TM 55-1510-215-10
guish. However, upon reaching the selected altitude, the
3-32.
Ground Proximity Altitude Advisory System
ALT annunciator will illuminate and stay illuminated for
(GPAAS).
two seconds, then extinguish. Upon deviating 300 feet
from the selected altitude the ALT annunciator will again
WARNING
illuminate and stay illuminated until the aircraft is 1000
feet from the selected altitude, whereupon it will again
extinguish.
The GPAAS will provide little, if any
(2) Altitude (drum) display. The drum display
warning for flight into abrupt vertical
provides a full five figured digital readout of altitude in
terrain approaching a sheer wall if
increments of 100 feet. Black and white cross-hatching
there is little gradually rising terrain
is provided in place of the first digit of the counter to
before reaching the steep terrain.
command attention at altitudes below 10,000 feet.
Altitudes below sea level are indicated by a wavey blue
The GPAAS will provide no warning
and white line which takes the place of the first digit of
for stabilized descent into terrain
the counter. In the event of power failure or a detection
while the aircraft is in the landing
of a malfunction by the failure monitor, a red and white
configuration, unless the aircraft is
striped warning flag obscures the numbers on the digital
following an operating electronic
counter.
glideslope or a correct minimum
descent altitude (decision height) has
(3) Barometric windows .
Dual baroscales,
been set on the radio altimeter
which permit barometric pressure settings in both
indicator.
millibars and inches of mercury, display in the barometric
windows.
These settings are inputted with the
a. Description.
The ground proximity altitude
barometric correction knob, which simultaneously sets
advisory system (GPAAS) is provided to aid the flight
millibars (MB) and inches (HG) of mercury.
crew in terrain avoidance.
(4) Barometric correction knob .
The
The GPAAS is a completely automatic system
barometric correction knob is used to set the desired
(requiring no input from the crew) which continuously
altimeter setting in the display windows. The knob
monitors the aircraft's flight path at altitudes of between
simultaneously sets the millibars and inches of mercury.
100 and 2000 feet above ground level (AGL).
(5) Needle indicator. The needle indicates
The GPAAS computer processes the data and,
aircraft altitude in hundreds of feet with subdivisions at
when conditions warrant, selects the appropriate
twenty-foot intervals.
One revolution of the needle
digitized voice advisory/warning message from its
equals 1000 foot of altitude change.
memory. This message is then announced over the
c. Operating Procedure.
pilot's and copilot's audio systems. If the condition is not
corrected, the GPAAS will rearm, and will again
1. Barometric Correction Knob - Set desired
announce and repeat the warning if the condition recurs.
altimeter setting in barometric window.
The GPAAS computer remains ready to announce a
Note that needle indicator operates
different message during the intervals between
properly.
repetitions. All messages are disabled below 100 feet
2. Warning Flag - Check not vi sible.
AGL.
The GPAAS system receives 28 VDC power through
NOTE
a 1-ampere circuit breaker placarded G.P.A.A.S.
POWER, located on the instrument panel.
If the altimeter does not read within
70 feet of field elevation when the
(1) GPAAS switch-indicator lights. A switch-
correct local barometric setting is
indicator is located on the instrument panel. The upper
used, the altimeter needs calibration
half of the switch-indicator (yellow) is placarded VOICE
or internal failure has occurred. An
OFF. The lower half is an indicator (red) only and is
error of greater than 70 feet nullifies
placarded VA FAIL.
use of the altimeter for IFR flight.
Depressing the upper (VOICE OFF) switch-indicator
disables the GPAAS voice advisory, and illuminates the
VOICE OFF indicator light.
The VA FAIL annunciator light (red) will illuminate
when the GPAAS fails.
Change 9 3-39
