ADC and AHRS roles and failures
Two units feed the PFD (PHAK ch.8):
| Unit | Inputs | Outputs |
|---|---|---|
| Air data computer (ADC) | Pitot pressure, static pressure, outside air temperature | Airspeed, altitude, true airspeed; also feeds the autopilot |
| Attitude and heading reference system (AHRS) | Accelerometers, magnetometer (heading) | Pitch and bank to PFD, slip/skid, heading |
- AHRS: solid-state replacement for free-spinning gyros. Works at any attitude without tumbling.
- Some modern attitude indicators are also built not to tumble.
Power failure:
- Gyro instruments: most aircraft have at least two power sources, so at least one bank source survives a failure (PHAK ch.8).
- Sources are separate electrical or pneumatic: no single failure prevents safe completion of the flight (IFH ch.5).
- Only some dials name their power source: consult the POH/AFM for each, to know what to do on failure (IFH ch.5).
- EFD relies on electrical power for AHRS, ADC and engine indicating system (IFH ch.11).
- Alternator or generator failure: abnormality in a conventional aircraft, emergency in an EFD aircraft (IFH ch.11).
- Standby battery powers the essential bus, keeping the PFD usable after the main battery is flat (IFH ch.11).
Display and unit failure:
| Failure | Result | Cite |
|---|---|---|
| Display | Each screen can show all critical information of the other: reversionary mode | IFH ch.5, ch.11 |
| AHRS | PFD loses all heading and attitude indications and all unusual-attitude protection | IFH ch.7 |
| PFD unavailable | Standby instruments only: attitude indicator, altimeter, airspeed indicator, magnetic compass | IFH ch.11 |
Trap: ADC = air data (airspeed, altitude). AHRS = attitude, heading, slip.
References
- PHAK ch.8 (FAA-H-8083-25), Electronic Flight Display, ADC, AHRS and glossary
- IFH ch.5, ch.7, ch.11 (FAA-H-8083-15), power sources, AHRS failure, reversionary mode, standby instruments