Display Unit Failures and Reversion Before Dispatch
During pushback and engine start, the first officer's navigation display (ND 2) blanks. The EICAS shows 'ND 2 FAULT' and the captain's PFD and ND remain normal. The crew completes the checklist and starts both engines; engine parameters are displayed on the upper EICAS. During taxi, the lower EICAS display flickers and then shows a 'DISPLAY UNIT' amber message. The captain's ND now shows the engine secondary parameters automatically. The aircraft has four display units and a reversion panel. The weather is CAT I, and the flight is a 2-hour domestic sector. The MEL allows one display unit failure with restrictions. The captain's PFD and ND are on DU 1 and DU 2; the FO's PFD is on DU 6.
1. How can the crew tell whether the blank ND is a display unit failure or a data bus/symbol generator failure?
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The blank ND 2 with the captain's PFD and ND still normal points to the display unit itself, not to a shared data source. ARINC 429 is a point-to-point bus, so a failure in one receiver does not affect the other displays. AFDX is a switched network with redundant networks, and a single display unit failure is isolated. If a symbol generator had failed, every display using that symbol generator would be affected, not just ND 2. If a data bus had failed, multiple displays or systems would show invalid data, or the affected bus would be flagged on the status page. The EICAS message 'ND 2 FAULT' is consistent with a DU fault. The crew should check the reversion panel and the status page to confirm that the source data are valid on another display, and then consult the QRH.
2. What automatic reversion has occurred, and what should the crew do before dispatch?
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The automatic reversion has moved the engine secondary parameters to the captain's ND, and the lower EICAS display has failed. This is a display reversion, not a loss of the engine data. The crew must ensure that the primary engine parameters, warnings, and the PFD/ND data are still visible. They should use the reversion panel to select the required pages and avoid losing the EICAS entirely. The MEL allows one display unit failure with restrictions, but a second failure may make the aircraft non-dispatchable. Before dispatch, the crew should confirm that the remaining displays can show all required information for the intended flight, including engine parameters, warnings, and navigation. If the lower EICAS is failed, the captain's ND must be used for engine secondary parameters, which may interfere with the navigation display. The QRH and MEL will specify the required equipment and any operational restrictions, such as day/VFR only or no CAT II/III.
3. If the captain's ND fails after take-off, what would be the consequence and how should the crew manage it?
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If the captain's ND fails after take-off, the aircraft would have two display units failed, and the remaining displays may not be able to show all required information. The crew should first maintain aircraft control and use the standby instruments if the PFD is affected. If the EICAS is lost, engine parameters may be available on the standby engine indicator or the remaining DU. The crew should use the reversion panel to prioritise the PFD and the engine warnings. If the required information cannot be displayed, the crew should declare an emergency and land as soon as possible. The AFDX network has redundant A and B networks, so a single bus failure should not cause this; if both networks fail, multiple systems are lost. The crew should not continue the flight with a display configuration that prevents monitoring of the engines and systems.