TM 1-1510-262-10
the system can no longer detect a Z4 (red level) target at a range where Z3 (yellow level) targets would normally be
detected without intervening weather. The pilot can then recognize that, from his present position, any precipitation
in the sector between the display weather and the yellow PAC Alert arc may be underestimated and should be
avoided.
NOTE
The PAC feature is intended for weather detection modes only. Using a weather detection mode
and downward tilt to produce a ground map will probably produce a display which makes correct
interpretation more dificult. The PAC circuit may interpret the return signals from ground targets as
intense storm targets and try to compensate for the attenuated signal, resulting in the yellow PAC
Alert arc appearing at the perimeter of the display. In MAP mode, the PAC Alert feature is automat-
ically disabled.
Table 3-3.
Relectivity Chart
PRECIPITATION RATE
COLORS
in/hr
mm/hr
Less than 0.03
Less than 0.8
Black (no display)
0.03 to 0.15
0.8 to 3.8
Green
0.15 to 0.5
3.8 to 12.7
Yellow
0.5 to 2.0
12.7 to 50.8
Red
Magenta
Greater than 2.0
Greater than 50.8
Greater than 0.02 with turbulence
Greater than 0.5 with turbulence
Magenta (in WX+T or TURB mode)
greater than 16.4 ft/sec
greater than 5.0 m/sec
(g) WX+T (Weather plus Turbulence). WX+T mode is the same as WX mode except that, along with
weather detection, precipitation-related turbulence is detected and displayed in the 5, 10, 25, and 50-nmi range
positions. (Turbulence cannot be detected at ranges greater than 50-nmi.) Detectable weather is color-coded; black
represents no detectable moisture, while detectable weather appears as one of four colors: green, yellow, red, or
magenta (least relective through turbulence). Relectivity levels (represented by Z1, Z2, etc) are used to relate
precipitation rate to return signal strength. Table 3-3 shows the relationship between relectivity levels, precipitation
rates, and display colors. In WX+T mode and with the range selected to one of the turbulence related positions (5,
10, 25, or 50), the weather radar indicator will display detected turbulence along with the weather detection. Detected
turbulence will be displayed in magenta and will consist of precipitation areas where horizontal wind velocity shifts
in excess of 16.40 feet per second (5 meters per second) are detected. For reference, the US National Weather
Service deines light turbulence as 0 to 19 feet per second (0 to 5.79 meters per second) and moderate turbulence
as 19 to 35 feet per second (5.70 to 10.67 meters per second). "WX+T" is annunciated at the left end of the range
arc on the MFD unless the 100, 200, 300-nmi range is selected. If the WX+T mode is selected along with a range
of 100, 200, or 300-nmi, "WX" is annunciated instead of "WX+T", and the system remains in weather only mode.
NOTE
Doppler turbulence detection techniques rely on the presence of at least light precipitation. The
TWR-850 system is not capable of detecting clear air turbulence.
(h) TURB (Turbulence Only). Selecting TURB mode removes all weather radar targets from the display
except for areas of detectable precipitation related turbulence that appears in magenta. On the WXP, this position of
the MODE switch is spring loaded (i.e., the operator must hold MODE switch in the TURB position). Releasing the
MODE switch on the WXP returns it to the WX+T position and restores the full weather radar display. TURB mode
is useful for closely analyzing areas of turbulence that have been detected while in the WX+T mode. By removing
the green, yellow, red, and magenta precipitation targets from the display, the areas of turbulence can be observed
alone. Turbulence detection is possible only in the 5, 10, 25, and 50-nmi range positions. Range selections of 100,
200, and 300 keep the system in WX (weather only) mode if TURB is selected. With the TURB selection, "TRB" is
annunciated at the left end of the half-range arc on the MFD. A special strap may be installed for the TWR-850 to
enable instant removal of weather display in TURB mode. If the strap is not used, weather display is removed at the
normal antenna sweep rate in TURB mode.
3-28
