band represents PAC Alert. Red is used for the third level of intensity, and magenta is used for the most intense
precipitation level and for turbulence.
(d) TGT (Target Alert). The TGT mode on the WXP-850A/B permits detectable weather targets (precipi-
tation or precipitation-related turbulence) to be detected and annunciated without the targets themselves appearing
on the weather radar display. This mode is normally used when other data (i.e., checklists, remote data, etc) is
being displayed on the MFD. When TGT mode is selected, the radar transmitter is energized, the antenna begins
to scan, and the TWR-850 automatically selects the range and tilt required (the pilot has no control of range and
tilt in TGT mode). A box with the letters "TGT" inscribed appears non-lashing to the right of the lubber line at the
left end of the half-range arc on the MFD. TGT mode automatically evaluates two target "windows" (refer to Figure
3-2) to provide search coverage from 7 to 200 nmi range (50 nmi maximum range for turbulence detection) and ±15
degrees of dead ahead, regardless of the range selected on the WXP. The target alert mode notiies the pilot of the
following two weather phenomena:
When red or magenta weather targets are detected within the entire range. (Rainfall rates greater than
0.5 in/hr or 12.7 mm/hr.)
When areas of precipitation-related turbulence are detected within the 50 nmi window. (Wind velocity
shifts in excess of 16.40 ft/sec or 5m/sec.)
A lashing box with the letters "TGT" inscribed appears to the right of the digital heading read out at the top of the
MFD if either of the preceding target conditions is detected within the target alert window. A lashing box with the
letters "TRB" inscribed appears when an area of turbulence is detected within the target alert window. If areas of
precipitation and turbulence are both detected within the target alert windows, the lashing annunciation alternates
between "TGT" and TRB", but at a faster rate of lashing. This lashing annunciation indicates to the pilot that areas
of severe precipitation and/or turbulence have been detected. When this occurs, the pilot should select one of the
WXP-850A/B weather and/or turbulence detection modes (WX , WX+T , or TURB ) and adjust the tilt as required.
RDR should be selected on the MFD, or the ND s should be placed in a format that allows weather to be displayed.
Momentarily holding the MODE switch in the TURB position will display only turbulence which has been detected,
and other returns are removed from the display. The ability of target alert to automatically control the position of the
antenna beam is possible because of the radar s capability to determine the aircraft s AGL (Above Ground Level)
altitude. To determine the aircraft s AGL, the radar beam automatically dips into the "ground" at the end of every third
antenna scan. The resulting return is used to calculate aircraft altitude AGL. This dipping of the antenna beam does
not affect the weather radar display, and therefore is not noticed by the pilot. Over large bodies of water, the radar s
ability to calculate the aircraft s altitude AGL is reduced or eliminated due to the low relectivity of such surfaces.
Under this condition, the target alert circuits choose a tilt setting of zero degrees. This tilt setting is maintained until
the radar beam begins to see "land" again.
The TGT mode should only be used after the integrity of the stabilization source has been veriied
and the antenna alignment check has been successfully performed. Failure to do so may result in
false target alarms and/or missing targets.
(e) MAP . Selecting MAP mode places the system in the terrain-mapping mode. MAP mode automatically
disables the PAC Alert and GCS features so that cities, rivers, lakes, or other prominent geographic features may be
displayed. MAP mode is most effective when one of the shorter ranges is selected and the antenna is tilted down.
MAP format, range annunciators, and ground targets will be displayed. "MAP" is annunciated at the left end of the
half-range arc on the MFD.
(f) WX (Weather only). Selecting WX mode allows the system to operate in the weather detection mode.
Detectable weather is color-coded; the black screen represents no detectable moisture, while detectable weather
above the minimum display threshold appears as one of four colors: green, yellow, red, or magenta (in the order
of least relective to most relective). Refer to Table 3-3 for a relectivity chart that shows the relationship between
relectivity levels, precipitation rates, and display colors. "WX" is annunciated at the left end of the half-range arc
on the MFD. With WX mode selected, the PAC (Path Attenuation Correction) feature is automatically enabled. The
purpose of PAC is to compensate for the radar beam absorption as it penetrates a given precipitation cell. This
overcomes the tendency in non-compensated radar to under estimate the true image of a precipitation cell simply
because the energy is absorbed as it penetrates the cell. When radar targets are of suficient magnitude (intensity
and depth) to exhaust the full range of attenuation correction, a condition known as PAC Alert exists. PAC Alert
highlights those sectors of uncertainty which are masked by intervening beam absorption. A yellow arc (bar) at the
perimeter of the radar display shows the bearing towards these areas. The yellow PAC Alert arc will appear when