Part 135 MEL dispatch: how to release legally with inoperative equipment

An aircraft with an inoperative item is not automatically grounded. Under 14 CFR 135.179, an aircraft may be operated with inoperative instruments or equipment when a Minimum Equipment List (MEL) is authorized under the operator's OpSpecs and the inoperative item is listed in that MEL as a "go" item — with or without limitations.

The MEL exists to give operators a documented, FAA-approved framework for making these decisions consistently and legally. The alternative — making the call ad hoc each time — is not sanctioned and exposes the operator to certificate action if the basis for the dispatch cannot be reconstructed.

MMEL vs. MEL — the distinction matters

The Master Minimum Equipment List (MMEL) is the FAA-approved baseline document for each aircraft make and model. It lists items that may be inoperative and still permit flight, and sets the maximum relief the FAA will grant for that aircraft type.

The Minimum Equipment List (MEL) is the operator's document — derived from the MMEL and approved as part of the OpSpecs. An operator's MEL may be more restrictive than the MMEL but never more permissive. If an item is not in the MMEL, it cannot be in the operator's MEL as a go-item.

An operator without an approved MEL must treat every inoperative item as a no-go unless the Kinds of Operations Equipment List (KOEL) or the Aircraft Flight Manual (AFM) specifies otherwise. This is a more restrictive baseline — the MEL is a relief from that baseline.

How MEL items work: categories and repair intervals

MEL items are assigned repair interval categories that set how long the item may remain deferred:

• Category A — Specific time limit stated in the MEL remarks (may be hours or calendar days, varies by item).
• Category B — Must be repaired within 3 consecutive calendar days, excluding the day of discovery.
• Category C — Must be repaired within 10 consecutive calendar days, excluding the day of discovery.
• Category D — Must be repaired within 120 consecutive calendar days, excluding the day of discovery.

"Excluding the day of discovery" means if an item is found inoperative on the 1st, a Category C item must be repaired by the 11th, not the 10th.

A repair interval that expires does not mean the item is repaired — it means the deferral is no longer authorized and the aircraft cannot be dispatched until the item is corrected or re-deferred under an approved extension procedure (if one exists in the MEL).

Who has authority to defer an item

Deferral authority under a Part 135 MEL is typically held by the Director of Maintenance (DOM) or a person authorized by the operator's maintenance program. The PIC does not have unilateral authority to defer an item simply by annotating the logbook — the MEL procedures must be followed, which typically requires a maintenance release or a deferred maintenance entry signed by an authorized person.

Some operators authorize the PIC to defer Category D items in the field, but this must be explicitly written into the MEL procedures and the OpSpecs. If it is not, the PIC cannot do it.

Required placard and crew notification

When an item is deferred under the MEL, the MEL typically requires:

• A placard on or near the inoperative item indicating its status.
• An entry in the aircraft maintenance logbook.
• Crew notification — the PIC must be informed of all deferred items before accepting the aircraft for flight.

A deferred item without a placard is not properly deferred. The placard is part of the crew-notification chain — it ensures that the next crew to accept the aircraft knows the item is inoperative without having to search the logbook.

How the pre-dispatch gate handles MEL items

The dispatch gate checks two things for MEL-deferred items: (1) whether the item is in the operator's MEL at all, and (2) whether the repair interval has expired. An item that is in the MEL and within its repair interval is a go with the applicable limitations. An item whose repair interval has expired, or an item not in the MEL, returns UNABLE.

The dispatch gate does not evaluate whether the MEL procedure was followed correctly — that is a maintenance-authority determination. What it checks is the expiry: is this deferral still legally active on the date of the proposed flight?

The most common MEL-related UNABLE in practice is a Category C item deferred on day 1 and not tracked — on day 11, the deferral has expired and no one noticed because the item was entered in the logbook but not in the dispatch system.

Items with no MEL relief

Not everything can be deferred. Required equipment under 14 CFR 135.149 and 135.177 (and the applicable aircraft certification basis) is not subject to MEL relief unless the MMEL specifically provides it. The emergency equipment required under 135.167 and 135.168 — life rafts, survival kits, ELT — generally has no MEL relief available.

If an item is required by the operating rules (not just the AFM) and the MMEL does not list it as deferrable, it is a no-go regardless of what the operator's maintenance personnel write in the logbook.