Honeywell PRIMUS 1000 Pilot's Manual page 215

The glideslope part of the ILS mode intercepts, captures, and tracks the
glideslope signal. When the glideslope mode is used with the localizer
approach mode, a fully coupled ILS approach can be flown. The mode
is interlocked so that glideslope capture is inhibited until localizer
capture has occurred.
With the localizer captured and outside the normal glideslope capture
limits, the PFD annunciates the following modes:
D
LOC
D
GS (white)
D
Any other vertical mode in use at this time is also displayed.
As the aircraft nears the glideslope, as shown in Figure 6–38, the
vertical beam sensor monitors true airspeed, vertical speed, and
glideslope deviation to determine the correct capture point.
At glideslope capture, the computer drops any other vertical mode that
was in use and automatically generates a pitch command to track the
glideslope signal.
A28–1146–134
REV 1 Jan/03
ILS Mode Capture, Profile View
Figure 6–38
R
PRIMUS 1000 Integrated Avionics System
Modes of Operation
6-51