EFIS and Standby Instrument Disagreement
A pilot is cruising at FL350 in a glass-cockpit business jet. The primary flight display (PFD) shows a left bank of 15° and a heading of 270°, while the standby attitude indicator shows wings level and the standby compass shows 265°. The autopilot is engaged in LNAV. The aircraft has just passed through an area of moderate turbulence. The air data computer (ADC) 1 has a fault flag on the EICAS. The first officer's PFD agrees with the captain's PFD. The standby airspeed indicator reads 250 kt, while the PFD reads 310 kt. The aircraft is in RVSM airspace.
1. Determine which indications are likely correct and describe the failure mode.
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The key clue is the ADC 1 fault flag. In a typical glass cockpit, the captain's PFD and the autopilot may be fed by ADC 1, while the first officer's PFD uses ADC 2. If both PFDs agree, the fault may be in a shared source or the flag may be advisory only.
However, the standby instruments are independent. The standby attitude indicator shows wings level while both PFDs show a 15° bank. That is a significant disagreement. The standby airspeed of 250 kt versus 310 kt on the PFD is also large.
Likely scenarios:
- ADC 1 fault causing erroneous air data on the captain's side, with the first officer's PFD cross-fed from ADC 1 or also affected.
- IRU/ AHRS fault causing the attitude and heading disagreement.
- Pitot or static blockage on the captain's side, causing airspeed error, with the standby system unaffected.
The standby instruments are the most trustworthy because they are independent and simple. The pilot should treat the standby readings as the reference until the fault is isolated. In RVSM airspace, an altitude or airspeed discrepancy of this magnitude is a serious event.
2. State the immediate actions and the ATC communication required, including RVSM considerations.
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Immediate actions:
- Disconnect the autopilot and hand-fly using the standby instruments.
- Cross-check the standby attitude, airspeed and altitude against the PFDs and identify which source is faulty.
- Select an alternate air data source if the system allows (e.g. cross-feed from ADC 2).
- Advise ATC of the instrument failure and the unreliable airspeed/altitude indication. Request a block altitude or leave RVSM airspace.
- Consider a diversion to the nearest suitable airport.
RVSM considerations:
- RVSM requires two independent altitude measurement systems and an autopilot with altitude hold. If one system is faulty, the aircraft may no longer meet RVSM requirements.
- ATC should be informed so that separation can be increased.
- The pilot should not continue in RVSM airspace with an unreliable altitude source.
Communication example: 'Mayday, Mayday, Mayday, [callsign], unreliable airspeed and altitude indications, request block altitude FL340 to FL360 and vectors to [airport].'