TM 1-1510-225-10
3C-63
Figure 3C-35. Mode 4 – Proximity To Terrain
(6) Terrain Clearance – Gear Down, Flaps
Up (Mode 4b). Mode 4b provides visual and aural
warnings for terrain clearance with gear down and
flaps less than 40%. When aircraft penetrates the
warning boundary at a speed below approximately 145
KIAS with the flaps not extended, the "Too low flaps"
aural warning will sound and the PULL UP glareshield
warning lights will begin to flash. When the boundary
is penetrated above approximately 145 KIAS the "Too
low – terrain" aural warning will be sounded and the
PULL UP glareshield indicator lights will begin to flash.
In the event that the landing gear is extended
and then retracted, at 200 feet AGL, the "Too low -
gear" aural warning will sound and the PULL-UP
glareshield indicator lights will begin to flash if the gear
is still retracted at that time.
Mode 4b is inhibited below 50 feet AGL for any
gear or flap position. The computer automatically
switches from mode 4b to mode 3 whenever the
boundary of mode 4b is passed in full landing
configuration (gear and flaps down), or whenever the
aircraft is below 50 feet AGL with the gear down.
(7) Descent Below Glideslope (Mode 5).
Mode 5 provides visual and aural warnings for descent
below glideslope when making an ILS approach.
Refer to Figure 3C-36. When the aircraft penetrates
the outer warning boundary while on an ILS approach,
a "Glideslope" aural warning will be given softly and
the BELOW GS glareshield indicator lights will be
continuously illuminated. If the inner warning
boundary is penetrated, the "Glideslope" aural warning
repeats faster and at a higher audio level. The
glideslope advisory alert may be canceled anywhere
below 1000 feet AGL by pressing the BELOW GS
glareshield switch-indicator. The deviation below
glideslope mode will automatically be reset by climbing
above 1000 feet AGL.
(8) Descent Below Selected Radio Altitude
(Mode 6). Mode 6 provides aural warning for
descending below the radio altitude decision height
selected on the radio altimeter indicator with the
decision height marker for altitudes between 1000 feet
AGL and 50 feet AGL with the landing gear down.
When the aircraft descends below the selected
radio altitude the "Minimums" aural warning will sound.
The mode will not function again until 1000 feet AGL
or 50 feet AGL have been transitioned or the landing
gear has been cycled up or down. There are no
glareshield warning lights associated with mode 6.
d. Emergency Operation. If an emergency
procedure makes it necessary to land with the gear up
or some situation makes it necessary to disable the
GCAS, pull the GPWS POWER circuit breaker located
on the copilot's sidewall circuit breaker panel.
