MEL & Dispatch — Flying with an ATA 21 Defect

This article covers ATA 21 MEL dispatch — after an ATA 21 component fails, can the aircraft keep operating, on what conditions, and for how long? An operator's MEL is built on the Airbus MMEL; the items below are illustrative of a typical operator MEL — always defer to your own operator MEL for the actual conditions.

1. The MEL — three elements & repair intervals

[!important]- The three MEL elements

Element	Meaning	Example
Repair interval	the maximum time allowed to fly with the defect	A / B / C / D
Installed / required	total fitted / required working for dispatch	2/1 = 2 fitted, 1 must work (1 may be inoperative)
Placard	whether a cockpit label is required	yes / no

[!note]- The repair-interval categories

Category	Interval
A	no fixed time — per the item's wording
B	3 consecutive calendar days
C	10 consecutive calendar days
D	120 consecutive calendar days

Counted from the day the defect is found. An operator may shorten an interval (never extend it).

[!tip]- "(o)" and "(m)"

(o) = the operating procedure — what the pilot does (e.g. PACK FLOW HI, close an ISOL VALVE). (m) = the maintenance procedure — what maintenance does (e.g. secure the PFCV closed). The pilot reads both: (o) for the flight limit + actions, (m) as the maintenance prerequisite.

ATA 21 MEL items group into five themes: control/light failures, SD-indication failures, component failures (PFCV / pack controller / cabin fan / recirculation / AEVC / cargo / zone controller), temperature regulation (trim-air / hot-air valves / zone-controller channels), and cargo ventilation.

2. Representative items

PFCV — pack flow control valve

A typical MEL: one PFCV inoperative (interval C, 2/1) — the valve must be secured closed and the associated pack treated as inoperative; must work for departure from base.

[!important]- Why the PFCV must be secured closed

A failed PFCV has an uncontrolled position (open / part / stuck). Securing it closed prevents an unwanted bleed flow, makes the pack unambiguously unavailable (with the linked "pack inoperative" item), and simplifies the pilot's mental model (one pack fully gone, not partial). "Must work for departure from base" = the dispatch with the defect is not a departure-from-base dispatch — it is typically an out-station provision (a failed PFCV away from base may be flown back to base + repaired), not a "depart from base with it broken".

[!warning]- Both PFCVs inoperative — dispatchable, but unpressurised flight only

A typical MEL allows both PFCVs inoperative (2/0), but under extreme conditions: unpressurised flight (per the no-cabin-pressurisation supplementary procedure), no ETOPS beyond 180 min, and must work for departure from base. Unpressurised flight → below FL 100 (cabin = ambient, no oxygen needed) → short, low-altitude operation. This is an out-station contingency (fly the aircraft back to base) — a real operator's SOP may be stricter.

Cabin fan

One cabin fan inoperative (interval C, 2/1): both packs working + PACK FLOW selector to HI. The cabin flow = pack supply + recirculation (the cabin fans); one fan lost → less recirculation → PACK FLOW HI (120 %) compensates. The cost: slightly more bleed consumption (a little more fuel).

Cabin pressure controller (CPC)

[!warning]- CPC 1 vs CPC 2 vs both — the MEL limits differ

Defect	ETOPS	High-altitude airport	Unpressurised
CPC 1 inop	not allowed	not allowed (from base)	—
CPC 2 inop	allowed	not allowed (from base)	—
both inop	≤ 180 min	not allowed (from base)	required

The intuition is "they are a redundant pair, losing either is the same". But the MEL is stricter for CPC 1 — in the A330 active/standby logic, CPC 1 is the primary, CPC 2 the backup. CPC 1 inop = running directly on the backup → less margin → no long oceanic. CPC 2 inop = the primary still works but no backup → ETOPS allowed but not a high-altitude airport. Teaching line: the MEL looks not only at "how many failed" but at "which one failed".

[!note]- Why high-altitude airports are restricted

A high-altitude airport (commonly field elevation ≥ ~8000 ft) → after takeoff the cabin altitude soon reaches the 9550 ft boundary → the CPC must control precisely to avoid an EXCESS CAB ALT warning. With the CPC backup lost, do not operate there. Such operations need dedicated training + an operations specification.

AEVC avionics extract fan

[!warning]- The same defect, different intervals for different flight limits

The avionics extract fan inoperative gives three MEL options:

Item	Interval	Altitude limit	Condition
A	C	none	both packs + both cabin fans + EXTRACT OVRD
B	C	29,500 ft	one pack + both cabin fans + EXTRACT OVRD
C	B (3 days)	33,500 ft	both cabin fans + EXTRACT OVRD

Choose by the situation: both packs working → option A (most flexible, no altitude limit); one pack lost + a long sector → option C (a short B interval but a higher altitude limit); one pack lost + a short sector → option B (a long C interval but a 29,500 ft limit). The MEL is a matrix — the crew/dispatch combine the current defect state × the mission to pick the best dispatch.

Other items

Recirculation valves (2/0, no special condition — one or both may be inoperative; recirculation still runs via the cabin fans). A pack-button FAULT light (2/0, placard yes — the light is dead but the button + pack work, the ECAM still prompts; distinct from a component failure). A zone-controller channel (2/1 — the other channel takes over, no special condition). The PACK FLOW selector (1/0 — but if stuck in LO, no dry-ice transport for vaccine cooling).

[!important]- Dry-ice ban — a MEL cargo limit

A PACK FLOW selector stuck in LO (80 % flow) → less bleed → less cargo ventilation → slow CO₂ removal from sublimating dry ice → possible CO₂ accumulation hazardous to occupants. The MEL bans dry-ice carriage — a "cargo–aircraft–MEL" interaction a pilot would not ordinarily think of. The MEL governs not only "can the equipment be used" but cargo conditions.

[!note]- A button light vs the component

A pack-button FAULT light failed = the LED is dead; the button + pack still work — the pilot just cannot see the FAULT annunciation (the ECAM still prompts). A component failure = the PFCV / pack controller physically fails (ata-21-21). The MEL distinguishes the indicator from the function: a dead light dispatches very freely (placard required); a failed function follows its own item.

3. Dispatch process & non-dispatchable defects

   defect found (ground / cruise)
     → ECAM text + STATUS/INOP → find the MEL item → read the four parts
       (interval / installed-required / flight limits / (o)+(m))
     → does the current sector meet the limits? + within the interval?
       yes → dispatchable: notify dispatch + maintenance ((m) done) + placard + fly per (o)
       no  → not dispatchable: repair

[!warning]- ATA 21 defects that are not MEL-dispatchable

Some ATA 21 defects do not have a MEL dispatch option — they must be repaired: an EXCESS CAB ALT trigger with no resolution (a real CPC or outflow-valve fault); both packs in OVHT (not OFF, OVHT); a safety valve stuck open + unrecoverable; the AEVC overboard valve stuck open + unrecoverable (affecting pressurisation). These directly threaten pressurisation safety / cabin ventilation → no dispatch option.

[!tip]- The MEL is not the pilot's own dispatch decision

The MEL is an approved document, but the actual dispatch: the pilot finds the defect → notifies dispatch; dispatch + the maintenance control centre assess; maintenance confirms (m) is done; dispatch judges against the MEL + operations specification + the mission (e.g. ETOPS); the pilot finally confirms. The pilot has a veto (may refuse if a MEL-dispatched flight is unsafe) but cannot widen the MEL (no "close enough").

Self-test

[!note]- Q1. What do the MEL repair intervals A / B / C / D mean?

A = no fixed time (per the item's wording); B = 3 consecutive calendar days; C = 10; D = 120. Counted from the day the defect is found; an operator may shorten (never extend) an interval.

[!note]- Q2. One PFCV stuck failed — the typical dispatch conditions?

The PFCV secured closed ((m)); the associated pack treated as inoperative (the linked pack-inoperative item); must work for departure from base (an out-station provision — fly back to base, do not depart from base with it broken); interval C (repair within 10 days). The flight then operates single-pack (the ONE PACK ONLY IF WAI ON STATUS) with the fuel/diversion considerations of single-pack operation.

[!note]- Q3. CPC 1 vs CPC 2 vs both inoperative — how do the MEL limits differ?

CPC 1 inop: no ETOPS + no high-altitude airport (from base). CPC 2 inop: ETOPS allowed + no high-altitude airport (from base). Both inop: ≤ 180 min ETOPS + unpressurised required + no high-altitude airport. CPC 1 (primary) inop is stricter than CPC 2 (backup) inop because losing the primary leaves only the backup; both inop degrades to manual + unpressurised.

[!note]- Q4. The avionics extract fan inoperative has three MEL options — which do you choose?

By the situation: both packs working → option A (interval C, no altitude limit, most flexible); one pack lost + a short sector → option B (interval C, 29,500 ft limit); one pack lost + needing 33,500 ft → option C (interval B, only 3 days, but the higher limit). A MEL matrix — combine the defect state × the mission for the best dispatch.

[!note]- Q5. One cabin fan inoperative — what does the pilot do after a MEL dispatch?

Per the (o): both packs must be working (no further pack fault), and the PACK FLOW selector to HI (120 % to compensate the lost recirculation). The cost: slightly more fuel (~1 % cruise), a little more cabin noise, more bleed load.

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

The dispatch logic follows the Airbus MMEL structure on which an operator MEL is built (the repair-interval categories A/B/C/D, the installed/required and placard convention, and the (o)/(m) operating/maintenance references), with the ATA 21 items presented illustratively: the PFCV secured-closed + pack-inoperative + departure-from-base item, the both-PFCV unpressurised item, the cabin-fan PACK-FLOW-HI item, the CPC 1 / CPC 2 / both-inoperative items with their differing ETOPS / high-altitude / unpressurised limits, the avionics-extract-fan three-option altitude matrix, the recirculation-valve / button-light / zone-controller-channel items, and the PACK-FLOW-selector dry-ice limit, plus the supplementary no-cabin-pressurisation procedure and the relevant AMM deactivation tasks. The specific conditions are illustrative of a typical operator MEL and vary by operator. The CPC primary/backup rationale, the AEVC matrix-selection logic, and the indicator-vs-function distinction are integrative syntheses. All engineering detail is from the A330 knowledge base; the items are presented generically without an operator name, revision, or fleet tail number, and no cross-type comparison is made.

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