15. The Mark III clock has an 8-day movement with chronograph action for the centre second-hand and subsidiary 12-hour time-of-trip dial. This also replaces the watch Mark V. It may be used in a manner similar to a stop-watch for timing over short distances, computing the average speed, and for the measurement of a period such as the time of a trip with or without landings. Mark III (with adaptor plate) and Mark IIIA clocks are interchangeable on the instrument panel, and, except for mounting details, are identical.


    16. The adaptor plate which is added to the Mark III clock conforms to the flange dimensions of the standard 3.75 in. instrument case. The clock is secured to this plate by means of a U-shaped clamp. Two studs projecting from the back face of the clock-case pass through clearance holes in the clamp. The adaptor plate fitted on the instrument panel is gripped between the flange of the case and the ends of the clamp by screwing up the nuts on the studs. 

    17. The movement is fitted in the case in such a manner that slight distortion does not effect the accuracy of time keeping within certain limits. The Mark IIIA has no adaptor plate and is fitted to the instrument panel in the usual manner.

    18. The Mark III and Mark IIIA clocks have similar operating knobs in the lower half of the case. Care should be taken in the operation of the knobs, as too much force may result in the distortion or loss of a knob. The delicate chronograph mechanism, though of a larger scale than that of an ordinary stopwatch, is liable to be damaged if the control knob is roughly handled.

    19. The time-of-trip hands and the centre secondhand are started, stopped, and returned to zero by successive pressings of the knob located at 4 o'clock. In action and purpose the knob is used in a manner similar to that of a stopwatch, the centre secondhand indicating to one-fifth of a second.

    20. The arrester knob at 6 o'clock is used to interrupt the running of the time-of-trip hands. This enables the actual flying time to be recorded on a trip involving intermediate landings. It is turned to arrest and restart these hands. A coloured tell-tale is coupled to the knob to indicate to the pilot whether the time-of-trip hands are set for recording or waiting. It shows red for recording and white for waiting or idle hands, and is visible through a hole in the main dial just above 6 o'clock.

    21. The arrester knob is linked to the chronograph knob at 4 o'clock. When the latter is pressed to return the time-of-trip hands to zero, the arrester knob is reset automatically and the arresting device put out of action. Thus, the hands are free and idle at the zero position and prepared for any subsequent record when the chronograph knob is operated again.

    22. The knob at 8 o'clock operates in a manner similar to the keyless movement of a watch. It is turned for winding, and pulled out and turned for setting the main hands.

    23. The lever for regulating the escapement is situated on the rear of the clock. The indicating hour and minute hands, the even numbered hours, the dot at 12 o'clock on the main dial, and time-of-trip hands are luminous in the luminous clocks.

    24. The specification requires that when tested (a) at normal temperature (10 deg. C to 20 deg. C) with the dial either vertical or horizontal, the clock shall not accumulate an error of more than one minute during any period of 24 hours within six days of winding, and (b) at -20 deg. C and at + 50 deg. C, within six days of winding, that it shall not accumulate an error of more than 0.5 minute during a four hours' run.


No. 20