The MEL Dispatch to a Fog-Bound Alternate
A Boeing 737-800 registered in Ireland operates a scheduled service from Dublin to Faro. Before departure, the operator's maintenance control defers the left engine-driven generator using the Minimum Equipment List, relying on the APU generator as the redundant source. The APU is serviceable. The MEL entry carries a 10-day repair interval and requires the APU generator to be operative. En route, the APU fails; the crew shuts it down and continues on the remaining main generator. Faro's weather is at the edge of the CAT I minima, and the destination alternate is Lisbon, which is forecast at 900 ft ceiling and 4 km visibility. The aircraft has 90 minutes of fuel beyond the planned alternate. The commander considers continuing to Faro and landing there, then positioning to a maintenance base. The operator's dispatch office is pressuring the crew to complete the schedule because the return rotation is sold out.
1. With the APU failed in flight, does the aircraft remain dispatchable to Faro under the MEL? Explain the airworthiness and operational consequences.
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No. The MEL entry permitted deferral of the engine-driven generator only on the condition that the APU generator is operative. That condition is the compensating redundancy that makes the dispatch legal. Once the APU fails, the aircraft no longer meets the MEL dispatch condition, so the original deferral is invalid.
The consequence is that the aircraft must be treated as having a single remaining generator source. Under the operator's approved configuration, that may require landing at the nearest suitable aerodrome, or may permit continuation only under a specific operational restriction. The crew cannot simply continue because the schedule is sold out; commercial pressure is irrelevant to the airworthiness determination.
The correct action is to declare the situation to ATC, advise the operator's maintenance control, and plan to land at the nearest aerodrome where repairs can be made. Faro may be acceptable as that aerodrome if it is the nearest suitable one, but the crew must not treat Faro as a normal destination with a Lisbon alternate. The commander's authority under the operator's manual to deviate from the schedule in the interest of safety is the controlling principle.
2. Lisbon is forecast at 900 ft ceiling and 4 km visibility. Does it satisfy the alternate requirements for this flight, and what does the commander need to check?
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The answer depends on the operator's approved alternate minima, which are derived from the applicable operational rules and the approach facilities available. As a general check, a Category I precision approach alternate typically requires a ceiling at or above the decision height plus a margin, and visibility at or above the approach visibility minimum plus a margin. A 900 ft ceiling and 4 km visibility may or may not meet those figures depending on the specific approach and the operator's table.
The commander must check: the approach type available at Lisbon, the published decision height and visibility, the operator's alternate minima table, and whether the forecast is valid at the expected time of arrival. He must also confirm that the alternate is not required to be used only if the destination is forecast below the applicable planning minima.
If Lisbon does not satisfy the alternate minima, the flight cannot depart or continue under the planned dispatch. Given the generator failure, the prudent course is to land at the nearest suitable aerodrome rather than press toward a marginal destination with a doubtful alternate. The commander should document the weather and the alternate assessment in the technical log.
3. What records and notifications must the operator make after landing with the generator and APU unserviceable?
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The commander must enter the defects in the aircraft technical log, describing the generator deferral, the APU failure and the resulting configuration. The operator's maintenance organisation must open a defect record and, if the MEL deferral is no longer valid, either restore the aircraft to the required configuration or obtain a new approved deferral.
Under the continuing airworthiness regime, the operator must ensure that any deferred defect is tracked within its approved interval and that the aircraft is not released to service unless it conforms to the MEL. If the aircraft is ferried to a maintenance base, that ferry must be conducted under an approved ferry flight authorisation, not as a scheduled service.
The operator should also review whether the event is reportable to the State of registry and the State of the operator under the occurrence reporting rules. The commander's report and the maintenance action should be retained. The dispatch pressure should be recorded so that the operator can address the commercial interference with safety decisions.