RAF issue Aircraft Clock, Mk 2
General
10. The clock has an 8-day movement with a bezel winding gear and replaces the watch Mark V. It has a centre second-hand, also dummy hour and minute hands. The dummy hands are used for marking the start of any particular period of time, for example when computing the period of a trip.
Description
11. With a view to interchangeability on the instrument panel, the clock has dimensions of the 2.5 in. standard type case as regards the flange and fixing holes. The flange is flush with the flat back of the case, and no hole is required through the instrument panel for the body of the case. The milled bezel is rotated for winding the clock, and for setting the indicating hour and minute hands when the setting lever is in the uppermost position. The setting lever is situated in the bottom right hand corner. When installing the clock, the fixing screw in this corner of the Case may be omitted if it interferes with the freedom of the setting lever. The lever for regulating the escapement is situated under a sliding dust-proof cover at the rear of the clock, and the clock must be removed from the instrument panel to adjust this.
12. In connection with the bezel-wind described above, it should be noted that too muchforce should not be applied when near the fully wound position. The possibility of damage to the winding mechanism or main spring is thereby minimized. The escapement is more likely to start from a stopped state if the spring is not fully wound.
13. The dummy hands are finished in red and are operated by the small knobs on the glass. The hands are assembled with sufficient friction to avoid movement under vibration. The luminous parts of the dial in the luminous clock are the hour and minute hands, the even numbers of the hours, and the scale marks at each hour point.
14. 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 period of four hours.