The GCAS is a completely automatic system
(requiring no input from the crew) which continuously
monitors the aircraft's flight path at altitudes of
between 50 and 2450 feet Above Ground Level (AGL).
advisory/warning messages to the crew in the event
the aircraft is flown along a projected flight path that
would result in contact with the terrain under any of six
modes of operation.
The operating temperature range of the
computer is -15 °C to 71 °C.
When operating in temperatures below
-15 °C, the system may be unreliable.
below 50 feet AGL.
System messages always precede the
"Whoop, Whoop" aural tone and "Pull Up"
warnings, thus allowing the crew time to
identify and correct the specific flight
situation. The system messages and
warnings are discussed in the various
In all GCAS warning modes, the warnings cease
and the system resets when the pilot establishes a
positive pull up and climb. The system consists of a
ground collision avoidance system computer located in
the nose avionics compartment, pilot's and copilot's
speakers/headsets, a flap override switch, a GPWS
INOP annunciator light, and a system circuit breaker,
placarded GPWS POWER, located on the copilot's
sidewall circuit breaker panel.
b. Controls, Indicators, and Functions.
The pilot and copilot are each provided with two GCAS
system switch-indicators located on their respective
sides of the glareshield. The left switch-indicator is red
and is placarded PULL UP in large letters and GPWS
TEST in smaller white engraved letters below. The
right switch-indicator is yellow and is placarded
BELOW G/S. The PULL UP indicator lights will flash
when the threshold activation limits of modes 1, 2, 3,
or 4 have been exceeded. The BELOW G/S indicator
lights will illuminate continuously when the threshold
activation limits of mode 5 have been exceeded.
Pressing the BELOW G/S switch-indicator when below
1000 feet AGL will deactivate the deviation below
glideslope mode. The glideslope mode will be re-
armed when the aircraft climbs back through 1000 feet
AGL. Pressing the PULL UP, GPWS TEST switch-
indicator will activate the GCAS self-test sequence.
(2) GCAS Aural Warning Indicators. The
following is a list of aural indications. Due to the
possibility of activating more than one condition at a
established. A voice annunciator of a higher priority
will immediately override a lower priority message in
progress. A lower priority message will function only
upon the cessation of a higher order warning.
1. Whoop, Whoop, Pull Up" (modes 1
2. "Terrain" (mode 2).
3. "Too Low Terrain" (mode 4).
4. "Too Low Gear" (mode 4).
5. "Too Low Flap" (mode 4).
6. "Minimums" (mode 6).
7. "Sink Rate" (mode 1).
8. "Don't Sink" (mode 3).
9. "Glideslope" (mode 5).
(3) GCAS Flap Override Switch. A guarded
switch placarded GPWS FLAP OVRD, located on the
pedestal extension, provides a means of disabling the
"Too Low Flap" aural warning to facilitate landing with
the flaps in the UP position. The GCAS "Too Low
Flap," aural warning will not be activated if the flap is in
the APPROACH or LANDING positions (40° or
Annunciator Light. Illumination of the cautionary
(yellow) annunciator light placarded GPWS INOP,
located on the caution/advisory annunciator panel,
warns the flight crew that the GCAS is not functional.
c. Normal Operation.
(1) Turn-On Procedure. The GCAS is
operable when the following conditions have been
1. Battery switch ON.
2. Inverter switch NO. 1 or NO. 2.
3. AVIONICS MASTER POWER switch
4. GPWS POWER circuit breaker In.