MEL Dispatch for ATA-34

The previous articles cover "how to fly it broken"; this one covers "can it be dispatched broken." The Minimum Equipment List translates each ATA-34 item into three things: can it go, how long, and what must be satisfied first.

[!warning]- Operator-specific content Repair intervals, installed/required quantities, and dispatch conditions are defined by each operator's MEL, derived from the Airbus MMEL, and vary by operator and regulator. The figures below illustrate typical provisions to convey the reasoning; always use the applicable operator MEL for an actual dispatch decision.

1. The three books — point, grade, isolate

An operator MEL for this chapter works in three coordinated parts:

• MEL entries (the pointer table) — indexed by ECAM caution, each caution resolves to one of three outcomes: no dispatch, not MEL-related (a crew action, not a fault — e.g. transponder manually to STBY), or refer to item 34-xx-xx.
• MEL items (the definition) — the actual dispatch item, with a repair interval, installed quantity, required quantity, and placard, plus operational (o) and maintenance (m) conditions.
• MEL operations (the appendix) — how to isolate the item after dispatch (with ground/flight differences).

The dispatch flow is ME → MI; the crew-isolation flow reads MO.

ME (entries)  = ECAM caution → dispatch outcome
   ├─ no dispatch
   ├─ not MEL-related
   └─ refer to item 34-xx-xx
        ↓
MI (items)    = repair interval / installed / required / placard + conditions
        ↓
MO (operations) = how to isolate after dispatch (ground / flight)

2. Repair intervals and the "no dispatch" red lines

The repair-interval categories translate roughly as A (a short, item-specified interval — often three calendar days, sometimes an item-specific figure), B (10 days), C (about 30 days / three calendar months), D (120 days); a dash means "must work, not dispatchable."

The no-dispatch entries for ATA-34 share one property: they have already damaged a flight-control law or a speed/AOA protection. Typically these are NAV ADR DISAGREE, dual and triple NAV ADR FAULT, NAV AOA DISAGREE, and NAV AOA EXCEEDED. Carrying such a residue aloft means any subsequent stacked failure could go uncontrollable, so the MEL closes the door. By contrast, single ADR, single IR, single AOA, and single transponder faults are dispatchable — they lose only redundancy, not the law.

One special-interval case is TCAS, which is often dispatchable for a limited number of legs (typically three) rather than a number of days, provided the aircraft is not departing its base and, in RVSM airspace, an ATC clearance is obtained. This matches its single-computer architecture: it is the last line of defence and can be briefly absent, ideally only ferrying home to a base with spares.

3. The ADIRS family — the source of a chain of knock-ons

The ADR and IR dispatch items expose the "bus fatalism" of the overview article as MEL clauses:

• ADR — a single ADR (side 1 or 3) is typically dispatchable for a limited period with ETOPS and RNP AR prohibited; ADR 1 (or 3) inoperative additionally forces "GPWS basic modes and predictive GPWS considered inoperative," because the EGPWS's inertial/air-data source is ADIRU 1. ADR 2 carries no such GPWS knock-on.
• IR — a single IR (side 1 or 3) dispatch typically requires a pre-flight check that the two flight-control Nz accelerometers and two rate gyros are working — independent inertial sensors, not part of the ADIRU, kept as the flight controls' non-ADIRU fallback. IR 1 inoperative additionally forces "predictive GPWS considered inoperative," and if the TCAS is affected, points to the TCAS item.
• AOA — one AOA sensor inoperative forces the associated ADR to be considered inoperative, cascading into the ADR chain (AOA → ADR → GPWS, a three-layer nesting).

4. The surveillance family — the one that must work, and "radar takes windshear with it"

• GPWS basic modes are typically the chapter's only "must work" item (installed 1 / required 1, no repair interval) — the CFIT defence is non-dispatchable, which is why the ADR 1 / IR 1 knock-ons above are so serious: they can pull this "must-work" item down.
• Predictive GPWS is dispatchable, typically with RNP AR prohibited and "not permitted to fail at a high-plateau airport of departure."
• Radio-altimeter auto callout is dispatchable with CAT 3 prohibited (an autoland's monitoring chain needs the callouts).
• Radio-altimeter system — one of two dispatchable with the unit disconnected, all ADIRU/SFCC/LGCIU working, a high-plateau restriction, and CAT 3 prohibited; RA 1 inoperative further considers GPWS basic modes inoperative.
• Weather radar — one or both can be inoperative for daylight-VMC (or no-hazardous-weather) operation, with ETOPS prohibited and the predictive windshear considered inoperative with it (PWS lives inside the radar), and at least one required at a high-plateau airport of departure.
• Predictive windshear — one or both may be inoperative provided the reactive windshear is working: prediction may be absent, but the last-line reactive protection must be present.

5. The radio-navigation family — VOR pulls the marker, GPS pulls ADS-B

• ATC transponder — a single unit dispatchable, ATC 1 with ETOPS prohibited (side 1 always carries more).
• DME / VOR — single-unit dispatchable; VOR 1 additionally forces the MARKER considered inoperative (the single marker set lives inside VOR 1).
• MARKER — dispatchable provided the approach procedure does not need a marker fix.
• ILS — a single unit dispatchable; ILS 1 forces GPWS G/S mode 5 and the ISIS ILS indication considered inoperative, plus CAT 2/3 prohibited (ILS 1 feeds mode 5 and is the ISIS's only LS source).
• GPS — a single unit dispatchable with RNP AR and oceanic/remote RNP 2 prohibited; both units carry a long PBN prohibition list, and dual GPS failure considers ADS-B Out inoperative (ADS-B's core is GPS position + integrity).
• ADS-B Out — the repair interval varies with airspace: least restrictive where the target route does not require it, most restrictive (a short interval, often three days) where the route requires it and subsequent flights touch a mandated airspace.

6. High-plateau restrictions and the MO appendix

The restriction "not permitted to fail at a high-plateau airport of departure" recurs in three items: predictive GPWS, the radio-altimeter system, and the weather radar (at least one required). The common reason: at a high-plateau airport (complex terrain, thin air, small go-around margins), terrain prediction (TAD), height above ground (RA), and weather avoidance (radar) shift from "nice to have" to "life-critical," so what other airports can dispatch without, a high-plateau leg cannot.

The MO (operations) appendix standardises "how to isolate after dispatch," with ground/flight differences. For the ADR pushbutton, for instance: in flight, a related ADR can be selected OFF via the IR mode selector, but with the side effect that the related IR also fails (the rotary cuts the whole ADIRU) — which is why the normal ADR isolation uses the ADR pushbutton, not the rotary; the MO spells out the cost of the "last-resort rotary" method.

Key takeaways

References

• Operator MEL / Airbus MMEL, ATA-34 items (ADR, IR, AOA, GPWS basic and predictive, radio altimeter and auto callout, TCAS, weather radar, predictive windshear, ATC, ADF, DME, VOR, MARKER, ILS, GPS, ADS-B Out) — the knock-on chains trace to the system mechanisms established in the ATA-34 system articles.

Note: dispatch categories, quantities, and conditions reflect operator- and regulator-specific practice and may not generalise; defer to the applicable operator MEL.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.