI, but not necessarily.
A difference of altitude indications of between 100 and 240 feet will cause the ALTITUDE comparison flags to appear.
- The difference required to get a warning increases with altitude. Approximately 100 feet at sea level will increase to 240 feet at 40,000 feet.
The Instrument Comparator threshold is doubled if the vertical speed exceeds 800 FPM.
A difference of heading indications of between 6º and 15º will cause the HEADING comparison flags to appear. The difference required for the HEADING flag increases as the angle of bank increases. A HDG flag in the HSI could also be present, but not necessarily.
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