Dispatch an aircraft with an MEL deferral and its maintenance clock starts, though many scheduling systems do not track it. The Minimum Equipment List defines when specified inoperative equipment may legally remain in service. That permission is time-limited. Each item has a category setting the maximum correction interval. Category A items must be fixed within the days stated in the MEL remarks. Category B items require correction within 3 consecutive calendar days. Category C permits 10 calendar days. Category D permits 120 calendar days.
For dispatch, the aircraft remains legal while the deferral window is open and the MEL's required operational or maintenance procedures are followed. For planning, that window adds a constraint to the maintenance plan. Flight schedules often hide it, and maintenance plans sometimes do too.
How MEL deferrals build up
Several open MEL items on an older aircraft in active service are common. Each was legally deferred, has its own expiry, and calls for a defined repair with particular parts and labor. One aircraft with three concurrent MEL items, the scheduled maintenance program, and the AD compliance register is manageable. Three concurrent items on 12 of 20 aircraft, each with different categories and expiry dates, becomes a major coordination task.
The accumulation pattern matters. An MEL deferral keeps the aircraft dispatchable while repair is arranged. If the required part has a six-week lead time, that window can allow sourcing without grounding the aircraft. But a Category C deferral with a seven-week lead time outlasts its window. The repair must be expedited at premium cost, or the aircraft must be grounded when the 10-day window ends.
Make that calculation when the deferral is entered, not six days later when the planner finds it. Order the part on day one and confirm delivery within the deferral window. If lead time is longer, escalate at once to operations and engineering to assess an alternative source or a ferry flight to a maintenance base. The decision needs days, not hours.
The visibility gap in scheduling
MEL items usually enter the MRO records system when deferred. The crew or line technician usually records them in the aircraft technical log as well. What may not occur automatically is carrying the deferral into the maintenance schedule, with its expiry shown as a planning constraint.
A planner reviewing the open MEL list daily will usually find expiring items with enough lead time. Weekly review, or alerts sent by the MRO system into a general notifications queue, can leave an item unnoticed until its window is tight. On a busy fleet with 15 or 20 open items, those narrow-window items face the greatest risk of being missed.
With MEL data connected to maintenance scheduling, every open deferral appears as a time-limited work item beside scheduled visits. The planning view identifies the aircraft, each expiry, and whether the flight schedule brings the aircraft near a maintenance base beforehand. If not, it flags the conflict for action: move the aircraft to an earlier visit, alter routing through a maintenance base, or start expedited parts procurement.
MEL clearance and work packaging
Each open MEL item carries a repair action requiring labor, possibly parts, and hangar time. When a scheduled maintenance visit is near, determine whether the item can be included and cleared there, or whether its deferral window closes first and requires earlier resolution.
Combining MEL clearance with scheduled maintenance is nearly always better than a separate line visit. The aircraft is already in the hangar, technicians are already assigned, and mobilization is shared. A Category D item with 90 days left that fits the next C-check in 75 days is suitable for bundling. A Category C item with 8 days left is not and needs a separate resolution window.
The scheduling system should automatically identify those bundling opportunities. For every open MEL item, compare expiry with the next planned visit. An expiry after that visit makes the item a bundling candidate for its work package. An earlier expiry requires its own resolution path and immediate flagging. The comparison is simple to automate but laborious across a fleet's open items.
CDL items and how they differ
The Configuration Deviation List is similar to the MEL but covers physical configuration items, including fairings, panels, and structural elements that may be missing or damaged within stated limits, rather than inoperative equipment. CDL deferral limits also come from the aircraft flight manual or maintenance procedures. CDL items create the same planning issue as MEL items, but may sit elsewhere in the records system. Complete scheduling integration must therefore read both sources.
In practice, handle MEL and CDL items the same way for scheduling. Each is an open work item with a compliance window, repair action, and parts and labor requirement. The scheduling logic remains the same regardless of regulatory category.
Where scheduling responsibility stops
The crew or licensed maintenance engineer assigns an MEL category at deferral, following the operator's approved MEL. The MEL remarks define dispatch procedures, which the crew carries out. Approved maintenance data defines the procedure for clearing the deferral.
Tailsight covers scheduling: it tracks deferral expiry, exposes conflicts with flights and maintenance, and flags items needing action before the next planned visit. The aircraft technical log and MRO records system remain authoritative for the open deferral, category, and clearance. The scheduling system reads those records to expose planning conflicts. We are not saying it replaces those records or assigns categories. We are saying it shows the scheduling consequence early enough to act, rather than just before the window closes.