RAF issue Aircraft Clock, 6A/7865
Introduction
1. The elapsed time clock, Pt. No. NL1359 S.C., is designed to provide an accurate time datum when flying on a Q.D.R., thus enabling the various stages of a let-down to be accurately timed, or for a given rate of turn to be maintained for a predetermined time.
2. The clock is of the 36-hour type and is of a similar pattern to the elapsed time clock, Pt. No. N.L.1395 (Ref. No. 6A/5184) but with differences in the design of the case and the rating at extreme temperatures.
Case and weight
3. A small (2.25) flanged case is fitted and the instrument weighs approximately 8 oz.
DESCRIPTION
4. The outer dial is graduated into 60 seconds corresponding to one revolution of the large hand. The inner dial is graduated into 12 minutes corresponding to one revolution of the small hand. All figures, markings and hands are white on a matt black background. A winding button and a reset button on the front of the case control the operation of the clock.
5. A 16 jewel, 36-hour movement is contained in the light alloy flanged case which has three clearance holes for mounting purposes. The movement is fitted so that minor inaccuracies of the aircraft instrument panel, which might cause 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.
6. 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
7. The clock is wound by the winding button situated at 4-30 o'clock. A friction overwind device incorporated in the button ensures that the main spring or the winding mechanism is not damaged by overwinding. Starting or stopping is effected by depressing the winding button; the first pressure starts the clock, the second pressure will stop it. Both hands are set to zero by depressing the reset button situated at 6 o'clock.
Note...
If the clock is restarted after being stopped
before the hands are returned to zero, the clock will record the elapsed time
from the stopped position.
8. The clock can be started at any time during flight
and allowed to run. When a marker beacon is identified, the reset button is
depressed, causing the hands to return to zero and to restart instantaneously.
thus recording the elapsed time from zero.