MEL — Dispatch and Operational Limits

The Minimum Equipment List (MEL) is the document that determines whether the aircraft can be dispatched with one or more items inoperative. For ATA 29 (hydraulics), the MEL is structured into multiple layers — one keyed to the ECAM warning the crew has just observed, one keyed to the specific component that has failed, and one explaining the procedural handling for false-warning conditions. Understanding the structure is what lets the crew or dispatcher answer the question "can this aircraft go?" in a defensible, documented way.

This article walks through the MEL's structure for ATA 29, the categories of repair interval used across all MEL items, the dispatch philosophy that hydraulic items follow (most real warnings = NO-GO; specific exceptions exist), and worked examples for the two extremes — an item that can be dispatched with conditions (Green electric pump) and an item that cannot be dispatched at all (the RAT).

It is not, and cannot be, a substitute for the operator's own MEL. Specific entry numbers, decision rules, and category definitions vary across operators, fleet configurations, and regulatory environments. The article gives the architecture; the operator's document is the authority.

1. The two MEL layers for ATA 29

The MEL's hydraulic section is built on two complementary lookup paths:

A330 MEL — ATA 29 (Hydraulics)
   │
   ├─ ME-29 — MEL Entries (keyed to the ECAM warning)
   │     Each entry corresponds to one specific ECAM warning the crew
   │     observed. Examples of entries (representative; the exact set
   │     and identifiers vary by operator):
   │       • HYD G/B/Y RSVR LO LVL
   │       • HYD G/B/Y RSVR OVHT
   │       • HYD G/B/Y RSVR LO AIR PRESS
   │       • HYD G/B/Y SYS LO PR
   │       • HYD G/B/Y PUMP LO PR (per pump)
   │       • HYD G/B/Y ELEC PUMP FAULT
   │       • HYD G+B / G+Y / B+Y SYS LO PR (dual-system combinations)
   │       • HYD MONITORING FAULT  (HSMU related)
   │       • HYD RAT FAULT
   │     Typically ~25 distinct entries covering the full ATA-29 alert set.
   │
   └─ MI-29 — MEL Items (keyed to the component or function)
         Subdivided into:
         •  MI-29-01 — Pushbutton-switch indicators
         •  MI-29-07 — SD HYD page indications
         •  MI-29-09 — ECAM warning handling (one entry per warning class,
         │            covers the false-warning procedural path)
         •  MI-29-10 — Primary components MEL (typically ~10 items:
         │            engine-driven pumps, depressurization function,
         │            system accumulator, brake accumulator, FSOV,
         │            case-drain filter, HP/LP filters, etc.)
         •  MI-29-20 — Auxiliary hydraulic components MEL (typically 5
                       items: Green electric pump, Green RAT, Blue
                       electric pump, Yellow electric pump, Yellow hand
                       pump)

The two paths reflect the two diagnostic starting points: "I saw warning X" or "Component Y has failed." Both lead to the same dispatch decision but through different lookups.

2. The crew's lookup flow

For any hydraulic event the crew observes, the procedural sequence is:

ECAM warning appears in flight
        │
        ▼
Crew completes the ECAM procedure
        │
        ▼
Status page documents the failed condition and any operational notes
        │
        ▼
On the ground, after the flight:
        │
        ▼
[Distinguish: real warning vs spurious warning]
   – Real:   the sensor signal reflects an actual system condition
   – Spurious: a sensor or wiring fault produced the warning without
              the underlying system condition existing
        │
        ▼
Look up the ECAM warning in ME-29
   – Real warning → most entries say "no dispatch" (see §3)
   – Spurious warning → entry points to the corresponding MI-29-09 item
                        for procedural handling
        │
        ▼
If dispatch is permitted under MI-29-XX, check:
   – Repair interval (A / B / C / D, see §4)
   – Number installed vs number required
   – Whether a placard is needed in the cockpit
        │
        ▼
Apply additional operator OM / regulatory requirements
        │
        ▼
Make the dispatch decision

The flow is procedural, not opinion-based. The crew does not "decide" whether a warning is real or spurious — the diagnostic criteria for that distinction are spelled out in the procedural articles (see Pump vs System Failure for the technique that distinguishes pump-level from system-level real conditions, and equivalent cross-checks for the reservoir-side cautions).

3. The "no-dispatch" philosophy for hydraulic real warnings

The dominant pattern across A330 hydraulic ME-29 entries is "real warning → NO DISPATCH". This is the design philosophy at the level of the MEL itself, not a single entry's quirk.

3.1 The pattern, with representative entries

The dispatch decision is dominated by the "no dispatch on real warning" answer. The crew or dispatcher reading the MEL for any of these warnings sees the same recurring text — "real warning → no dispatch."

3.2 Why the philosophy is restrictive

The reasoning, consistent across the warning set:

• Hydraulics is a flight-safety-critical core system. A real hydraulic warning means a documented loss of function in a system the aircraft depends on for flight control, gear, brakes, and reversers.
• Real warnings indicate actual system degradation. Unlike spurious warnings (which are sensor or wiring artefacts), a real warning means the architectural margin has been eroded. The aircraft is not in the as-designed state.
• Limited tolerance for cascading failures. With one system degraded, the crew is one failure away from a dual-system condition. Dispatch under these terms accepts that further failure during the flight could exceed the architecture's design envelope.
• Limited operational impact of repair. A330 hydraulics is largely maintained at line-level; repairing a real hydraulic fault is typically done within hours, not days. The cost of refusing dispatch is moderate compared to the safety benefit.

3.3 The one notable exception — electric pump faults

A consistent exception across operator MELs:

HYD G/B/Y ELEC PUMP FAULT — real warning → see MI-29-20 (auxiliary hydraulic components) for dispatch with conditions.

The exception exists because the electric pump is a backup component, not a primary. Per FCOM, "as a general rule, do not manually select a HYD ELEC PUMP ON, except temporarily, to retract the spoilers if they remain out after a hydraulic failure." That is, the electric pump's normal contribution to flight is small (it does not power the system in cruise), and its loss does not significantly degrade the architecture.

Loss of the electric pump primarily affects:

• Specific automatic triggers (Green ELEC PUMP for gear retraction; Blue for rudder authority on Eng 1 + PRIM 1/3 fault; Yellow for slats/flaps on Eng 2 fault).
• Manual selection availability.

These are degraded — not eliminated. The system retains its EDP and (for Green) its RAT. Dispatch with the electric pump inoperative is therefore reasonable under defined conditions.

4. Repair interval categories

The MEL uses a small set of repair-interval categories applied uniformly across all MEL items (not just ATA 29). The categories represent the maximum permissible elapsed time between fault detection and corrective action while the aircraft remains in service.

The interval starts from the day the fault is recorded. The aircraft can continue in service throughout the interval as long as MEL conditions are met. At the end of the interval, the fault must be corrected or the aircraft cannot be dispatched until the repair is made.

For ATA 29, Category C (10 days) is the most common where dispatch with a deferred hydraulic item is permitted. The 10-day window allows for parts ordering, scheduling, and maintenance turnaround without aircraft grounding.

5. The MI-29 component-MEL table — typical structure

Each component-level MEL item (in MI-29-10 or MI-29-20) follows a standard tabular structure:

The crucial comparison is between Installed quantity and Required quantity:

• If Required = Installed → all instances must be operational → no dispatch with the item inoperative.
• If Required < Installed → some instances can be inoperative within the limit → dispatch with restrictions.
• If Required = 0 → the component can be entirely inoperative → dispatch with restrictions, often with a placard requirement.

6. Worked example — Green electric pump (dispatchable)

The Green electric pump MEL entry, as a representative example:

Interpretation:

• The aircraft can be dispatched with the Green electric pump completely inoperative. (Required quantity = 0 means no operational electric pump on Green is needed for dispatch.)
• Repair must be made within 10 days.
• A placard is posted in the cockpit to remind the crew that the Green electric pump is not available, so that:
  • Manual selection attempts will not produce pump operation.
  • The automatic gear-retraction trigger after an engine failure will not fire — gear retraction with one engine inoperative depends on the surviving EDP only.
  • The procedural awareness is in place for the abnormal cases where the electric pump would normally contribute.

The dispatch is conditional, not unconditional. The 10-day window is a tolerance, not a license — the aircraft is operating with reduced redundancy throughout.

7. Worked example — RAT (not dispatchable)

The Green RAT MEL entry, by contrast:

Interpretation:

• The aircraft cannot be dispatched with the RAT inoperative. (Required quantity = Installed quantity = 1.)
• No repair interval applies — the item cannot be deferred at all. Repair must be completed before the next flight.
• No placard is needed because the situation does not permit operation in this state.

Why the RAT is treated as no-dispatch

The RAT is the architectural answer to the worst-case hydraulic scenario:

• Dual engine failure → Green system has no EDP source.
• Combined reservoir loss (Green + Blue, or Green + Yellow) → Green's recovery path needs the RAT.

The RAT also supplies the emergency generator (ATA 24), giving electrical AC EMER bus availability in dual-engine-out conditions.

Allowing dispatch with the RAT inoperative would mean accepting flight without the architectural last-resort recovery for both hydraulic and emergency-electrical scenarios. The regulatory and operational frameworks do not permit this — the RAT is treated as a safety-critical item that must be available for every dispatch.

8. Other components — summary view

Without listing every component (which varies by operator MEL), the general pattern across MI-29-10 and MI-29-20:

The pattern recurs: primary sources, monitoring functions, and the RAT are no-dispatch items; backup pumps, accumulators, and filters are typically dispatchable under conditions.

9. Spurious warnings — the MI-29-09 path

Where a warning is determined to be spurious (sensor fault, wiring fault, transient — see Pump vs System Failure for diagnostic criteria), the ME-29 entry typically refers the crew/dispatcher to the corresponding MI-29-09 sub-item.

The MI-29-09 items cover spurious warnings for each ECAM warning class:

• 29-09-01 — Electric pump FAULT warnings (G/B/Y)
• 29-09-02 — Pump LO PR warnings (Blue ENG 1, Yellow ENG 2)
• 29-09-03 — Green pump LO PR warnings (multiple combinations)
• 29-09-04 — Reservoir LO AIR PRESS warnings
• 29-09-05 — Reservoir LO LVL warnings
• 29-09-06 — Reservoir OVHT warnings
• 29-09-07 — System LO PR warnings
• 29-09-08 — Monitoring fault (HSMU)
• 29-09-09 — RAT fault

Each item lists the diagnostic confirmation, the allowed dispatch conditions, the repair interval, and any operational restrictions. Spurious warnings often permit dispatch with conditions where the same warning treated as real would not.

The procedural distinction matters because a sensor or wiring fault is a maintenance-side issue with no real impact on hydraulic system performance, while a real warning is a hydraulic-side issue with operational consequences.

10. Beyond the MEL — operator OM and regulatory considerations

The MEL is not the only document governing dispatch. The crew and dispatcher work within several overlapping frameworks:

Several considerations follow:

• The operator MEL is more restrictive than the MMEL. Operators can choose to enforce shorter repair intervals, more restrictive conditions, or block dispatch on items the MMEL would allow.
• The OM may add operational restrictions. For example, a dispatched MEL item might require weather minima above a certain threshold, a specific route avoidance, or extra fuel margin.
• Regulatory requirements may override MEL provisions. Certain conditions defined in national or international regulations cannot be deferred under MEL.

The dispatch decision is the intersection of all these constraints. A condition that the MEL alone would permit may still be no-go because of OM or regulatory restrictions. The crew and dispatcher work with the most restrictive applicable rule.

11. Practical takeaways for the pilot

The MEL is not casually used. A few practical points:

1. Do not skip the MEL procedure. Even if the ECAM procedure is complete in flight, the post-flight MEL check is mandatory. The MEL determines whether the aircraft can continue in service.
2. Distinguishing real from spurious warnings is procedural, not opinion-based. The diagnostic criteria are documented; the crew applies them, the dispatcher confirms them, and maintenance verifies them on the ground.
3. The "no-dispatch" list dominates ATA 29. Most hydraulic real warnings produce a "no dispatch" outcome. The crew should expect this rather than be surprised by it.
4. ELEC PUMP FAULT is the notable exception. A real ELEC PUMP FAULT is dispatchable under conditions — the only common ATA-29 real warning with this disposition.
5. The operator's OM is usually more restrictive than the MEL. Read the OM dispatch chapter alongside the MEL — they sometimes contradict, and the more restrictive applies.
6. Placards in the cockpit must be respected. A placard means a specific MEL item is in deferred status. The crew adapts operations accordingly and does not remove or obscure the placard.
7. The dispatch decision is documented. Every deferred item, every condition, every restriction — recorded in the aircraft's technical log so the next crew can review the state on arrival.

Self-test

[!note]- Q1. What are the two main layers of the ATA-29 MEL, and which one does the crew typically consult first?

The MEL is structured as ME-29 (entries keyed to ECAM warnings) and MI-29 (items keyed to components or functions). The crew typically consults ME-29 first, because the starting point is "what warning did I see?" The ME-29 entry either gives the dispatch decision directly (most commonly "no dispatch" for real warnings) or refers the crew to a specific MI-29 sub-item (MI-29-09 for the procedural handling of spurious warnings, or MI-29-10/MI-29-20 for component-level dispatch with conditions). The two layers are complementary — ME-29 by warning, MI-29 by component — and lead to the same final decision through different lookups.

[!note]- Q2. The MEL Category C means a repair interval of 10 consecutive calendar days. What does this mean in practical terms, and when does the clock start?

Category C means the aircraft can be dispatched with the affected item inoperative for up to 10 calendar days from the date the fault was recorded. The clock starts on the day the fault is entered in the technical log, regardless of whether the aircraft flew that day. The 10-day window allows for parts ordering, scheduling, and maintenance turnaround. At the end of the 10 days, if the fault has not been corrected, the aircraft cannot be dispatched until the repair is made. The crew is not required to ground the aircraft mid-flight if the 10-day window expires; that requirement applies at the next dispatch decision. Operators may apply shorter intervals than the MMEL Category C — the operator MEL is more restrictive than the manufacturer's baseline.

[!note]- Q3. The Green electric pump has Required Quantity = 0 and Repair Interval Category C. The RAT has Required Quantity = 1 and Repair Interval "—". What does this tell you about how the architecture views each component?

The Green electric pump is treated as a backup or supplementary component — the aircraft can fly with zero operational Green electric pumps for up to 10 days. The architecture's hydraulic generation is provided primarily by the two Green EDPs; the electric pump contributes only under specific automatic-trigger conditions. The dispatch posture reflects this hierarchy: lose the backup, accept the conditions, repair within the interval. The RAT is treated as a safety-critical last-resort component — the aircraft must have one operational RAT for every dispatch, and the dispatch cannot be deferred even briefly. The architecture's dual-engine-out recovery and the dual-reservoir-low recovery both depend on the RAT; allowing dispatch without it would accept a flight without the architectural last-resort. The two MEL postures encode the two architectural roles.

[!note]- Q4. Why does the A330 MEL treat most hydraulic real warnings as "no dispatch"?

The pattern reflects four converging considerations: (1) hydraulics is a flight-safety-critical core system, with failure consequences extending to flight controls, gear, and braking; (2) a real warning indicates documented system degradation, meaning the architecture is no longer in its as-designed state; (3) the architecture's redundancy is engineered for the case where the aircraft starts with all systems healthy — accepting dispatch with one system already degraded means accepting that subsequent failures during flight could push the system beyond its design envelope; (4) the operational impact of repairing a real hydraulic fault is typically modest (hours at line-maintenance level), so the cost of refusing dispatch is small compared to the safety benefit. Combined, the design philosophy supports a restrictive dispatch posture for real warnings, with the few exceptions being limited to backup components (electric pumps, filters, accumulators with restrictions).

[!note]- Q5. The crew lands with a HYD G RSVR LO LVL warning that has been determined to be spurious (the SD HYD page showed normal quantity throughout, but a level sensor failure triggered the warning intermittently). What does the dispatch flow look like for this aircraft on the next flight?

The flow:

1. The crew enters the spurious warning in the technical log, noting that the SD HYD page indications throughout the flight showed normal quantity and confirming no actual fluid loss occurred.
2. Maintenance investigates and traces the spurious warning to a level-sensor or wiring fault.
3. The dispatcher looks up the ME-29 entry for HYD G RSVR LO LVL. The "spurious warning" path refers to MI-29-09-05.
4. The MI-29-09-05 sub-item describes the dispatch conditions for a spurious LO LVL warning — typically Category C with operational restrictions (e.g., crew briefed on the sensor fault, sensor replaced or inspected at next maintenance opportunity, alternate cross-check of fluid quantity during pre-flight).
5. The aircraft can be dispatched within the conditions of MI-29-09-05, with a placard if required.
6. The fault must be repaired within the Category C repair interval (10 days). The procedural distinction between real and spurious matters because the spurious path permits dispatch where the real path does not. The diagnostic criteria for this distinction are spelled out in the related procedural articles and applied through documentation, not crew opinion.

References

Per the manufacturer's MMEL for ATA 29 items and the typical structure of operator MEL implementations (the MEL document is the dispatch authority); FCOM PRO-ABN-HYD (provides the procedural context for the warning conditions that the MEL responds to); operator OM dispatch chapter (often more restrictive than the MEL alone). Cross-references to the diagnostic technique in Pump vs System Failure for distinguishing real from spurious warnings.

This article gives the structural overview of the ATA-29 MEL and is independent study material. It is not a substitute for the operator's own MEL, which is the authoritative document for any dispatch decision. Always defer to the current operator MEL, OM, and regulatory framework for operational use.