RAF issue Aircraft Clock, 6A/8972
Introduction
1. The time of flight clock, Pt. No. N.L.6520 VC, is designed to provide a very accurate time datum for the purpose of astro-navigation and, when required, to record the time taken to complete a flight.
2. The clock is of the 8-day type, capable of maintaining its accuracy for 7 days after being fully wound. It is of a similar pattern to the time of flight clock, Pt. No. N.L.6520 (Ref. No. 6A/5183), but with differences in the dial presentation and the design of the case due to an external dial light which is fitted when the clock is installed in the aircraft. The oval dial of the instrument is designed to give conventional presentation at the angle the clock is viewed in the aircraft.
Case and weight
3. A large (3.25) S.A.E. case is fitted, and the instrument weighs approximately 1 Lb.
DESCRIPTION
4. A 12-hour dial with a centre-seconds hand shows the time of day and a small, subsidiary, AM/PM dial at 12 o'clock provides a clear indication of the hour on a 24 hour basis as shown by a single hand. The hand is driven at half the speed of the hour hand and thus will make one revolution in 24 hours. The time of flight is shown on a subsidiary, conventional 12-hour dial at 6 o'clock. All figures, markings and hands are white on a matt black background.
5. A black/white flag indicator at 3 o'clock provides recording or waiting indication. A three pressure-push button at 6 o'clock is used in conjunction with the flag indicator during the time of flight operations, and a winding button at 4 o'clock is used for winding or starting the clock.
6. The 23 jewel. 8-day lever movement is contained in a light alloy, large (3.25) S.A.E. case which has three integral 4 B.A. stiff nuts for mounting purposes. The movement is fitted so that minor inaccuracies of the aircraft instrument panel, which might cause slight distortion of the case when the clock is fitted to the panel, will not affect the accuracy of timekeeping sufficiently to create errors greater than the permitted tolerances. The back of the case is fitted with a corrosion inhibitor. A hole in the flange at 1-30 o'clock provides a mounting for the pillar securing the external dial light.
7. A temperature compensated balance assembly and a Breguet overcoil hairspring are incorporated in the movement. The balance compensates for changes in temperature which might affect the moment of inertia of the balance and the modulus of elasticity of the balance spring. The overcoil is provided to maintain a constant centre of gravity.
OPERATION
8. The clock is wound by the winding button situated at 4 o'clock. A friction overwind device incorporated in the button ensures that the main- spring or the winding mechanism is not damaged by overwinding.
9. The time of day hands and the AM/PM indicator can be reset by pulling out the winding button. This action stops the clock and automatically returns the sweep centre hand to zero' thereby enabling the clock to be accurately set to an external time datum.
10. The action of depressing the winding button on receipt of the signal automatically starts the clock. The resetting of the time of day hands will not affect the time of flight hands.
11. The hands of the time of flight dial are inoperative while the clock is stopped and are controlled by the three-pressure pushbutton, situated at 6 o'clock, while the clock is running. The button is coupled with the flag visible at 3 o'clock. The sequence of the button is as follows:-
(1) The first pressure starts the
time of flight hands and a black flag will be shown.
(2) The second pressure stops the
time of flight hands and a white flag will be shown.
(3) The third pressure returns the
time of flight hands to zero and a white flag will be shown.