The A320 has a built-in independent clock which sends time data to several systems. It sends time to the centralized fault data interface unit, 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 battery hot bus.
The clock performs four functions :
- It displays “UTC” (GMT) time in hours, minutes and seconds on the center counter.
- It displays elapsed time (ET) (from engine startup) in hours and minutes on the lower counter.
- It drives the chronometer (CHR), which measures a time interval (from the pushing of the CHRONO button) in minutes and seconds.
- It can replace the UTC with the date.
UTC (GMT) COUNTER
This counter displays the present time in 24 h format from 0 to 23 h 59 min 59 s.
ELAPSED TIME (ET)
This counter registers the elapsed time up to 99 h and 59 min.
CHRONO (CHR) COUNTER
This counter registers elapsed time from 0 to 99 min 59 s. It is controlled by the CHR pushbutton.
First push : starts the CHR counter
Second push : stops the CHR counter, keeps the display at its last indication.
RESET (RST) PUSHBUTTON
When pressed, the CHR counter restarts from 0 provided that the chrono has been first stopped with the CHR pushbutton.
- “RUN” : the ET counter starts
- “STP” : the ET counter stops counting
- Spring loaded “RST” : the ET counter is blanked. The selector returns to its STP position when the selector is released.
Note: A cumulative elapsed time can be realized by alternatively setting this switch in “RUN” and “STP” position.
- First push : sets the clock to date mode. The UTC time display is replaced by the date (day month year).
- Second push : sets the clock to time mode. The date display disappears.
- Note: in order to select the date mode, the UTC selector must be set on “GPS” or “INT” position.
- “GPS” : Time (or date, if selected) is displayed, and this data is synchronized on GPS information.
- If the signal between the GPS and the clock is not detected, dashes are displayed. Only the “INT” and “SET” positions are then available.
- If the signal is detected, but GPS data is invalid, the clock automatically runs on its internal time.
- The clock will automatically resynchronize on the GPS information, as soon as the GPS data becomes available.
- “INT” : Internal time (or date, if selected) is displayed.
- The clock’s internal time is initialized with the latest valid GPS information.
- If there is no valid GPS information at power up, the internal time will be 00:00:00, until the clock is initialized.
- “SET” : Allows the internal time and date to be initialized.