The clock system has the function to show the flight time, universal time coordinate (UTC), local (LOC) time, date and chronograph time for the flight crew.

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The clock is located on the main instrument panel and is held in a mount. There is an electrical receptacle on the rear of the clock. The clock has three digital liquid-crystal displays (LCDs). The top display gives the flight time (in hours and minutes from 00:00 to 99:59). The middle display gives either the UTC (in hours, minutes, and seconds from 00:00:00 to 23:59:59), local time (in hours, minutes, and seconds from 00:00:00 to 23:59:59) or date (in day, month and year). The bottom display gives the chronograph time (in minutes and seconds from 00:00 to 99:59).

There are four pushbuttons on the clock face. The GPS/MAN pushbutton changes the mode of operation. It sets the global positioning system (GPS) mode or the manual mode. The FLT RST/+ pushbutton resets the clock. The CHR/-pushbutton operates the chronometer. The MODE pushbutton is to navigate through the three functions of the middle display. It sets the UTC, local time, or date.

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On pre IS100-31-0001, the clock system operates on 28 V dc power supplied by the L ESS BUS as the primary power source. There is a standby power source to supply some functions when there is no primary source. The standby 28 V dc power comes from the L BATT BUS. The display lighting is supplied by the 5 V dc from the panel integral-lighting system.

On post IS100-31-0001, the clock system operates on 28 V dc power supplied by the L ESS BUS as the primary power source. The standby 28 V dc power also comes from the L ESS BUS. The display lighting is supplied by the 5 V dc from the panel integral-lighting system.

Flight time (FLT) represents the time that the aircraft is airborne as determined by the status of the in-air/ on-ground discrete from the proximity-sensor electronic unit (PSEU). The FLT starts counting from zero (00:00) when the in-air/on-ground input transitions from ground (on ground) to open (in air). The FLT stops counting when the in-air/on-ground input transitions from open (in air) to ground (on ground). The FLT resets to 00:00 by pressing FLT RST switch for a period of 2 or more seconds. The chronograph (CHR) begins counting upon depression of the CHR switch for a period of 200 milliseconds or more. The first actuation starts the CHR from zero (00:00). The second switch actuation stops the CHR time accumulator. The third switch actuation causes the CHR display to blank and resets the CHR timer.

The GPS/MAN pushbutton selects the time source. When in GPS mode, the chronometer uses the GPS ARINC 429 as the time source to provide UTC, if the 429 bus is valid. The chronometer also uses the GPS time to continually synchronize its internal time base. If the GPS ARINC 429 input should fail after the chronometer is in GPS mode, the chronometer uses its internal time base. If the 429 bus input should recover, the chronometer returns to using the 429 data from the GPS. GPS mode is indicated by an illuminated GPS legend on the display. When in manual mode, the chronometer uses it internal time base as the source to provide UTC. Manual mode is indicated by absence of the GPS indication on the LCD. While in manual mode, the chronometer ignores the GPS ARINC 429 signal and the UTC display shows the time as calculated by the internal time base.

The chronometer can be used at all time and in all modes by the flight crew to measure a period of time.

The MODE pushbutton provides two functions:

• UTC/LOC/DATE display mode switching
• UTC/LOC/DATE set mode

The MODE pushbutton allows selection of the UTC, LOC, or DATE display modes. Upon depression of the switch for a period of 200 milliseconds, the chronometer toggles between the UTC, LOC, and DATE display modes. The UTC/LOC/DATE display is always illuminated, except in the case of a failure. If the MODE pushbutton is depressed for more than 2 seconds or longer, the chronometer enters the UTC/LOC/DATE set mode. This is indicated by a flashing digit display that corresponds to the mode selected. The setting can only be changed when in MAN mode. The chronometer is supplied by 28 VDC L MAIN BUS for calculations, synchronization with GPS, and display. In case of L MAIN BUS power loss, the L BATT BUS is used only to supply internal time accumulation and processing functions necessary to maintain the specified accuracy of all chronometer functions. Any hard failure in the chronometer including loss of the internal time base or a failure in the microprocessor program will blank the displays.

Strapping

The I/O BUS LOW SPEED SEL (P1-P) and STRAP CMN (P1-C) is open to configure the ARINC 429 bus interface as high speed.

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The clock system has interfaces with the components/systems that follow:

• Remote Data Concentrator (RDC)
• Proximity-Sensor Electronic Unit (PSEU)
• Panel Integral Lighting
• Global Positioning System (GPS)

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The chronometer has a power-up BIT and also performs periodic BIT. The power-up BIT does not affect the internal operation of time keeping functions.

If a failure is detected or power is lost, the clock displays blank.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A121	DIGITAL CLOCK	ZONE(S) 211	31-21-01

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