Electronic Instrument Systems
Typical arrangement and layout of electronic instrument systems — the EADI/PFD and EHSI/ND, the symbol generators that drive them, display reconfiguration and the independent standby instruments.
Electronic instrument systems — summary notes
Free- EFIS (Electronic Flight Instrument System) replaced the electromechanical "basic six" with two integrated displays per pilot, driven by symbol generators (SGs) that gather data from the air data computers, inertial reference units and radio-navigation receivers and paint it as symbology.
- The EADI/PFD (Electronic Attitude Director Indicator / Primary Flight Display) carries the ATTITUDE group: artificial horizon, flight-director bars, airspeed and altitude tapes, vertical speed, Mach, radio altitude, the rising runway and the ILS localiser/glideslope deviation scales.
- The EHSI/ND (Electronic Horizontal Situation Indicator / Navigation Display) carries the NAVIGATION group: compass rose or arc, track and heading, waypoints, VOR/ADF bearing pointers, wind vector, and overlaid weather radar and terrain (EGPWS) returns. Typical selectable modes are ROSE, ARC, MAP and PLAN.
- Each display unit is interchangeable and reconfigurable: if one DU fails, its content can be transferred to the surviving DU (composite mode), and a display switching panel selects the alternate symbol generator.
- Standby instruments — an ISIS (Integrated Standby Instrument System) or separate attitude, airspeed and altimeter — remain independent of EFIS so a total EFIS loss still leaves flyable instrumentation.
- ⚠ Exam trap: weather radar and the map are on the ND/EHSI, never on the PFD/EADI. The PFD shows what the aircraft is doing; the ND shows where it is going.
A symbol generator fails in flight. What is the effect on the affected pilot's displays, and what action restores a usable picture?
Both of that pilot's display units lose their symbology because a single SG drives them. Selecting the alternate/opposite symbol generator on the display switching panel re-drives the units from a healthy SG; if only one display unit remains serviceable, composite mode combines the essential attitude and navigation symbology onto that single unit.
EFIS concept map
FreeElectronic Instrument Systems
Electronic instrument systems quiz
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