Electronic Display Units in EFIS: Redundancy, Switching, and Priority Logic

022-13-01Pablo Asensio Martínez2026-03-271 min

Modern EFIS (Electronic Flight Instrument Systems) utilize Display Units (DUs) (CRT or LCD screens) to present flight information. A key feature of these systems is the redundancy and interchangeability of the displays to ensure critical information remains available in case of a hardware failure.

Typical Layout (Airbus/Boeing)

  • Outboard DU: Usually the PFD (Primary Flight Display).
  • Inboard DU: Usually the ND (Navigation Display).
  • Center DUs: Upper and Lower for Engine/Systems (EICAS/ECAM).

Display Failure Logic (Automatic Switching)

Because PFD information (Attitude, Airspeed, Altitude) is critical for safe flight ("utmost importance"):

  1. Outboard DU Failure (PFD Screen Fails):

    • The PFD image automatically transfers to the Inboard DU (ND screen).
    • The ND information assumes a lower priority and is displaced (or can be selected on another screen).
    • Principle: You lose the Nav display to save the Primary Flight display.

  2. Inboard DU Failure (ND Screen Fails):

    • The PFD remains on the Outboard DU.
    • The ND screen goes blank (ND info lost on that screen).

  3. Upper Center DU Failure (Engine/Warning Fails):

    • The Primary Engine/Warning display automatically transfers to the Lower DU.
    • Information previously on the Lower DU (e.g., secondary engine params, checklists) may be compacted or displaced.

Independence

  • The Captain's side and First Officer's side are generally independent for this switching.
  • If the Captain's PFD screen fails, it switches to the Captain's ND screen (not the Copilot's).