05/19/16
Overview
A dual clock system is supplied with the aircraft avionics to provide the current time and date in local or Greenwich Mean Time (GMT), chronometer functions and elapsed time (ET). The digital clock system also transmits time-related information to all other aircraft avionics systems requiring a time reference. This information is transmitted via an ARINC 429 data bus to Integrated Avionics Computers (IACs).
One digital clock is located on the pilot instrument panel and the other is located on the copilot instrument panel. Display lighting is supplied by 5 VAC.
At cold weather operations the clock display may be affected as the normal operating temperature of the aircraft clock is -15 to +55 °C. The clock will operate to -40 °C but the display may be affected and the response time can also increase up to one second.
Service Bulletin 700-31-012 introduced two digital clocks that utilize inputs from the global positioning system (GPS) to automatically set and synchronize time settings.
Digital Clock
Pre SB 700-31-012 Clock
The digital clocks (prior to A/C 9140) operate on 28 VDC power supplied by the aircraft power system. Each digital clock has two liquid crystal display (LCD) readouts, an analog sweep second hand, and a selector knob.
The upper four-digit LCD readout displays Greenwich Mean Time, local (LOC) time, or the date. The lower four-digit LCD readout displays the elapsed time (ET) and the chronometer (CHR) function. The selector knob enables the operator to select between GMT, local time, or the date for the upper LCD. The analog sweep hand indicates chronometer changes from 00 to 60 seconds.
Digital Clock with GPS Input (Optional)
Post SB 700-31-012 Clock
The two digital clocks with global positioning system (GPS) inputs are a Service Bulletin option for A/C 9002-9139 and are standard for 9140 and subs.
The clocks utilize inputs from the GPS to automatically set and synchronize time settings. The pilot’s clock is still powered by the battery bus and the copilot’s clock by the DC bus 1 but there is no battery backup.
When the aircraft is powered up outside the hangar and a valid GPS signal is available, the clocks will display the correct time and transmit time data to the IACs. If the aircraft receives an invalid GPS signal upon power up, the clocks will enter the time data setting mode and will not transmit time data. However, if the clocks are set manually, the time data will be transmitted. The local time will need to be set each time the aircraft is powered up.
The clock has two LCD displays that show digits in while on a black background. There is no analog sweep. The upper six digit LCD readout displays date (DT), GPS, internal (INT), or local time (LT). The lower four digit LCD readout displays elapse time (ET) or chronometer (CHR).
For aircraft that have the SBAS GNSSU(s) installed the clock is configuration strapped to enable the reception of high speed (100kHz) ARINC 429 data from the SBAS GNSSU. (The original GPS unit uses a low speed bus (12.5 kHz)).
System Operation
Digital Clock
Pre SB 700-31-012 Clock
Two liquid crystal displays are situated in the center of the clock face. The upper display provides pilot-selectable GMT time, local time or date, and is controlled by the rotary switch in the lower left corner of the clock. The lower display provides the elapsed time and chronometer functions, and is controlled by the ET and CHR buttons on the clock face.
The clocks are set to the current time by rotating the clock front panel switch to SET. The switch must be pushed in at the same time as it is rotated into or out of SET. The parameter to be set is selected by pressing the ET button. The selected parameter will flash on the display, and appear in the order of GMT minutes, GMT hours, LOC minutes, LOC hours, day, month, and year.
When the selected parameter is flashing on the display, it can be incremented by pressing the CHR button. Only after setting the clock and rotating out of SET will the colon appear between the hours and minutes digits. The colon in the upper display is only present when the clock has been set and is running. The colon in the lower display is present only when the Elapsed Time is running.
Upper LCD Display
By selection, the upper LCD display readouts may display any one of the three functions: Date, Local Time and GMT Time.
Date
- Displayed by selecting the selector knob to DATE
- The date readout alternates between the month/ date, and year display. The month and day are displayed together for one second, and then the year is displayed for one second. The two left-hand digits are extinguished when the year is displayed
- The month (from 01 to 12) is displayed in the two left-hand digits of the LCD readout; the day (from 01 to 31) and year (from 00 to 99) are displayed in the two right-hand digits
Local Time
- Displayed by selecting the selector knob to LOC
- The local time is indicated from 0 to 23 hours and 59 minutes; two fixed dots separate the hours and minutes in the LCD readout
Greenwich Mean Time
- Displayed by selecting the selector knob to GMT
- The GMT time is indicated from 0 to 23 hours and 59 minutes; two fixed dots separate the hours and minutes in the LCD readout
Lower LCD Display
The lower display in the clock face presents the chronometer and elapsed time information. These functions do not operate until the GMT time has been established. The presence of the colon in the lower display indicates the elapsed time counter is operational and counting.
The operation of the CHR and ET buttons depends upon the current display and the state of the WOW discrete. Elapsed Time The ET displayed on the lower LCD readout corresponds to the flight time and is indicated from 00 to 99 hours and 59 minutes; two fixed dots separate the hours and the minutes.
Elapsed Time
The ET displayed on the lower LCD readout corresponds to the flight time and is indicated from 00 to 99 hours and 59 minutes; two fixed dots separate the hours and the minutes. The ET is incremented only when the aircraft takes off (weight off wheels), and can only be reset to zero when the aircraft is back on the ground.
Chronometer
The CHR function measures shorter duration events that occur during the ET measurement. The CHR is indicated from 00 to 99 minutes in the right-hand digits of the LCD readout. Chronometer and elapsed time (ET) pushbutton functions are detailed as follows:
Pushbutton Operation
| CURRENT DISPLAY | BUTTON PRESS | IN AIR/WOW | RESULTING DISPLAY |
|---|---|---|---|
| ET | CHR | N/A | Chronometer appears. Left digits blanked. Right digits zero. Sweep seconds start. |
| CHR | CHR | N/A | Chronometer stops. |
| STOPPED CHR | CHR | N/A | Chronometer resets. ET display appears. |
| CHR | ET | N/A | ET display appears. |
| COUNTING ET | ET | AIR | CHR display appears. |
| STOPPED ET | ET | GND | ET resets to zero. |
| ZERO ET | ET | GND | CHR display appears. |
Digital Clock with GPS Input
The operating modes are controlled by a MODE pushbutton, an ET pushbutton and a CHR pushbutton. The MODE pushbutton enables the operator to switch between date (DT), GPS, internal time (INT) or local time (LT) on the upper LCD display.
The CHR button selects the chronometer mode on the bottom LCD. The display shows two minutes digits, a colon and two seconds digits. The first push of the CHR pushbutton will start the chronometer to zero and cause the LCD to go blank. The elapsed time (ET) display corresponds to the aircraft’s flight time. The ET is displayed in the lower LCD and displays hours and minutes separated by a fixed colon in the run mode. The run mode is selected when weight on wheels not active condition is detected. When weight on wheels is detected, the ET counter stops and the fixed colon is removed. Pushing the reset (RST) pushbutton the ground will zero the ET counter and blank the display.
Time Display
UTC from the GPS or the internal computed (INT) is displayed on the upped LCD in hours minutes and seconds. A fixed colon separates the hours from minutes and comes on only when the function is initiated. UTC time can be manually set if GPS is not detected. GPS mode is initiated whenever a valid GPS time signal is detected, and will override any existing UTC manually set time.
Local time
Local time (LT) is displayed in the upper LCD display in hours, minutes and seconds. A fixed colon separates the hours from minutes and comes on only when the function is initiated. Local time need to be set manually each time the aircraft is powered up.
Date Display
Months, date, and years are displayed in place of the UTC on the upper LCD when the date mode is selected. In this selection the colon is not displayed. Leap years are taken into account.
Time Setting
GPS mode is automatically initiated if the GPS data from the IAC is valid. In GPS mode the clock is automatically updated.
If GPS data is not valid, the clock display automatically switches to internal (INT) mode and displays internally computed time based on the last valid GPS time. The clock may be set manually by pressing the MODE pushbutton for a minimum of 2 seconds to select the set mode. If starting from GPS mode, the clock will enter LT set mode and bypass INT and DT set modes. If the clock is not in GPS mode, then INT set mode will be initiated.
When entering the INT set mode the UTC minute digits are flashing and the INT flag is lit. The UTC minutes can now be set by pressing the ET pushbutton to increase the number displayed and pressing the RST pushbutton will increase the numbers displayed. The counter advances one pulse at each press.
Press the MODE pushbutton until the UTC hours digit is flashing and the INT flag is lit. Adjust the UTC hours using the same procedure as the setting the UTC minutes.
Press the MODE pushbutton again to display the flashing month digits with the date flag lit. Adjust the month using the same procedure as the setting the UTC minutes.
Press MODE pushbutton again to display the flashing days digits with the date flag lit. Adjust the day using the same procedure as the setting the UTC minutes.
Press MODE pushbutton again to display the flashing LT minutes digits with the LT flag lit. Adjust the LT minutes using the same procedure as the setting the UTC minutes.
Press the MODE pushbutton again to display the flashing LT hours digits with the LT flag lit. Adjust the LT hours using the same procedure as the setting the UTC minutes.
Press the MODE pushbutton once more to return to the flashing UTC minutes.
Press and hold the MODE pushbutton for more than two seconds to start the clock operation and exit the set mode.
Signal Interface
The digital clock on the pilot instrument panel is used as the primary source for time reference and the digital clock on the copilot side is used as the secondary source. The primary source time reference is fed to IAC 1 and IAC 3, and the secondary source time reference is fed to IAC 2 via the ARINC 429 bus.
The IACs distribute GMT to the various systems requiring time information (e.g. central aircraft information maintenance system (CAIMS), flight management system (FMS), and the flight data recorder (FDR). IAC 1 and IAC 3 synchronize to the UTC from clock 1 and IAC 2 synchronizes to the UTC from clock 2. The clocks are not synchronized to each other.
Digital Clock
The clock receives air/ground information from the landing gear electronic control unit (LGECU) to control the elapsed time function. The LGECU supplies main landing gear WOW(A) to the pilot’s clock and WOW(B) to the copilot’s clock. WOW(A) and WOW(B) is GND when the aircraft is on ground and is OPEN when the aircraft is in air.
Each digital clock is equipped with a 28 VDC battery input which is used when the primary 28 VDC input is interrupted. When primary power is interrupted, each digital clock enters a lower power mode where the correct operation of the time base is continued.
Digital Clock with GPS Input
The digital clock with GPS input receives its GPS signal from the IAC 1, 2, or 3. When the clock is in the GPS mode, the UTC time and date are synchronized on GPS information via the ARINC 429 data bus from the IAC. The IAC receives the GPS data from the global navigation system sensor unit (GNSSU).
Power Input
Digital Clock
The pilot’s clock receives +28 VDC power from the battery bus and the copilot’s clock receives +28 VDC power from DC BUS 1. Both clocks receive backup power from the +28 VDC battery direct bus.
Loss of primary power will blank all displays and stop the motion of the sweep second hand if the chronometer is operational. The date (month/day/ yr), local time (hours/minutes), and GMT time (hours/minutes) are not displayed, but continue to increment within the internal circuits of the clock as long as backup power is available.
The CHR and ET function do not continue to increment within the internal circuits of the clock. In this mode, the digital clock is neither required to display the time nor transmit time-related information on the ARINC 429 data bus.
Restoration of primary power will cause the ET and CHR functions to reset to zero (including the sweep second hand). GMT time, LOC time, and DATE will all be restored to current values and displayed according to previous switch selection.
Digital Clock with GPS Input
The pilot’s clock is still powered by the battery bus and the copilot's clock by the DC bus 1 but there is no battery backup. When the aircraft is powered up outside the hangar and a valid GPS signal is available, the clocks will display the correct time and transmit time data to the IACs.
If the aircraft receives an invalid GPS signal upon power-up, the clocks will enter the time setting mode and will not transmit time data. However, if the clocks are set manually the time data will be transmitted. The local time will need to be set each time the aircraft is powered up.
System Test
The clock does not have a user-commanded built in test to verify operation of the two liquid crystal displays. The clock’s operation can be monitored by the crew through observation of the displays and by comparing the two clock’s values.
Abnormal conditions are indicated by the lack of the colon in the upper time display.
Digital Clock (without GPS) Troubleshooting Tips
- Abnormal conditions are indicated by the lack of the colon in the upper LCD display
- Time reference stops transmitting to IACs when function selector is in SET
- Date and time must be reset after avionics battery is removed