cancels GCS . At system power-up, GCS is not selected. GCS should only be used to help differentiate weather
returns from ground returns (clutter). A special strap may be installed for the TWR-850 to enable the instant
removal of ground return display. If the strap is not installed, ground return display is removed at the normal
antenna sweep rate.
If the antenna is tilted down too much and GCS is turned on, a phenomenon known as a "GCS
wedge" may occur. This wedge is displayed as a black area (an area showing no return) located
approximately 10 degrees of dead ahead, and having the same depth as the area of ground return
that was being painted prior to turning GCS on. To eliminate this wedge, simply turn GCS off, or
raise the angle until the wedge disappears.
(6) STB (Stabilization) Pushbutton. The stabilization button enables or disables the radar antennas pitch
and roll stabilization circuits. The control is a latching push-on/push-off STB button on the WXP-850A/B. Stabiliza-
tion is normally enabled (button in). The stabilization feature can be disabled when a failure in the horizontal and/or
vertical reference system causes the radar system to become unusable. When stabilization is not selected, "USTB"
(unstabilized) is annunciated on the MFD. If stabilization is selected, there is no stabilization-related annunciation.
If an attitude fault is detected by the RTA-85X, a lashing "USTB" is annunciated on the MFD. If stabilization is on
and the RTA-85X reaches saturation limits due to unusual attitude maneuvers (greater than ±45 degree pitch or
roll), "SAT" is annunciated in this position.
(7) HLD (HOLD) Pushbutton. Pressing the non-latching HLD button on the WXP-850A/B (located con-
centric with the GAIN control) interrupts updating of the weather display on the MFD so that a given display can
be studied more closely. Updating is resumed when the HLD button is pressed again; when a MODE, RANGE,
GAIN, GCS, or SEC change occurs; or when power to the system is cycled (turned off and then turned on again).
The system powers up with HLD mode not selected. When HLD is deselected, the radar resumes display updating
from the current antenna scan position. In HLD mode, the weather radar annunciator in the upper left corner of the
MFD alternates between HOLD and the previously selected mode (i.e., HOLD-WX-HOLD-WX, etc). The alternating
words serve to remind the pilot that the system is in the HLD mode and the weather radar display is not being
(8) AUTO (AUTO tilt) Switch. AUTO tilt is selected by pulling outward on the AUTO switch (located con-
centric with the TILT control) on the WXP-850A/B. Auto tilt is designed to reduce pilot workload by automatically
adjusting the current displayed tilt setting following altitude or selected range changes. In effect, auto tilt reduces
the number of times manual tilt adjustment is required. When auto tilt is enabled, manual tilt commands from the
TILT control remain operational. Pushing the AUTO switch back in turns off the auto tilt feature and returns the
antenna to the setting of the manual TILT control. The normal procedure for using auto tilt is as follows:
(a) Select WX or MAP mode and the maximum range desired.
(b) Adjust the TILT control so that a light band of ground return is visible at the outer perimeter of the display.
(c) Engage AUTO tilt. When auto tilt is on and a different range is selected or the aircraft changes altitude,
the RTA-85X automatically tilts the antenna to maintain the same relationship between beam delection
Altitude information is internally derived. Pushing the AUTO switch back in turns off the auto tilt feature. Auto tilt is
annunciated on the MFD with an "A" preceding the tilt angle (i.e., A+10.7, A-5.3, etc). The tilt adjust resolution is the
same as with the TILT control.
(9) SEC (Sector) Pushbutton. The sector scan switch is an on-latching momentary switch located con-
centric with the RANGE control on the WXP-850A/B. Sector scan is selected by pressing in on the SEC switch.
When SEC is selected, the antenna azimuth scan angle is reduced from ±60 degrees on either side of the aircraft s
nose to ±30 degrees. With the antenna scan angle reduced by 50 percent, the weather radar display update rate is
doubled. This feature can be effectively used for a more careful study of dynamic conditions, like a more effectively
be used for a more careful study of dynamic conditions, like turbulence, where the increased update can be benei-
cial. In a dual WXP-850A/B installation, either panel can select or cancel the reduced sector scan function for the
TWR-850 system. The system powers up with SEC mode not selected. SEC mode is displayed by decreasing the
range arc from the normal 120 degrees to the reduced sector scan arc of 60 degrees on the display. The function
ability of the SEC mode is also affected by strapping options.
b. Fault Monitor. The fault monitoring function built into the TWR-850 system has priority over any selected
mode. The word FAULT is annunciated and alternates between FAULT and the selected mode annunciation (i.e.,