Patek Philippe ANNUAL CALENDAR CHRONOGRAPH Mode D'emploi page 4

The CH 28-520 IRM QA 24H chronograph
caliber is a self-winding mechanical movement with
a flyback column-wheel chronograph, power-reserve
indicator, Annual Calendar, and day/night display.
The Annual Calendar Chronograph is based on the
venerable column-wheel principle for controlling the
start/stop commands. However, it differs from clas-
sic Patek Philippe column-wheel chronographs by
virtue of a remarkable innovation: the clamps do not
control the engagement of a lever-mounted clutch
wheel with the chronograph train, but instead act on
a disk clutch that transmits (chronograph running) or
interrupts (chronograph stopped) the power between
the fourth wheel and the chronograph hand. This is
a highly reliable solution that makes it possible to
use the chronograph hand as a continuously running
seconds hand without affecting the rate accuracy of
the watch. For this reason, the subsidiary seconds
dial customarily found in many chronographs is no
longer needed on Patek Philippe's Annual Calendar
Chronographs.
5