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Flight Instruments - Clock

Note: There are two types of clock that may be fitted to the 757. The only difference is whether the clock can indicate the current date (DATE switch fitted).

Two electronic clocks are installed, with two digital displays on each clock. Coordinated universal time (UTC), local time, or date may be set on the upper time display. The lower ET/CHR display is used for either elapsed time or the chronograph. Separate controls are provided for each display. The chronograph has priority on the display and must be zeroed to read the elapsed time.

Clock Chronograph (CHR) Switch - initiates start, stop and reset functions of the CHR display and second hand. Subsequent pushes:

  • override any existing ET display
  • controls chronograph second hand.

The CHR can be controlled by either a push-button switch on the clock or by a remote switch on the glareshield. The clock accepts a signal from either switch with equal priority. The first switch actuation starts the CHR display after resetting the ET display to zero. The second switch actuation holds the CHR time and the sweep second hand. A third actuation of the switch resets the CHR time to zero, displays the current ET if running, and returns the sweep-second hand to zero.

Time/Date Window

  • Displays time (hours, minutes - 24 hour format) when time is selected with the date switch.
  • Alternately displays day-month and year when date is selected with the date switch.

Elapsed Time (ET) Selector - Controls the Elapsed Time function:

  • RUN - starts the elapsed time display.
  • HLD - stops the elapsed time display.
  • RESET - (spring-loaded to HLD) returns ET display to zero.

Date Switch (as installed) - displays day and month (dd mm), alternating with year (yy). Subsequent push - returns display to time.

GMT/SET Selector - Sets the time and date when the date switch is set to manual:

  • RUN - starts the time indicator.
  • First position:
    • HLD (Hold) - stops the time indicator and sets the seconds to zero when time is selected with the date switch.
    • HLDY (Hold, Year) - advances years when date is selected with the date switch.
  • Second position:
    • SS (Slow Slew) - advances minutes when time is selected with the date switch.
    • MSM (Minutes Slew, Month) - advances months when date is selected with the date switch.
  • Third position:
    • FS (Fast Slew) - advances hours when time is selected with the date switch.
    • HSD (Hour Slew, Day) - advances days when date is selected with the date switch.

The time references for the clocks are connected to the Hot Battery Bus but the displays are powered from other busses. When electrical power is removed from the airplane, the clock is blank and the chronograph does not operate. When power is restored, the clock displays the correct time.

The Captain's clock provides GMT to other systems such as the FMCs and Flight Recorder. If the Captain's clock fails, the First Officer's clock provides this data.

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