Intermittent ADIRU Fault and Chafed Avionics Harness
An A320 has an intermittent ADIRU 2 fault. On three consecutive flights, the ECAM shows 'IR 2 FAULT' for 2-5 seconds during climb, then clears. The fault occurs when the aircraft passes through 18,000 ft and cabin temperature is 30°C. The operator's MEL allows dispatch with one ADIRU inoperative for 10 days if the remaining two are serviceable. The aircraft is due for a 3-hour flight over remote terrain at night. Maintenance finds no fault in the BITE, but a wiring harness near the avionics bay door has chafing evidence and a 0.5 ohm resistance change when the door is moved. The connector has 3 broken strands out of 19. The technician wants to defer per MEL, but the captain asks about RVSM and MNPS. The avionics engineer must decide.
1. Does the MEL permit dispatch with the intermittent ADIRU 2 fault? Distinguish an intermittent fault from an inoperative unit.
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The MEL normally covers a unit that is known to be inoperative and can be positively isolated, with the remaining systems providing the required redundancy. Here the ADIRU 2 fault appears for 2-5 seconds and then clears; the unit is not positively inoperative and may produce erroneous data intermittently. The 10-day relief assumes the failed unit is deactivated and the other two are serviceable. The intermittent fault also occurs at a specific phase, climbing through 18,000 ft, and temperature, 30°C, which suggests a connection or thermal issue rather than a stable unit failure. The wiring finding makes that more likely: chafing at the avionics bay door and a 0.5 ohm change when the door moves is an active defect. The MEL does not cover a known chafed harness. Therefore dispatch under the ADIRU MEL is not justified until the fault is found and corrected.
2. How should the wiring finding be handled, and what acceptance criteria would you apply?
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Three broken strands out of 19 is about 16% of the conductor, which exceeds the common 10% limit for wire damage; even if the limit were higher, the chafing and movement-related resistance change show an unstable connection. I would replace or repair the harness using approved wiring practices, not simply tape or tie it back. Inspect the connector for arcing, heat damage, and bent pins; check the backshell and clamp; verify bonding and continuity; and perform an insulation resistance test. Then run the ADIRU through a ground test while moving the door and, if possible, a pressurization or temperature test that reproduces the fault environment. Acceptance requires stable resistance, no chafing, a clean BITE history, and no fault during the test. The repair should be recorded as a wiring repair, and the original chafing cause should be corrected so it does not recur.
3. If the repair cannot be completed at this station, what approvals and restrictions would you require for a one-time ferry?
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A one-time ferry would require a special flight permit or equivalent approval, and I would not carry passengers. The ADIRU 2 circuit should be positively isolated if the fault can feed erroneous data, and the flight manual procedures for an IR fault should be followed. I would restrict the flight to day/VFR if possible, avoid RVSM and MNPS airspace unless the remaining systems and approvals clearly support it, and require a maintenance observer aboard. The route should stay near suitable airports, and the crew should use standby instruments and cross-check altitude and heading. If isolation of the ADIRU is not possible, or if the fault affects multiple systems, even the ferry should be refused. This is not a normal MEL dispatch.