MEL repair intervals: the day-of-discovery rule and the CDL trap
An MEL deferral is only legal until its repair interval runs out. That sounds simple until you notice that the expiration your scheduling or tracking software shows and the expiration the MEL actually allows are not always the same date. When they differ, the aircraft is either grounded a day early for no reason, or flown a day late without anyone realizing it.
The four repair categories
| Category | Repair interval |
|---|---|
| A | As stated in the remarks of the specific MEL item |
| B | 3 consecutive calendar days |
| C | 10 consecutive calendar days |
| D | 120 consecutive calendar days |
The rule that trips people up: the day of discovery doesn't count
For the calendar-day categories, the day the discrepancy is discovered and recorded is excluded. The clock starts at 0000 the next day.
Take a Category B item written up on January 26 at 1000. The three days are January 27, 28 and 29, and the deferral expires at 2359 on January 29, not at 1000 on the 29th or at midnight on the 28th.
Most software counts from whatever date and time you type in. Some handle the exclusion correctly and some don't, and the difference is easy to miss when the screen shows a tidy due date. Check your MEL preamble and your operator's procedures for the exact wording that applies to you.
The fix: enter the true date of discovery, then check the expiration the system calculates against the rule, every time. Test your software once with a known example so your whole desk knows how it counts.
The CDL trap
Configuration Deviation List items are where this gets expensive. Many tracking systems treat a CDL entry like a Category D item and default it to 120 days. But many operators' manuals put a much shorter limit on CDL items (a 10-day, Category C-style limit is common).
If your manual says 10 days and your software says 120, the software will happily show the aircraft as legal for almost four months after the item should have been fixed. No one is trying to break a rule; the default just goes unchallenged.
The fix: find the CDL limit in your GOM, GMM or MEL procedures, write it on the desk quick reference, and set every CDL entry's expiration by hand.
Who owns the date
Software is a tracking tool, not the authority. The MEL definitions and your approved procedures decide when a deferral expires, and Maintenance Control owns getting that date right. A controller who checks each expiration in the ten seconds after entering it will never be the one explaining an exceedance.
General guidance for orientation only. Your MEL, approved manuals and the regulations govern; confirm against them before acting.