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A Minimum Equipment List (MEL) deferral permits an aircraft to continue operating with specific equipment inoperative, subject to defined conditions, operational limitations, and time-limited rectification intervals. This page provides the technical reference for MEL categories, interval calculations, regulatory requirements, and the deferral lifecycle as implemented in PlaneConnection.
This reference covers the data model and rules enforced by PlaneConnection. For step-by-step procedures, see Manage MEL Deferrals. For the broader discrepancy tracking reference, see MEL Deferrals.

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MEL Categories and Rectification Intervals

The FAA Master MEL (MMEL) defines four deferral categories, each with a mandatory rectification interval. PlaneConnection enforces these intervals with countdown timers and automated alerts.
CategoryIntervalCalendar DaysDescriptionAlert Behavior
AAs specified in remarksVariesRepair interval is defined in the specific MEL item remarks or the MMEL. Typically the most time-critical items. Some Category A items must be rectified before the next flight.Alert at interval specified in MEL remarks
B3 consecutive calendar days3Must be rectified within 3 consecutive calendar days, excluding the day of discovery.Alert at 1 day remaining
C10 consecutive calendar days10Must be rectified within 10 consecutive calendar days, excluding the day of discovery.Alert at 3 days remaining
D120 consecutive calendar days120Must be rectified within 120 consecutive calendar days, excluding the day of discovery. Least urgent category.Alert at 14 days remaining

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Interval Calculation Rules

Deferral intervals are measured in consecutive calendar days, not flight days, duty days, or business days. The day the item is discovered (deferred) does not count toward the interval.
Example: A Category B item deferred on Monday does not start counting until Tuesday. The 3-day interval expires at the end of Thursday (Tuesday, Wednesday, Thursday = 3 calendar days). PlaneConnection calculates the expiration date as: 
expiration_date = deferred_date + (interval_days + 1) calendar days
The additional day accounts for the exclusion of the discovery date. The system displays both the expiration date and a real-time countdown showing days remaining.

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Category A Special Handling

Category A items have no fixed interval — the rectification window is defined in the remarks column of the specific MEL entry. Common Category A patterns include:
  • “Rectify before next flight”
  • “Rectify within 1 calendar day”
  • “As specified in the operator’s approved MEL”
When creating a Category A deferral in PlaneConnection, the operator must manually enter the expiration date or interval based on the approved MEL entry. The system validates that an expiration is provided but does not impose a default interval.

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Regulatory Basis

MEL deferrals operate within a regulatory framework that spans both operating rules and maintenance requirements.

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Primary Regulations

RegulationScopeRequirement
14 CFR 91.213All operationsEstablishes when an aircraft may be operated with inoperative instruments and equipment. Requires reference to an approved MEL or the aircraft equipment list.
14 CFR 135.179Part 135 operationsRequires Part 135 certificate holders to have an approved MEL based on the FAA Master MEL (MMEL) for each aircraft type. The MEL must be carried aboard the aircraft.
14 CFR 91.403All operationsEstablishes the owner/operator’s responsibility for maintaining the aircraft in an airworthy condition, including compliance with MEL deferral deadlines.
14 CFR 91.213(d)Operations without MELSpecifies conditions under which aircraft without an approved MEL may operate with inoperative equipment (placard, equipment list, and kind-of-operations check).

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Supporting Guidance

DocumentPurpose
MMEL / MEL Policy Letter 25FAA guidance on MEL revision, amendment procedures, and category definitions.
OpSpec D095Operations Specification authorizing the operator to use an MEL. References the specific MMEL revision applicable to each aircraft type.
AC 91-67Advisory Circular providing guidance on MEL and MMEL policies and procedures.
PlaneConnection does not replace the operator’s FAA-approved MEL document. The system tracks deferral records, calculates expiration dates, and supports dispatch decisions. Operators must always refer to their approved MEL for specific deferral conditions, placards, (O) operating procedures, and (M) maintenance procedures.

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Deferral Lifecycle

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Status Flow

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Status Definitions

StatusDispatch ImpactDescription
OpenEvaluated per flightDiscrepancy reported but no deferral decision made. Must be evaluated before each dispatch.
DeferredConditionally dispatchableDeferred under MEL with category, conditions, and time limit. Aircraft may dispatch subject to MEL operational limitations.
In ProgressDepends on work scopeCorrective action underway. Dispatch availability depends on whether work requires aircraft grounding.
ResolvedNo impactItem repaired, tested, and returned to service. No dispatch restrictions.
ExpiredAircraft groundedDeferral interval has expired without rectification. Aircraft must not dispatch until the item is resolved.

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Required Deferral Fields

When deferring a discrepancy under the MEL, the following fields are required to create a compliant deferral record:
FieldTypeRequiredDescription
mel_categoryEnum (A, B, C, D)YesMEL category per MMEL. Determines rectification interval.
mel_referenceStringYesReference to the specific MEL item number (e.g., “28-20-01”).
ata_chapterStringYesATA 100 chapter code identifying the affected system (e.g., “28 — Fuel”).
deferred_dateDateYesDate the item was deferred. Interval counting begins the following day.
expires_atDateAutoCalculated from category interval and deferred date (manual for Category A).
operational_limitationsTextNoAny operational restrictions or conditions while the item is deferred.
descriptionTextYesDetailed description of the inoperative equipment and its condition.
severityEnumYesSeverity level: minor, major, critical, or grounding.

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Severity and Dispatch Interaction

The severity level works alongside the MEL category to determine dispatch availability:
SeverityMEL-DeferrableDispatch Rules
MinorYesDispatch normally with deferral notation in aircraft records.
MajorYesDispatch with MEL conditions applied. Brief flight crew on limitations.
CriticalConditionalDispatch only with explicit approval from Chief Pilot or Director of Operations.
GroundingNoAircraft cannot be deferred. Must be repaired before any further flight.

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Extension Requests

In certain circumstances, an operator may request an extension of a deferral interval beyond the standard MEL category limits. Extensions require:
  1. Justification — documented reason why the item cannot be rectified within the standard interval (parts availability, MRO scheduling, etc.)
  2. Approval authority — Director of Maintenance or Chief Inspector must approve the extension
  3. Revised expiration — new expiration date recorded in the system
  4. Operational review — any additional operational limitations during the extension period
Extensions beyond MEL category limits may require coordination with the operator’s Principal Inspector (PI) at the local FSDO. PlaneConnection records the extension but cannot validate FAA approval of extended intervals.

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Deferral Statistics

PlaneConnection tracks the following MEL metrics on the Maintenance dashboard:
MetricDescription
Active deferralsTotal currently deferred items across the fleet.
Expiring soonItems within 25% of their remaining interval (e.g., 1 day left on a B item).
ExpiredItems past their rectification deadline. Requires immediate action.
By categoryBreakdown of active deferrals by MEL category (A, B, C, D).
By aircraftCount of active deferrals per tail number for fleet-wide visibility.

MEL Deferrals Reference

Discrepancy record fields, severity levels, and corrective action tracking.

Aircraft Statuses

How aircraft operational status interacts with MEL deferral dispatch decisions.

Manage MEL Deferrals

Step-by-step guide to creating, extending, and resolving MEL deferrals.

Work Order Statuses

Lifecycle of the work orders created to rectify deferred items.
Last modified on April 5, 2026