RAF issue Aircraft Clock, 6A/8821
Introduction
1. The Type PWV308/326 time of flight clock (Ref. No. 6A/8821), in addition to its normal function, computes the lapse of time between intermediate stops during a flight or the total flying time on a flight which involves intermediate stops. After being fully wound, the instrument is capable of operating for six days without accumulating an error of more than one minute during any 24-hour period.
2. The main dial incorporates a subsidiary 12 hour dial which registers the time of flight; both dials are white with Arabic numerals. Figures for all even numbered hours on the main dial, dots indicating all hours on the main dial, and the main and time of flight hour and minute hands are black. The clock weighs approximately 14 oz and is fitted with a large (3.25) S.A.E. case.
DESCRIPTION
3. A self starting, fully jewelled, 8-day lever movement is contained in a dustproof, light alloy case and is fitted so that slight distortion of the case does not affect the accuracy of timekeeping sufficiently to create errors greater than the permitted tolerances. The movement incorporates a bimetallic balance and a Breguet overcoil hair spring.
4. The bimetallic balance compensates for any change in temperature which affects the moment of inertia of the balance and the modulus of elasticity of the spring. The overcoil promotes concentricity of the hairspring and maintenance of a constant centre of gravity. The chronograph hands travel in conjunction with a centre-seconds hand which indicates to one fifth of a second.
5. Three knobs, located at 4, 6 and 8 o'clock, control operation of the clock and a green/red flag indicator at 6 o'clock provides recording or waiting indication. A lever for regulating the escapement is situated at the back of the case. 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 flight is in operation, the time of flight hands are also set proportionately.
6. 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 flight 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 flight cams and immediately meshes the drive so that all the hands are traveling.
7. Second pressure withdraws the drive and applies a brake to the centre-seconds wheel, so stopping the travel of the time of flight 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.
8. The arrester knob at 6 o'clock is used to interrupt the travel of the time of flight hands, so that on a flight which involves intermediate landings, the time of flight 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 the coloured flag indicator which is visible through an aperture above 6 o'clock on the main dial. When the chronograph knob is pressed to return the hands to zero, the arrester knob is reset automatically and the arresting device is put out of action.
OPERATION
9. Operation of the chronograph mechanism is summarized as follows:-
Chronograph knob at 4 o'clock
(1) 1st pressure starts T of F (time of flight) seconds-hand.
(2) 2nd pressure arrests T of F seconds-hand.
(3) 3rd pressure returns T of F hand and seconds-hand to zero.
Arrester knob at 6 o'clock
Knob permits travel of T of F hands to be
arrested and restarted without operation of the chronograph knob.
Winding knob at 8 o'clock
(1) Pull to reset hands.
(2) Push back to winding position after resetting.
Flag indicator
Green: Recording when
seconds-hand is traveling, waiting when time of flight and seconds-hands are
returned to zero.
Red: Waiting.
Note...
If the
flag is red before the hands are reset to zero, it will automatically return
to green when the chronograph knob is pressed to return the hands to zero.
INSTALLATION
10. The clock is secured to the aircraft instrument panel by four 6-32 UNC-2B screws on a P.C.D. of 3.5 in.