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with +5° being a good compromise setting. Below 10,000 feet, the flight
crew is busy with a variety of tasks from completing checklists to talking
with approach/departure control. Setting a +5° tilt and leaving it set
through 10,000 feet reduces cockpit work load. The +5° setting will
eliminate most ground clutter and detect the majority of the weather in
the immediate vicinity of the aircraft (figure 4-62). The two topics that
follow (Climb and Descent) explain the logic behind these guidelines,
and when a +2° tilt setting and a +7° tilt setting might be appropriate.
Figure 4-62 Recommended Tilt For Low Altitude
1st Edition, 1st Revision
18 Sep 03 4-63
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
CLIMB
It is typical for a two engine air transport category aircraft to climb
out after takeoff at approximately 240 knots with a 3000 fpm rate of
climb. This equates to a 7° climb angle from the horizontal (figure
4-63). Therefore, a +7° tilt setting keeps the radar aligned along the
aircraft flight path, alerts the crew to potential penetration of vaulted
thunderstorm areas (♦page 5-15) and eliminates ground clutter.
Figure 4-63 Climb Out Flight Path
The drawback to a +7° tilt is that weather detection is limited to the
general vicinity of the aircraft. This can be shown using the general
formula that says 1° of tilt gives you 100 feet per NM of beam position
change. For instance, with a +7° tilt the center of the beam is at 24,500
feet at 35 NM.
TECH DETAIL
1° of tilt at 35 NM yields 3,500 feet of beam position change.
Multiply 3,500 by 7 (due to the 7° of tilt). The result is the 24,500
feet used in the above paragraph.
This means that if the radar top of the thunderstorm is less than 24,500
feet, it may not be displayed on the radar. At 50 NM, the center of the
beam is at 35,000 feet, and the majority of the weather at this range will
not be visible due to the fact that the radar is looking at the top (♦page
5-5) of storms at this range. Since the radar beam is approximately 3.5°
wide (♦page 6-20), a +5° radar tilt angle provides a good compromise
because it keeps the outer edge of the radar beam pointed close to the
aircraft flight path and provides marginally better weather detection
ranges.
1st Edition, 1st Revision
4-64 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
CAUTION
Caution: If the tilt setting is too high, the radar beam may scan
above the radar tops (♦page 5-5) of thunderstorms that are a threat
to the aircraft. If the tilt is too low, the radar may not detect vaulted
thunderstorm energy (♦page 5-15).
N NOTE
Automatic MultiScan operation optimizes weather returns during a
climb by using one beam to scan along the aircraft flight path to
detect vaulted thunderstorm energy and a second beam set at a
lower tilt setting to detect weather at extended ranges and prevent
over-scanning of weather in the vicinity of the aircraft that is a
turbulence threat. The result is that the flight crew no longer needs
to compromise between tilt settings that scan weather along the
aircraft flight path and settings that provide extended range weather
detection and prevent over-scanning.
1st Edition, 1st Revision
18 Sep 03 4-65
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
DESCENT
Below 10,000 feet, a +5° tilt angle remains the best compromise for
descent if cockpit work load is heavy. This tilt angle will detect most
weather while at the same time eliminating the majority of ground clutter.
The benefit to this method is that the tilt setting can be set and forgotten
during the critical approach and landing phase of flight. However, it
is possible to descend into thunderstorms that are developing below
the aircraft flight path and are under the radar beam. Therefore, an
alternate tilt procedure for descent below 10,000 feet is to initially set a
+2° tilt setting, then gradually raise it to +5° as the aircraft descends to
lower altitudes (figure 4-64).
Figure 4-64 Recommended Tilt Settings For Descent
Figure 4-65 shows the effect on the display when tilt is not raised as
the aircraft descends. In the left picture, the aircraft is at 4,000 feet
with +2° tilt setting. Ground clutter is just beginning to be visible on
the display. In the right picture, the aircraft is at 2,000 feet with +2° tilt
setting. Clutter is now able to mask weather.
1st Edition, 1st Revision
4-66 18 Sep 03
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Collins Weather Radar operator’s guide(22)
