When an aircraft arrives at the gate with a write-up in the logbook, the question is never simply whether a mechanic can clear the discrepancy — it is whether the flight can legally and safely depart with that item deferred. Under 14 CFR 121.628, certificated air carriers operating under Part 121 must have a formal, FAA-approved process for handling inoperative instruments and equipment, and that process places the aircraft dispatcher at the center of every go/no-go decision involving a deferral. Understanding this regulation — and the industry guidance that supports it in Advisory Circular 120-77 — is essential for any dispatcher certificate candidate or working dispatcher.
The regulation exists because an aircraft's airworthiness is not static. Between scheduled maintenance visits, components fail, sensors drift, and avionics malfunction. A structured deferral system allows operations to continue safely while managing known deficiencies, but only when the people responsible for the flight — pilots, mechanics, and dispatchers — communicate and agree on the implications of every deferred item before departure.
The Regulatory Framework of 14 CFR 121.628
Section 121.628 prohibits a Part 121 air carrier from dispatching or releasing an aircraft with inoperative instruments or equipment unless specific conditions are met. The regulation establishes a hierarchy of authority that governs what can be deferred and under what circumstances:
- An approved Minimum Equipment List (MEL) must exist for the aircraft type, and the specific inoperative item must be listed within it. The MEL is developed by the carrier, approved by the FAA's Flight Standards District Office (FSDO) or certificate-holding district office (CHDO), and tailored to the carrier's specific operation.
- The aircraft must be operated under all applicable conditions and limitations specified in the MEL for that deferred item. These may include required placarding, operational procedures, crew awareness, or maintenance actions that must be completed before flight.
- The certificate holder's operations specifications (OpSpecs) must authorize the use of the MEL. The MEL itself is incorporated by reference into the carrier's OpSpecs, giving it regulatory force.
- Items not listed in the MEL cannot simply be ignored. If an item is required by the aircraft's type certificate, airworthiness directives, or the applicable FARs and it does not appear in the MEL, the aircraft is not airworthy and cannot be dispatched.
The regulation also addresses items that do not have an MEL entry but are not required for the specific flight. In such cases, the carrier may defer the repair under a Configuration Deviation List (CDL), which governs missing or damaged exterior components — panels, fairings, and similar structural or aerodynamic elements — provided the CDL is also FAA-approved and incorporated into OpSpecs.
The Dispatcher's Role in MEL Coordination
The dispatcher's authority under 14 CFR 121.533 establishes joint operational control of every flight with the pilot-in-command. That joint authority does not pause when a maintenance issue arises — it becomes even more critical. When a mechanic defers an item under the MEL, the dispatcher must evaluate how that deferral affects the planned flight, including the route, the alternate requirements, the fuel load, and any operational restrictions imposed by the MEL.
Advisory Circular 120-77 amplifies this coordination requirement. The AC describes the MEL as a document that must be actively managed, not simply filed. It emphasizes that the dispatch release — the legal authorization for the flight — must reflect the actual aircraft configuration, including all active deferrals. A dispatcher who signs a release without knowing the current MEL status of the aircraft has failed in their fundamental duty. In practice, this means:
- The dispatcher must have access to the aircraft's current MEL status, typically through the carrier's maintenance tracking system or a computer-based deferral log.
- Before issuing a dispatch release, the dispatcher must confirm that all active deferrals are permissible for the intended flight — including the route, weather, alternate airports, and en route conditions.
- If an MEL item imposes an operational restriction (such as prohibiting flight into known icing conditions, or limiting the aircraft to a lower maximum altitude), the dispatcher must plan the flight accordingly and communicate those restrictions to the crew.
- If the MEL requires a specific maintenance action (a so-called Category A or Category B limitation with a required procedure) before flight, the dispatcher must confirm with maintenance that the action has been completed and documented before releasing the flight.
MEL Categories and Time Limits
The FAA's Master Minimum Equipment List (MMEL) — published for each aircraft type — provides the template from which individual carrier MELs are derived. Items in the MMEL and carrier MEL are assigned repair interval categories that govern how long a deferral may remain open:
- Category A: Items with specific time limits stated in the MEL itself (e.g., a certain number of flight hours or calendar days). The time limit begins when the discrepancy is first deferred.
- Category B: Items that must be repaired within 3 consecutive calendar days, excluding the day the malfunction was recorded.
- Category C: Items that must be repaired within 10 consecutive calendar days, excluding the day the malfunction was recorded.
- Category D: Items that must be repaired within 120 consecutive calendar days, excluding the day the malfunction was recorded.
These time limits are not suggestions — they are regulatory requirements once the deferral is entered in the aircraft records. The dispatcher must be aware of any active deferrals that are approaching their expiration. A flight dispatched after a deferral has expired is operating an unairworthy aircraft, regardless of whether the item was previously approved for deferral. Carriers typically build automated alerts into their maintenance tracking systems to warn dispatchers of approaching time limits.
The Dispatch Release and Airworthiness Confirmation
The dispatch release required by 14 CFR 121.687 must include a statement of airworthiness. For domestic operations, the dispatcher signs this release jointly with the captain. That signature is a legal representation that the aircraft is airworthy for the intended flight. When deferrals are active, that statement is only valid if:
- Each deferred item is listed in the carrier's approved MEL with an entry that covers the aircraft's specific configuration.
- All required maintenance actions (M-items) and operational procedures (O-items) specified in the MEL for each deferred component have been accomplished.
- The flight conditions — weather, terrain, route, and airport requirements — are compatible with all active deferral restrictions.
- The deferral has not exceeded its applicable repair interval category time limit.
If any of these conditions cannot be confirmed, the dispatcher must not issue the release. The flight must be delayed until the discrepancy is resolved or the aircraft is ferried to a maintenance facility under an appropriate authorization.
Why This Process Matters Operationally
The coordination requirement in 121.628 is not bureaucratic overhead — it prevents accidents. Multiple accident investigations have identified cases where flights departed with deferred equipment that created hazards the crew was either unaware of or unprepared to manage. When dispatch is integrated into the MEL process, a second set of eyes evaluates whether the sum of all active deferrals still leaves the flight with adequate safety margins for the specific route and conditions of that day.
Consider a scenario: an aircraft has an inoperative weather radar receiver deferred under the MEL. Under many carrier MELs, flight into known or forecast areas of convective activity may be prohibited or require specific crew procedures when weather radar is deferred. If the dispatcher is reviewing a route through an area of forecast afternoon thunderstorm development and does not know about the radar deferral, the crew may face a hazardous situation without the primary tool for thunderstorm avoidance. The coordination requirement in 121.628 is designed to prevent exactly this outcome.
Key Numbers and Rules
- Category B deferrals: 3 consecutive calendar days (excluding day of entry)
- Category C deferrals: 10 consecutive calendar days (excluding day of entry)
- Category D deferrals: 120 consecutive calendar days (excluding day of entry)
- MEL authority flows from OpSpecs; no MEL is valid without OpSpec authorization
- The dispatcher's signature on the release is a joint legal certification of airworthiness under 14 CFR 121.533 and 121.687
- Items not in the MEL require return to airworthy condition before dispatch — no informal deferrals are permitted
- CDL items cover missing or damaged exterior components; MEL items cover inoperative instruments and equipment
Common Test Traps
- Confusing MEL and CDL scope: The MEL covers inoperative equipment; the CDL covers missing or damaged external parts and surfaces. Exam questions will present scenarios that blur this line.
- Misreading category time limits: The day the discrepancy is recorded does NOT count. A Category B item entered on Monday expires at the end of Thursday (days 1, 2, 3 = Tuesday, Wednesday, Thursday).
- Assuming the mechanic's sign-off is sufficient: A maintenance release does not substitute for the dispatcher's review. Both functions must independently confirm airworthiness before the release is issued.
- Forgetting operational restrictions: Approving a deferral is not the end of the analysis. The dispatcher must evaluate whether any restrictions attached to the deferral affect the planned flight — route, altitude, weather avoidance, or alternate requirements.
- Treating expired deferrals as still valid: Once a deferral's repair interval expires, the aircraft is unairworthy regardless of prior authorization. The dispatcher must track expiration dates as part of pre-departure review.