MCDU Pages and Data Formats

Article 03 covered the MCDU as a machine; this article covers its language — the screen's anatomy, the scratchpad's grammar, the five field states, the data-format rules that cause most FORMAT ERRORs, and the message system's queue and quiet hours. (The mechanics are attributed summaries of the AMM's operational-use chapter; format specifics beyond the highlights live in the FCOM's reference chart.)

1. Anatomy of a screen

Fourteen lines of 24 characters: line 1 the title (columns 23/24 reserved for the horizontal slew arrows); line 14 the scratchpad (23/24 for vertical slew); the twelve between form six pairs of small-font label line over large-font data line — 1L/1R own lines 2–3, down to 6L/6R owning lines 12–13 (per AMM 22-74). Pages organise by mode key (INIT, F-PLN, PERF, DATA, RTA…), navigation being automatic switches, LSK prompts or mode keys — and some pages have no exit prompt at all: only a mode key leaves them. The DATA key is the tool drawer. Per FCOM DSC-22_20-20-10:

DATA INDEX 1/2 PAGE [1L] POSITION MONITOR [2L] IRS MONITOR ... [3L] GPS MONITOR This key calls up the GPS MONITOR page. [4L] A/C STATUS This key calls up the aircraft status page.

— plus CLOSEST AIRPORTS and EQUI-TIME POINT (article 21), PRINT and AOC (article 28); page 2 holds the stored elements (pilot waypoints/navaids/runways/routes — article 04's quota). The position trio is article 20's; A/C STATUS is article 04's databases and factors.

2. Input grammar — the scratchpad's rules

From the AMM's operational-use mechanics (attributed summary, figures as printed):

• A 22-character ceiling — the 23rd keystroke is silently ignored (no beep; the key plays dead). Press an LSK to move the entry into a field; the scratchpad clears; the computer validates, and a rejected entry leaves the field unchanged with an error in the scratchpad. Two recovery paths: CLR the message (the original entry returns for editing) or simply type over it. Pressed the wrong LSK and got refused? The entry is still there — press the right one.
• Two-level judging, in order: wrong width/format/type → FORMAT ERROR; right format, out-of-bounds value → ENTRY OUT OF RANGE. First "is the sentence grammatical", then "is the number sane".
• Slash positioning: multi-field lines separate by slashes, and the number of leading slashes equals the number of fields skipped — in a wind-direction/speed/altitude line, changing only the altitude is typed //altitude; only the speed, /speed.
• CLR, three ways: a tap deletes the last character; held, it deletes continuously — and held 2 s on an empty entry, it clears the whole scratchpad. With the scratchpad empty, pressing CLR arms a "CLR" token: the next LSK clears that field (fields with defaults revert to them; clearing a default is a no-op). The system distinguishes the CLR key from a typed C-L-R.
• The page-change trap: certain field entries auto-jump the page and delete the scratchpad on the way; ordinary page changes preserve it — and keystrokes during the page-change transient are ignored.

3. Display semantics — five states and three font exceptions

The data line speaks five states (article 03's colours, completed): cyan data = modifiable here; green = displayed but not modifiable here; cyan brackets [ ] = an optional seat waiting for you; amber boxes □ = mandatory — the function will not run without it (INIT's FROM/TO, PERF TO's V-speeds); white dashes = the computer's own slot, data missing or being computed (article 24: dashes mean thinking).

The font rule (data large, labels small) has three deliberate exceptions: ① a modifiable field holding a default/predicted value shows small font while the system's value stands, large once you overwrite it — the general law behind article 22's "large blue is yours, small blue is its guess"; ② of two interdependent values (a hold's time/distance), the independent one is large, the derived one small; ③ where active and alternatives share a page, the chosen one is large. (Stars and arrows take their prompt's colour; a slash takes the colour of the data to its right.) The reading to internalise: small font does not mean unimportant — it means system-filled: a small-font CG or wind reads "I have not yet spoken".

4. Data formats — the chart and the traps

Per FCOM DSC-22_20-20-30:

The following chart lists all the data the flight crew may enter on the MCDU. It also shows the acceptable format for the various data items, the acceptable range, the units of entry, and the MCDU pages on which the data can be entered. The following codes are used to indicate various data formats: A : letters N : numbers X : letters and numbers

The 23-page chart is a lookup reference — this article carries the trap highlights (AMM table, same source): altitudes need their leading zeros (0500, not 500) and round to 10 ft on entry; flight levels may drop the zero; speeds are three digits (90 kt won't go in — the floor is 100); Mach needs its decimal point (.78); QNH auto-recognises both unit bands (745–1050 hPa / 22.00–31.00 inHg); latitude/longitude need their direction letters (N/S/E/W; minutes cap at 59.9); wind has two different ceilings — 200 kt maximum on the WIND pages but 500 kt on the F-PLN B/VERT REV side (a 250 kt jetstream entry is refused on the WIND page); route reserve takes two formats (NNN.N = tonnes, /NN.N = percent, 0–15 %); CG 8.0–50.0 %MAC; DH 0–700 ft and accepts the literal "NO"; THS as UP/DN with 0.1 increments (sent to the FWCs for the takeoff-config comparison); and the runway's L/R/C suffix is mandatory only when same-numbered parallels exist.

5. The message system — two colours, two types, one queue

Colour is priority (per AMM 22-74): amber = important (navigation-related, input-demanding, or echoed on the EFIS); white = an operational receipt — the direct consequence of your own keystroke. Type is lifecycle:

• Type I — immediate results of an action (NOT ALLOWED, FORMAT ERROR…): no queue, gone as soon as you type or press an LSK/mode key.
• Type II — system-generated (GPS PRIMARY LOST, STEP DELETED…): a LIFO queue of five; a sixth arrival drops the bottom one; a repeated message is not duplicated (it re-tops instead); and it shows itself only when no Type I is up and the scratchpad is empty — it steps aside while you type and reappears after.

And the quiet hours — condition A: in takeoff, go-around or approach below 800 ft (the AMM marks the figure TBC), messaging is minimised — only spontaneous important navigation messages and immediate receipts pass; the rest are held until the condition clears. So the MCDU going quiet at low height is design — and the burst of messages after a go-around is stored inventory, not fresh news. Two registry entries cashing earlier references: SETTING SPD/RTA = the system is re-computing the managed profile for an RTA, target momentarily frozen (article 22); MCDU OVERHEATED = the power-saving blank screen's explanation card (article 03).

6. The page-family map

The INIT A → B fuel flow (merging article 21 and article 25): INIT A (company route/city pair, CRZ FL, CI, tropopause, cruise temperatures) → INIT B (ZFW/ZFWCG in amber boxes — mandatory; BLOCK, or the FUEL PLANNING prompt to have the FM back-compute minimum fuel; TRIP WIND) → after engine start the page yields to FUEL PRED (GW/CG, EFOB/EXTRA). The PERF phase pages: one per phase, the current phase's title green; CLB/DES carry the frozen ECON and the preselects (article 25); APPR holds QNH/temperature/wind/DH-MDA/landing configuration — the granary of ground-speed mini and the capability computation (articles 12/17); GO AROUND holds THR RED/ACC. RTA is article 22's; DATA's two pages are section 1's. This series deliberately does not transcribe every field: the authority for page furniture is the FCOM page-description chapter plus the format chart — the teaching layer tells you which page owns which job and where to look it up.

7. Operating discipline

The ten-thousand-foot rule, verbatim. Per FCTM AOP-20:

Below 10 000 ft, entries should be restricted to those that have an operational effect: ‐ PERF APPR ‐ DIR TO ‐ NAVAIDS ‐ Late change of runway ‐ Activate SEC F-PLN ‐ ENABLE ALTN. Time consuming entries must be performed at all times: ‐ By the PM upon PF request, or ‐ By the PF after a temporary transfer of controls to the PM.

A six-item whitelist below 10 000 ft — each one something that not doing causes an operational problem — and at any altitude, the person typing must not be the person flying. The FCTM pairs it with anticipation. Per FCTM AS-FG-10-1:

With the FMS, anticipate flight plan updates by preparing: ‐ EN ROUTE DIVERSIONS ‐ DIVERSION TO ALTN ‐ CIRCLING ‐ LATE CHANGE OF RWY. This enables the MCDU to be used for short-term actions.

Do the long typing high; leave the low altitudes to short, pre-planned actions (article 06's two standing questions supply the monitoring frame). Troubleshooting hooks: an unresponsive keyboard — count your characters (the 22 ceiling); an uncleareable field — it is green, or holds a default; a message storm — check the timing (condition A just cleared); a dark screen — a keystroke revives it (article 03).

[!warning]- Four misconceptions this article corrects (1) The 23rd character is not beeped away — it is silently ignored; an unresponsive scratchpad usually means a full one. (2) Small font is not fine print — it means the system filled the value and you haven't spoken; overwrite it and it grows. (3) The MCDU going quiet below 800 ft (takeoff/approach/go-around) is deliberate, and the post-go-around message burst is stored inventory, not a fresh cascade. (4) The wind-entry ceiling differs by page — 200 kt on the WIND pages, 500 kt on F-PLN B/VERT REV; a refused jetstream entry is a format rule, not a failure.

Self-test

[!note]- Q1. Which two screen lines does LSK 3L own — and what defines a "blank" field?

Lines 6–7 (the third label/data pair). Blank = cyan brackets (optional, awaiting entry) as opposed to amber boxes (mandatory) or dashes (computer's slot).

[!note]- Q2. What happens on the 23rd keystroke?

Nothing — it is ignored without any annunciation. The scratchpad holds 22 characters.

[!note]- Q3. The judging order of FORMAT ERROR versus ENTRY OUT OF RANGE?

Format first (width/type/grammar), range second (value sanity). A three-digit rule violation draws FORMAT ERROR before the value is ever considered.

[!note]- Q4. In a wind direction/speed/altitude line, what does "//250" change?

The altitude — two leading slashes skip two fields. One slash would target the speed.

[!note]- Q5. Amber box versus cyan brackets — and the three origins of small font?

Box = mandatory (function withheld until filled); brackets = optional seat. Small font: a standing system default/prediction; the derived one of an interdependent pair; the non-selected alternative on a shared page.

[!note]- Q6. The Type II queue's capacity, its overflow victim, and whether repeats stack?

Five, last-in-first-out; a sixth message drops the bottom one. Repeats do not stack — the message re-tops instead.

[!note]- Q7. Condition A's three phases and height — what is suppressed, what passes?

Takeoff, go-around, approach below 800 ft (figure marked TBC in the AMM). Routine messages are held; spontaneous important navigation messages and immediate operational receipts pass.

[!note]- Q8. The two altitude-entry format rules — and wind's two ceilings?

Leading zeros are mandatory (0500) and entries round to 10 ft. Wind: 200 kt maximum on the WIND pages, 500 kt on F-PLN B/VERT REV.

[!note]- Q9. The below-10 000 ft whitelist — and the two lawful routes for a time-consuming entry?

PERF APPR, DIR TO, navaids, late runway change, activate SEC, ENABLE ALTN. Time-consuming entries: the PM on the PF's request, or the PF after temporarily handing over control.

Key takeaways

References

Screen anatomy, page organisation, input grammar (character ceiling, validation order, slash positioning, CLR behaviours, page-change effects), field states, font exceptions, format rules and the message system (colours, types, queue, condition A) per AMM 22-74 (operational use — attributed summaries of the description text; figures as printed, the 800 ft condition marked TBC in the source). DATA index pages, the SETTING SPD/RTA and MCDU OVERHEATED messages, and the data-format chart per FCOM DSC-22_20-20 (the chart being the lookup authority for field-level specifics). The below-10 000 ft whitelist, time-consuming-entry rule and the four anticipations per FCTM AOP-20 and AS-FG-10-1. The page-family map is an integrative synthesis.

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.