The question that actually gets asked in the hangar is never “what is a category C item”. It is “when does this expire, and can I push it”. Both answers are precise, both are written down, and both are split across two documents that most people only have one of.

The categories live in CS-MMEL, the obligations live in Air Ops

This split explains most of the confusion around rectification intervals.

GM1 ORO.MLR.105(e);(f) of the Easy Access Rules for Air Operations says so directly: the definitions and categories of rectification intervals are provided in CS-MMEL. So if you are looking in the air operations rules for how long a category C item can run, it is not there and never was.

What Air Operations governs is everything the operator must do: establishing the intervals, running a rectification programme, and the conditions under which an interval may be extended. Two documents, two jobs.

The four categories

CS MMEL.130 sets them out:

Category A is the odd one out and the one most often mishandled. It has no standard duration at all — the condition is written into the MMEL entry itself, and it may be expressed in flights, flight hours or cycles rather than days. You cannot look up a category A deadline; you have to read the item.

Category D carries qualifying criteria too: the absence of the item should not adversely affect crew workload, crews should not rely on the function routinely, and crew training and procedures should not depend on using it.

The detail that moves every date by one day

Each interval in CS MMEL.130 runs excluding the day of discovery.

So a category B item found on a Monday does not expire on Wednesday. Day one is Tuesday, day two Wednesday, day three Thursday. A category C item discovered on the 1st runs to the 11th, not the 10th.

This is the single most common counting error, and it cuts both ways. Counting from the day of discovery makes you conservative by a day, which costs availability. Counting to the wrong end date makes you non-compliant. Neither is a good outcome, and both come from the same four words.

Your MEL can be stricter than the MMEL, never looser

Before any of the above applies to your fleet, one constraint from ORO.MLR.105(e)(1): the operator shall establish rectification intervals for each item, and the rectification interval in the MEL shall not be less restrictive than the corresponding rectification interval in the MMEL.

The category durations above are therefore ceilings, not entitlements. If your own MEL sets a category C item at 5 days, 5 days is your number — and it becomes the basis for any extension calculation too.

Extending an interval: two gates, not one

Almost every discussion of the rectification interval extension covers the operator conditions and stops. There is an earlier gate.

Gate one: does the MMEL even allow it?

CS MMEL.135 states that the MMEL preamble indicates if extension of the rectification intervals of category B, C and D is applicable.

Two things follow. Category A is excluded — there is no extension mechanism for it. And the extension is not automatic even for B, C and D: it exists only if the MMEL preamble provides for it. The type certificate holder can decline to offer it, which is exactly the kind of detail that only surfaces when someone reads the preamble.

Gate two: the operator conditions

ORO.MLR.105(f) then permits, subject to competent authority approval, a procedure for the one-time extension of category B, C and D intervals, provided that:

• the extension is within the scope of the MMEL for the aircraft type;
• it is, as a maximum, of the same duration as the rectification interval specified in the MEL;
• it is not used as a normal means of conducting MEL item rectification, and is used only when events beyond the control of the operator have precluded rectification;
• a description of specific duties and responsibilities for controlling extensions is established;
• the competent authority is notified of any extension; and
• a plan to accomplish the rectification at the earliest opportunity is established.

So the arithmetic is simple: a 10-day category C interval can be extended by at most a further 10 days, once. A 3-day category B item gets at most another 3 days. And if your MEL tightened the interval, the extension shrinks with it.

“Events beyond the control of the operator”

This phrase does a lot of work, and GM1 ORO.MLR.105(f) is helpfully narrow about it. Extensions should only be applied under certain conditions, such as a shortage of parts from manufacturers or other unforeseen situations (e.g. inability to obtain equipment necessary for proper troubleshooting and repair).

A slot that was hard to find, a maintenance plan that slipped, a part that was ordered late — none of these are events beyond the operator’s control. They are planning outcomes. An extension log full of them is a finding waiting to be written.

What the authority expects to receive

AMC1 ORO.MLR.105(f) sets out the administrative side, and it is more demanding than “tell them”.

The operator’s procedures should give the competent authority the name and position of the nominated personnel responsible for controlling extensions, and those personnel should be listed by appointment and name. They must be adequately trained in technical and operational disciplines, with operational knowledge of how flight crew and maintenance personnel actually use the MEL, plus engineering competence.

On timing, the AMC expects notification within one month of the extension, or within the timescales set by the approved procedure. And the notification should specify the original defect, all such uses, the reason for the extension, and the reasons why rectification was not carried out within the original interval.

That last requirement is the one with teeth. “All such uses” means the authority is looking at your pattern, not the single event.

When the extension runs out too

ORO.MLR.105(e)(3) is unambiguous: after expiry of the rectification interval specified in the MEL, the operator may only continue to operate the aircraft when the defect has been rectified, or the interval has been extended in accordance with (f). There is no third option inside the MEL.

Outside it, there is exactly one route, and it is narrow. ORO.MLR.105(j) allows an operator, subject to a specific case-by-case approval by the competent authority, to operate with inoperative items outside the constraints of the MEL but within the constraints of the MMEL — again only where events beyond the operator’s control have precluded MEL compliance, with named personnel controlling it and a plan to return to MEL constraints at the earliest opportunity.

A worked example

Illustrative only — your own MEL and MMEL govern the actual dates.

A category C item is found on 3 March. The MMEL allows 10 calendar days; the operator’s MEL has not tightened it.

• 3 March is the day of discovery and is excluded. Day one is 4 March.
• Ten calendar days takes the interval to 13 March.
• The part is on back-order from the manufacturer and cannot be obtained. The MMEL preamble provides for extension of category C.
• A one-time extension of at most 10 further days is available, taking it to 23 March.
• The authority is notified within one month, with the defect, the reason, and why rectification did not happen in time.
• After 23 March: rectify, or seek a case-by-case approval under ORO.MLR.105(j). Nothing else.

The short version

Categories and durations come from CS MMEL.130. Every interval excludes the day of discovery. Your MEL may be stricter but never looser. Extension needs the MMEL preamble to allow it and the operator conditions in ORO.MLR.105(f) to be met, is one-time, and can never exceed the original interval. Category A cannot be extended at all.

Getting this right across a fleet

Rectification intervals are where MEL theory meets the despatch desk, and where the cost of a misread document is measured in aircraft availability. Our Minimum Equipment List (MEL) Customisation course covers how the intervals are set in your own MEL, the regulatory review behind them, and the approval process for the document as a whole.

Further reading

• What Is a Minimum Equipment List? — what the MEL is and what ORO.MLR.105 requires it to contain
• CS-MMEL: what it is and why it matters — the specification the categories come from
• MEL Customisation: 6 important steps — building the operator document
• Why do we need MEL training? — who needs to understand all this

Frequently asked questions