The Electrical Clock

The electrical clock is the sole "independent instrument". A clock on the right of the panel looks simple, yet it hides three layers that are not: it is the source of aircraft-wide UTC (fed to maintenance, recorders, flight management, the ATSU); it has two running references — GPS and internal; and its timekeeping accuracy directly bears on whether ATC datalink (CPDLC) may be used. This article makes those three clear.

1. The independent clock and four functions

Per FCOM DSC-31-55:

"A fully independent clock is on the right side of the control panel. It sends time to the Central Maintenance Computer, the Flight Data Interface Unit, and the Flight Management and Guidance Computer. The clock has two electrical supplies, one of which is a direct connection to the aircraft's battery hot bus. The clock performs four functions: - It displays "UTC" (GMT) time in hours, minutes, and seconds... - It displays elapsed time (ET) (from engine startup) in hours and minutes... - It drives the chronometer (CHR)... - It can replace the UTC with the date."

"Fully independent" is the key qualifier: it does not depend on the EIS/FWS, self-contained. It is the source of aircraft-wide time — UTC fed to maintenance, the recorder interface and flight management (the AMM adds the ATSU). Of the four functions UTC is the lead (the aircraft time reference), ET times the flight, CHR is a stopwatch, and the date can temporarily replace the UTC display.

2. Power supply — one wire direct to the battery, so it does not forget the time on main-power loss

The most notable design of the four functions is that direct-to-battery wire. Per AMM 31-21-00:

"This enables time to be counted in memory throughout the period that aircraft is parked."

There are two supplies: one main, one direct to the battery hot bus. On main-power loss. Per AMM 31-21-00:

"(1) Loss of main electrical power supply (aircraft bus) - the UTC window is blanked. - the CHRONO is blanked - time is still counted in memory through the A/C battery supply and the internal time base. (2) Loss of aircraft battery power supply — The time counting function is no longer available. All displays are blanked. (When the power is restored the time must be reset)."

Two levels of power loss, two outcomes: lose main power and the display blanks but time is still counted in memory (the battery holds it); lose the battery and it stops entirely and must be reset. This is the value of that direct-to-battery wire — overnight, with main power off, the clock does not zero; power up next day and it is still correct. It is why UTC is usually already correct on a cold power-up (the memory has been running).

3. The UTC selector — GPS / INT / SET

How the clock runs is set by the UTC selector. Per FCOM DSC-31-55:

"(8) UTC selector — GPS: The time (or date) is displayed, and this data is synchronized with GPS information. Note: If the signal between the GPS and the clock is not detected, or if the signal is detected but the GPS data is invalid, the clock automatically runs on its internal time... will automatically resynchronize with the GPS... as soon as the GPS data is available. INT: The internal time (or date) is displayed. Note: The clock's internal time is always synchronized with the latest valid GPS information unless the pilot enters a different internal time, via the SET function... SET: Allows the internal time and date to be initialized."

The relation: GPS is preferred (auto-synced to satellites); INT is the internal time base (which normally follows GPS anyway); SET is manual setting. The fault tolerance: at GPS, if the signal is lost/invalid, it falls back to the internal time base and re-syncs automatically when GPS returns — so GPS does not fear a brief loss of satellites. On power-up with no valid GPS, the internal time starts from 00:00:00 until initialised or GPS is available.

4. ATC datalink accuracy — the hazard of a manually-set INT

This is the most hidden layer — clock accuracy gates whether CPDLC works. Per FCOM DSC-31-55:

"If the clock is set to internal (INT) mode and the flight crew manually sets the time and date, the clock does not comply with the time precision required for ATC datalink communication (+/- 1 s UTC). This may lead to the rejection of messages, or to the acceptance of obsolete messages... To comply with the time precision requirement, the flight crew must: - Use the clock in the GPS mode, or - Use the clock in the internal (INT) mode and synchronize the clock and GPS at least once a day."

Core conclusion: CPDLC requires UTC accuracy of ± 1 s; a manually-set INT time cannot meet it and makes CPDLC reject / accept obsolete messages. So for datalink operation (oceanic, CPDLC clearances), the clock is either at GPS, or at INT but synced to GPS at least once a day. This is the often-missed coupling of "clock accuracy = datalink availability" — if CPDLC misbehaves, do not forget to check the clock has not been set adrift by hand.

5. GPS synchronisation in INT mode

How the "sync once a day" is actually done. Per FCOM DSC-31-55:

"When the clock is set to INT mode, the UTC time is only based on the internal clock and is not synchronized with the GPS. To reset the drift... the flight crew must perform the following actions: - Set the UTC selector of the clock to the GPS mode and keep this setting during at least 10 seconds - Then reset the UTC selector of the clock to INT mode. This synchronization ensures that the UTC time drift is below +/- 1 s UTC."

The sync action: turn the UTC selector to GPS and hold at least 10 s, then back to INT. Why hold 10 s? To give the clock time to capture a valid GPS time and align the internal base. After it, the INT drift is pulled back within ± 1 s and CPDLC works again.

6. ET, CHR, date and photocell dimming

The remaining controls, briefly:

• ET (elapsed time) — a three-position selector RUN (count) / STP (stop, frozen) / RST (reset to zero, spring back to STP); totalises to 99 h 59 min. It can accumulate by alternating RUN/STP.
• CHR (chronometer) — the CHR key starts on the first press, stops on the second; the RST key resets; to 99 min 59 s.
• Date — the DATE key, pressed while time is shown, switches to date (month/day + year in the seconds field); date mode requires GPS or INT.
• Photocell dimming — a photocell on the face detects cockpit luminosity and auto-adjusts the clock's internal lighting — so the clock dims itself at night.

One SET time limit: initialisation must be completed within 1 minute, else the CFDS must be reset to synchronise the lower-ECAM time display (a maintenance action).

7. A configuration note

Some operators' MEL documents a limitation specific to certain clock part numbers, whereby in one month of the year, if the clock is in GPS mode, the ET and CHR functions may be intermittently abnormal; the crew then select INT to override ET/CHR and the clock is considered operative (INT still requiring the daily GPS sync of §5 for datalink accuracy). Refer to the operator MEL for the specific part-number applicability.

Self-test

[!note]- Q1. What are the clock's four functions, to which systems does it feed UTC, and what does "fully independent" mean? UTC, elapsed time, chronometer, date. It feeds UTC to the CMC, FDIU, FMGEC (and ATSU). "Fully independent" — self-contained, not depending on the EIS/FWS.

[!note]- Q2. What are the two supplies, and the outcome of losing main power versus the battery? Main and a direct-to-battery hot-bus wire. Lose main power: display blanks but time is still counted in memory. Lose the battery: it stops and must be reset.

[!note]- Q3. GPS / INT / SET — what does each do, and what happens at GPS if the signal is lost? GPS synced to satellites, INT the internal base (normally following GPS), SET manual. At GPS, loss/invalid falls back to internal and re-syncs automatically when GPS returns.

[!note]- Q4. Why does a manually-set INT time affect CPDLC, and how is the requirement met? CPDLC needs ± 1 s UTC; a manually-set INT time cannot meet it and causes rejection/obsolete messages. Meet it by using GPS, or INT synced to GPS at least once a day.

[!note]- Q5. What is the exact GPS-sync action in INT mode, and why hold 10 s? Turn the UTC selector to GPS and hold at least 10 s, then back to INT — long enough to capture a valid GPS time and align the internal base.

Key takeaways

References

• FCOM DSC-31-55 — independent clock, four functions, two supplies, UTC selector GPS/INT/SET, ATC datalink accuracy, GPS synchronisation.
• AMM 31-21-00 — clock mechanism, 3-ATI case/photocell, direct-to-battery supply, power-loss behaviour, internal time base default, ET/CHR/date controls.

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.