Wiki·rules·rules/mel-deferral-rules.md

MEL Deferral Rules

The Minimum Equipment List (MEL) is the regulator-approved document that says: "this aircraft can fly with item X inoperative, provided you do Y, for Z amount of time." Every deferral recorded in DP_DPDEF_DISC_DEFERRAL_DTL must reference a specific MEL item.

Categories (from CFG_CFGMELC_MEL_CATEGORY)

The MEL classifies each possible inoperative item into a category that defines its deferral window:

CategoryDeferral limitTypical use
AAs specified in item (usually "no deferral — fix before next flight")Critical systems: primary hydraulics, fire detection, certain autopilot functions
B3 consecutive calendar daysSingle-redundant systems where one side has failed
C10 consecutive calendar daysMulti-redundant systems with partial degradation
D120 consecutive calendar daysCosmetic / convenience items: cabin lighting, some passenger entertainment

Calendar days are counted from the day after the deferral is opened.

Required fields for a valid deferral

A row in DP_DPDEF_DISC_DEFERRAL_DTL must carry:

  • DPDEF_MAINT_REPORT_NO — the parent discrepancy's maint report number
  • DPDEF_DEF_ITEM_TYPE — MEL / CDL / NEF (non-essential equipment)
  • DPDEF_DEF_LIMIT_TYPE — CAL (calendar days), FH (flight hours), FC (flight cycles)
  • DPDEF_DEF_LIMIT_BASIS — the numeric limit
  • DPDEF_DEFERRAL_REASON — operational justification
  • A valid MEL reference — links to CFG_CFGMEL_MDL_CONFIG_MEL for the specific aircraft model's MEL

If any of these is missing, the deferral is invalid and the discrepancy reverts to UNRESOLVED. This is enforced at both the ERP and the regulator-audit level.

Limit arithmetic

A deferral expires at the earliest of its limits:

expiry_date = min(
    opened_date + calendar_limit,        (if CAL)
    first_date_where(aircraft_cumulative_FH >= opened_FH + FH_limit),
    first_date_where(aircraft_cumulative_FC >= opened_FC + FC_limit)
)

Usage is tracked in DP_MRUSG_USAGE_DEF_PARAM_VAL. The ERP increments the used counter from parameter updates on AMEPackage closeouts (flog_tlgpm_param_upd_dtl).

Extensions

A deferral can be extended once, to the next-highest category's limit, with engineering approval. Extensions land as new revisions in DP_DEFHST_DISC_DEFERRAL_HST with a revised DPDEF_DEF_LIMIT_BASIS. Each extension:

  • Requires a separate authority signature
  • Cannot change the category (B stays B, can't become C via extension)
  • Must be audited at renewal

Cross-forward rules

Some MEL items have carry-forward conditions — when an aircraft is out of service for a visit, open deferrals may be carried forward into the post-visit state without re-certification. This is tracked in DP_CFWDDTL_MEL_PART_CF_DTL (empty in this drop, but the schema is defined).

Hard-stop conditions

Regardless of category, the aircraft cannot depart if:

  • The deferral is past its expiry
  • The deferral lacks a valid MEL reference
  • Multiple related deferrals combine to degrade redundancy below minimum (concurrence check — not fully enforced in data, checked by engineering review)

Related rules

See also