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PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00
Radar Facts
5-57
Step Procedure
5 When flying at high altitudes, tilt downward
frequently to avoid flying above storm tops.
Studies by the National Severe Storms Laboratory (NSSL)
of Oklahoma have determined that thunderstorms
extending to 60,000 ft show little variation of turbulence
intensity with altitude.
Ice crystals are poor reflectors. Rain water at the lower
altitudes produce a strong echo, however at higher
altitudes, the nonreflective ice produces a week echo as
the antenna is tilted up. Therefore, though the intensity
of the echo diminishes with altitude, it does not mean
the severity of the turbulence has diminished.
NOTE: If the TILT control is left in a fixed position at
the higher flight levels, a storm detected at
long range can appear to become weaker
and actually disappear as it is approached.
This occurs because the storm cell which
was fully within the beam at 100 NM gradually
passes out of and under the radar beam.
6 When flying at low altitudes rotate tilt upward
frequently to avoid flying under a thunderstorm.
There is some evidence that maximum turbulence exists
at middle heights in storms (20,000 to 30,000 ft); however,
turbulence beneath a storm is not to be minimized.
However, the lower altitude may be affected by strong
outflow winds and severe turbulence where thunderstorms
are present. The same turbulence considerations that
apply to high altitude flight near storms apply to low
altitude flight.
7 Avoid all rapidly moving echoes by 20 miles.
A single thunderstorm echo, a line of echoes, or a
cluster of echoes moving 40 knots or more will often
contain severe weather. Although nearby, slower moving
echoes may contain more intense aviation hazards, all
rapidly moving echoes warrant close observation. Fast
moving, broken to solid line echoes are particularly
disruptive to aircraft operations.
8 Avoid, the entire cell if any portion of the cell is red
or magenta by 20 NM.
The stronger the radar return, the greater the frequency
and severity of turbulence and hail.
Severe Weather Avoidance Procedures
Table 5-12 (cont)