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will be displayed as either yellow or red, and a yellow return will be
displayed as red.
N NOTE
In oceanic regions, some island or sea clutter may be displayed
when the MAN GAIN is set above CAL.
1st Edition, 1st Revision
4-36 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Automatic Operation
Figure 4-39 GAIN Set to MAX
During automatic operation rotating the gain knob counterclockwise
decreases receiver sensitivity. MIN gain (figure 4-40) is selected when
the gain knob is rotated fully counterclockwise and represents an
approximately one and a half color level decrease in the color of the
displayed weather (♦page 6-4). Consequently, red returns may be
displayed as yellow or green, yellow returns will be displayed as green
or disappear entirely, and green returns will no longer be displayed.
1st Edition, 1st Revision
18 Sep 03 4-37
MULTISCAN OPERATION COLLINS
MultiScan Automatic Operation MultiScan™ Radar
Figure 4-40 GAIN Set to MIN
CAUTION
Always return gain to CAL after a below gain setting has been
used. Gain settings below may cause thunderstorms to appear less
intense than is actually the case (♦page 6-8).
Note that if a thunderstorm remains red when MIN gain is selected
it indicates a storm exhibiting extreme reflectivity and is potentially
a substantial threat to the aircraft. However, the radar should only
be operated at the MIN gain position for short periods of time to help
identify thunderstorm cores and areas of extreme reflectivity. The gain
control should then be returned to the calibrated position. With gain set
to MIN it is possible that a thunderstorm that just crosses the red color
threshold will be displayed as green. Using MIN gain exclusively thus
increases the possibility of inadvertent thunderstorm penetration.
VARIABLE TEMPERATURE BASED GAIN
During automatic operation MultiScan uses variable gain based on
atmospheric temperature profiles to compensate for variations in
1st Edition, 1st Revision
4-38 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Automatic Operation
geographic location, time of day and altitude so as to optimize gain
settings and weather returns in all phases of flight. Gain is held constant
below the freezing level. As the aircraft ascends through the freezing
level and the temperature decreases below 0° C, gain is increased.
When temperatures fall below -40° C cloud tops are composed entirely
of ice crystals and exhibit minimal reflectivity (♦page 5-5). Variable
temperature based gain increases the gain by approximately one color
level in this region to provide more accurate high altitude weather
returns.
PATH ATTENUATION COMPENSATION (PAC) ALERT
If intervening rain fall creates an attenuated area, sometimes known as
a radar shadow (♦page 6-27), PAC Alert places a yellow arc on the outer
most range scale to warn the pilot of the attenuated condition (figure
4-41). PAC Alert is operative whenever the radar is being operated in
CAL gain and the aircraft is within 80 NM of a thunderstorm.
WARNING
NEVER FLY INTO A RADAR SHADOW!
CAUTION
PAC Alert is disabled for all non-CAL settings. N NOTE
PAC Alert is activated during both automatic and manual radar
operation.
1st Edition, 1st Revision
18 Sep 03 4-39
MULTISCAN OPERATION COLLINS
MultiScan Automatic Operation MultiScan™ Radar
Figure 4-41 PAC Alert on Weather Display
OVERFLIGHT PROTECTION
Current generation radars tend to over-scan the reflective portion
of thunderstorms (♦page 5-5) at cruise altitudes. When this occurs,
the actual thunderstorm top may still be in the aircraft flight path and
inadvertent penetration of a storm top is possible. Figure 4-42 shows
traditional manual operation of the radar and illustrates this phenomena.
In the first picture the air crew has selected an 80 NM range scale
and adjusted tilt to place ground clutter in the outer most range scale
(♦page 4-69). Two cells are clearly visible (circled) at 60 NM. In the
second picture, the 40 NM range scale has been selected and the
thunderstorm cells are now at 30 NM. Note that as the aircraft nears the
thunderstorms the radar beam narrows and begins to move higher into
less reflective areas of the cells. As a result, the intensity of the storms
is decreased and the returns appear as green cells only. In the third
picture the wet top (radar top) of the storms have fallen below the radar
beam and the cells have disappeared entirely from the radar display
although the actual (visual) top remains in the aircraft flight path.
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Collins Weather Radar operator’s guide(17)
