RAF issue Aircraft Clock, 6A/3157
Description
1. The Type V.308 clock (Ref. No. 6A/3157) is designed for use in gas-turbined fighter aircraft. In addition to its normal function, the clock computes the time of trip between intermediate points during flight, or the total flying time on a flight which involves intermediate stops. The main dial incorporates a subsidiary 12-hour dial which registers the time of trip; both dials have Arabic numerals. All even-numbered hours, the dot at 12 o'clock, the main and time of trip hour and minute hands are luminized. The weight of the clock is 14 oz approximately.
2. The clock has an 8-day, Le Coultre, jewelled lever movement which incorporates a bimetallic balance and a Breguet overcoil hairspring. The movement provides a chronograph action for the centre-seconds hand and the time of trip hands. The centre-seconds hand indicates to one-fifth of a second.
3. Three knobs, located at 4, 6 and 8 o'clock, control the operation of the clock. The lever for regulating the escapement is situated at the back of the clock. The knob at 8 o'clock operates in a manner similar to that of a keyless movement of a watch. It is turned for winding, and pulled out and turned for setting the main hands. It should be noted that if the main hands are set while the time of trip is in operation, the time of trip hands are also set proportionately.
4. The action of the chronograph knob at 4 o'clock is similar to that of a stop watch: successive pressings of the knob start, stop and return to zero the time of trip and centre-seconds hands. Starting with the hands in the zero position. the first pressure of the knob withdraws the strikers from the centre-seconds and time of trip cams and immediately meshes the drive so that all hands are travelling.
5. The second pressure withdraws the drive and applies a brake to the centre-seconds wheel, so stopping the travel of the time of trip and centre-seconds hands. The third pressure of the knob releases the brake from the centre-seconds wheel, and this operation is immediately followed by the strikers contacting both of the cams and returning the hands to zero. The action of the chronograph knob does not affect the travel of the main hands.
6. The arrester knob at 6 o.clock is used to interrupt the travel of the time of trip hands; so that on a flight which involves intermediate landings, the time of trip hands may be arrested during the period in which the aircraft is not airborne, and restarted when the flight is resumed, thus recording the actual flying time for the whole trip. The knob is coupled to a coloured flag which is visible through an aperture just above 6 o'clock on the main dial. The colours "green" and "red" indicate that the hands are set for recording and waiting respectively. When the chronograph knob is pressed to return the hands to zero, the arrester knob is re-set automatically and the arresting device is put out of action.
Operation of the chronograph mechanism
7. From the foregoing paragraphs, the operation of the chronograph mechanism can be summarized as follows. Starting with the time of trip hands and the centre-seconds hands at zero, and the coloured flag at "green":-
(1) Press the chronograph knob (at 4 o'clock). This
starts the centre-seconds hands at the time of trip hands. The flag remains;"green".
(2) To stop the time of trip hands but not the centre-seconds
hand, turn the knob at 6 o'clock clockwise. the colour of the flag will
then be "red".
(3) To restart the time of trip hands. turn the knob at 6 o'clock
until the flag becomes "green".
(4) To stop the time of trip and centre-seconds hands, press the chronograph
knob again.
(5) To return the centre-seconds and time of trip hands
to zero, press the chronograph knob a third time. The flag will now be "green".
Note...
If the flag is "red" before the hands are re-set to zero, it will
automatically return to "green" when the chronograph knob is pressed.