MEL Dispatch — Indicating/Recording

The first thirteen articles built the three domains of ATA-31 — display, warning, recording. This article lands them on "may we dispatch with it broken". The MEL is operator-specific, and exact conditions and rectification intervals vary; what follows is the dispatch logic typical of an A330 operator's MEL for the ATA-31 hardware — DMC, DU, FWC, SDAC, ECP, clock, recorders — framed generically. Always use the current operator MEL for actual dispatch.

Note: MEL provisions, categories and intervals are operator- and authority-specific. Treat this as the reasoning behind the ATA-31 dispatch items, not as a dispatch document.

1. The no-dispatch items — why only these

An ATA-31 MEL typically presents two kinds of entry: ME (indexed by the ECAM caution — "seeing this caution, may we go?") and MI (indexed by equipment — "with this item broken, how do we dispatch?"). The items that usually preclude dispatch, and why:

• EIS DMC 1 or DMC 3 fault — recall the reconfiguration article: DMC 1 drives the captain's side, DMC 3 drives ECAM and is the auto-takeover source when DMC 1 fails; losing either eats through the redundancy (whereas a DMC 2 fault is switchable to 3, so DMC 2 is dispatchable).
• Dual FWC / dual SDAC fault — the alerting philosophy article's "transparent to a single failure" holds only for one; with both gone the whole warning system is dead.
• OEB/FWC discrepancy — the two FWCs hold inconsistent OEB data; warnings may be wrong, so no dispatch.
• DFDR — some operators' MEL requires the DFDR operative for dispatch (installed 1, required 1). This is stricter than the "may be deferred for a few days" of a typical MMEL; follow the operator MEL.

Single-side faults are generally dispatchable (single FWC, single SDAC, DMC 2), which is the whole point of the dual/triple architecture.

2. EIS — DMC and DU dispatch, with interlocks

DMC (installed 3, required 2): a DMC 2 fault is dispatchable, typically requiring the F/O EFIS DMC selector pre-set to 3 (the F/O side takes DMC 3 in advance — echoing reconfiguration).

DU (installed 6, required 5) is the big interlock item. A failed PFDU/NDU/SDU is dispatchable but drags a long navigation interlock: typically no RNP AR, maximum landing capability CAT 2, no CAT 3; where the PF side is affected, a string of RNP/RNAV operations is prohibited (echoing the crosstrack-error basis of the ND article — you cannot fly to a precision you cannot display). An NDU 1 failure additionally tends to prohibit ETOPS (NDU 1 being one of the three screens kept on emergency power — see reconfiguration). An SDU failure typically requires the CDSS (cockpit-door surveillance) to be considered inoperative (the SD DU carrying the CDSS video — see the SD article).

3. FWS — FWC, SDAC and ECP

FWC (installed 2, required 1): FWC 2 dispatchable, at the price of landing capability — typically maximum landing capability CAT 3 SINGLE. The MASTER lights and their cancel functions (installed 2, required 1) allow one inoperative. The altitude warning (installed 2, required 1) allows one inoperative provided an autopilot works; both altitude warnings inoperative typically prohibits RVSM (echoing the C-chord altitude alert of the PFD article). SDAC (installed 2, required 1): SDAC 2 dispatchable.

The ECP keys carry the sharpest "must-work vs may-not-work" split:

Memorise the three that must work: ALL, EMER CANC, RCL — precisely the most indispensable of the five direct-wired keys in the ECP article (ALL is the only way to find a page with a dead ECP, EMER CANC suppresses spurious alerts, RCL recalls the cleared/inhibited). STS, T.O CONFIG, CLR and the system-page keys may be inoperative (substitutes exist: STS by RCL+CLR, T.O CONFIG by a manual configuration check, a system page by paging with ALL).

4. Recorders and clock

Recorders: the DFDR must work (§1); the DAR / DMU (data management) / QAR are generally "one or more may be inoperative"; the FDIU may be inoperative but then the DFDR and QAR are considered inoperative (the FDIU being their interface — see the recorders article). CVR faults refer to ATA-23. Clock: dispatchable but typically requiring two time sources to survive — the SD permanent-data UTC (from the CMS — see the SD article) and at least one ND chronometer — i.e. the clock body may fail so long as time and a chronometer are readable elsewhere. Some operators' MEL adds a part-number-specific February GPS-mode note (select INT to override, see the clock article).

5. Permanent data, E/WD memo and switching panels

SD permanent data (installed 7, required 1): all but TAT may be inoperative — TAT is the only permanent-data item that must work (it bears on ice protection and cannot be lost). E/WD memo: may be inoperative provided troubleshooting confirms the affected memo is a false warning. Switching panels (ATA-31-68): the AIR DATA / ATT HDG selectors refer to ATA-34, AUDIO to ATA-23, FM to ATA-22 — shared-panel entries refer out to their source chapters; the EFIS DMC selector F/O side may be inoperative at NORM with the EFIS display working; the ECAM/ND selector may be inoperative at AUTO/NORM with the automatic-transfer functions checked.

Self-test

[!note]- Q1. Which ATA-31 faults typically preclude dispatch, and why can DMC 2 be dispatched but not DMC 1/3? DMC 1/3, dual FWC, dual SDAC, OEB/FWC discrepancy, DFDR. DMC 1 drives the captain, DMC 3 drives ECAM and is DMC 1's auto-takeover — losing either eats the redundancy; DMC 2 is switchable to 3.

[!note]- Q2. What interlocks does a failed PFD/ND screen bring, and what does an NDU 1 failure add? No RNP AR, maximum landing CAT 2, no CAT 3; PF-side prohibits a string of RNP/RNAV. NDU 1 additionally prohibits ETOPS.

[!note]- Q3. Which three ECP keys must work, and why those three? ALL, EMER CANC, RCL — the most indispensable of the direct-wired keys: ALL finds a page with a dead ECP, EMER CANC suppresses spurious alerts, RCL recalls cleared/inhibited alerts.

[!note]- Q4. What is the DFDR's typical provision, and what does an FDIU failure drag in? The DFDR typically must work. An FDIU failure requires the DFDR and QAR to be considered inoperative (the FDIU is their interface).

[!note]- Q5. What two time sources must survive for the clock to be dispatched, and which permanent-data item alone must work? The SD permanent-data UTC (from the CMS) and at least one ND chronometer. TAT is the only permanent-data item that must work.

Key takeaways

References

• Based on the ATA-31 dispatch items of a typical A330 operator MEL (ME entries and MI items 31-01/05/06/07/08/20/30/53/55/56/62/63/68 and their operational procedures), generalised and anonymised. Content cross-checked to the system articles of this chapter.
• Always defer to the current operator MEL for actual dispatch.

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