Intermittent Electrical Fault After Component Swap
A B1.1 engineer replaces a failed fuel boost pump on a twin-engine aircraft. After the swap, the left boost pump low-pressure warning illuminates intermittently in flight, roughly every 20 minutes, and clears when the aircraft climbs. Ground runs show normal pressure and no warning. The engineer suspects the new pump, but the wiring harness connector was reused and shows slight fretting discolouration on two pins. The aircraft has flown 4 sectors since the swap. The minimum equipment list allows dispatch with one boost pump inoperative under specified conditions, but the warning is intermittent rather than failed. The operator wants the aircraft released for a 2-hour flight with the warning deferred.
1. Assess whether the intermittent warning can be deferred under the MEL and what fault-finding is required.
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The MEL relief applies to a pump that is inoperative, not to an intermittent low-pressure warning of unknown cause. An intermittent warning indicates an unresolved fault that could be a genuine low-output pump, a wiring/connector issue, or a sensor problem, and the MEL conditions may not be met because the system state is indeterminate. The engineer must not use the MEL to defer an intermittent defect. The fretting discolouration on two pins is a strong lead: vibration and altitude-related temperature changes can open a marginal contact, which fits the climb-related clearing. The engineer should perform resistance and voltage-drop checks across the connector, inspect the pins under magnification, and if fretting is confirmed, replace the connector or pins per the standard practices. Only after the fault is positively identified and either rectified or shown to match an MEL item should dispatch be considered.
2. Describe the reasoning needed to distinguish a pump fault from a wiring fault before further flight.
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The engineer should compare the new pump's output against the right pump at the same conditions, check the low-pressure switch setting and its wiring, and monitor the warning during a ground run with the connector flexed and the harness loaded. A pump fault would typically show low pressure on a gauge or a consistent warning, whereas a wiring fault would follow vibration, harness movement, or temperature. The fact that the warning clears in the climb suggests a connector or harness issue rather than pump output, because pump output does not normally improve with altitude. The engineer should record the resistance measurements before and after flexing; a change confirms the connector. Replacing the pump again without this evidence risks a repeat. The release decision should wait until the fault is reproduced and localised.