Deferred Defect on a Revenue Flight
An A320 operator's aircraft is dispatched with a deferred item under the MEL: the No. 2 engine anti-ice valve is inoperative, placarded, with an operational restriction noted. The MEL entry permits dispatch with the valve failed closed, provided the flight is not conducted into known or forecast icing conditions and a specific performance penalty is applied. The forecast for the destination is +2°C with rain; the alternate is +4°C with mist; the departure is +1°C with freezing rain reported. The captain notes the anti-ice system is otherwise serviceable and proposes to depart, arguing the forecast temperatures are above freezing. The maintenance release shows the defect was deferred 9 days ago and the MEL category is C (10 days).
1. Evaluate whether dispatch is permissible under the MEL and the airworthiness framework.
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Dispatch is not permissible as planned.
- MEL conditions are mandatory: The entry permits dispatch only if the flight will not enter known or forecast icing conditions. Freezing rain at departure is forecast icing; +2°C with rain at destination is also within the icing-risk envelope (freezing rain/drizzle can occur at or slightly above 0°C, and the MEL restriction is about forecast icing, not merely sub-zero temperatures).
- Category C: 10 calendar days from the day of deferral; day 9 is within the interval, so time is not the limiting factor - the operational restriction is.
- Performance penalty: Even if the restriction were satisfied, the specified performance penalty must be applied and verified.
The captain's 'above freezing' reasoning misreads the restriction. The correct action is to rectify the defect, or replan to a routing/forecast that clearly avoids icing, or use another aircraft. Deferral does not convert an unairworthy configuration into an airworthy one beyond the MEL's stated conditions.
2. What records and continuing-airworthiness obligations attach to this deferral?
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The deferral must be:
- Recorded in the aircraft technical log and on the deferred-defect list, with the MEL reference, category, date/time of deferral, and expiry.
- Placarded in the cockpit as required by the MEL, so the crew is aware.
- Tracked so that rectification occurs before the Category C interval expires; if it cannot be rectified in time, the aircraft must not be dispatched.
- Reflected in maintenance planning and, where the MEL requires, in the operator's reliability/continuing-airworthiness programme.
- Closed out with a certification of rectification by appropriately approved maintenance personnel, with the release updated.
The operator's MOE/MEL procedures and the continuing-airworthiness management exposition govern who may defer, who may extend (normally not permitted beyond the category without approval), and how the defect is cleared.