Deferred Defect Versus Airworthiness Directive Deadline
A regional operator's twin turboprop is at a line station with 36 hours remaining before its next scheduled overnight check. During turnaround inspection a mechanic finds the left engine fire-detection loop showing a continuous fault on the BITE panel. The aircraft's approved minimum equipment list permits dispatch with one fire-detection loop inoperative, subject to an operational procedure and a Category B rectification interval of 3 calendar days. Separately, the maintenance tracking system shows an airworthiness directive applicable to the fire-detection control unit with a compliance date 48 hours away; the aircraft is currently compliant. The station has no replacement control unit, but a serviceable unit is at the main base, a 90-minute ferry flight away. The flight crew is pressing for release because the next rotation carries 52 passengers and the alternative is a 6-hour bus transfer. The mechanic holds an airframe and powerplant certificate but the operator's approved maintenance program assigns fire-detection system work to the avionics shop.
1. Can the aircraft be dispatched under the MEL, and what is the controlling constraint? Explain the interaction between the MEL interval, the AD compliance date, and the operator's approved maintenance program.
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Dispatch is legally possible only if every condition is satisfied simultaneously, and the controlling constraint is not the MEL interval but the AD deadline and the task-authorization limit.
First, the MEL is a dispatch relief document, not a maintenance authorization. It permits operation with the listed item inoperative provided the stated operational procedure and rectification interval are met. Category B gives 3 calendar days, which appears to cover the 36-hour window - but the interval starts at the time of defect recording, and the clock is calendar-based, not flight-hour-based.
Second, the AD remains binding regardless of MEL status. The AD's compliance date is 48 hours away; if the aircraft flies the rotation and the rectification slips, the operator risks operating past the AD deadline. More importantly, if the AD requires terminating action on the same control unit, the MEL cannot be used to defer work that the AD mandates by a specific date. The AD date is hard; the MEL interval is discretionary relief.
Third, the approved maintenance program assigns fire-detection work to the avionics shop. The line mechanic's certificate authorizes the work in general terms but does not override the operator's program, which is the basis of the operator's approval. Performing the task outside the authorized shop, or signing it off, would be a program violation even if the technical work were correct.
Practical conclusion: dispatch under MEL is defensible only if the avionics-authorized rectification can be completed before the AD date, or if the AD itself is not affected by the inoperative loop. The safe and compliant path is to ferry the aircraft to base under an appropriate permit or MEL provision and complete the control unit replacement before the AD date, rather than launch the passenger rotation on a hope that the 3-day interval absorbs the AD.
2. The crew argues that the MEL explicitly allows the flight. How should the mechanic respond, and what documentation should be raised before any flight occurs?
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The mechanic should acknowledge that the MEL does allow dispatch with one loop inoperative, but explain that the MEL is subordinate to the type certificate, the AD, and the operator's approved program. The MEL cannot extend an AD deadline, and it cannot authorize a task the program reserves to another shop.
Before any flight the mechanic should: record the defect with time and BITE evidence; verify the AD status and the exact terminating action required; confirm whether the inoperative loop affects the AD compliance method; raise a deferral record citing the MEL item and the operational procedure; and, if the aircraft is to move, ensure the move is authorized under the correct provision (ferry permit or MEL) with the right sign-off. The deferral record must show the start time of the interval so the 3-day clock is auditable.
The mechanic should also escalate to the maintenance controller or chief inspector, because the decision crosses the boundary between line release and program authorization. A verbal assurance from the crew is not a basis for release; the release must rest on a documented airworthiness determination. If the AD cannot be met before the flight, the aircraft should not fly the passenger rotation, and the operator should arrange the bus transfer or a substitute aircraft.