JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Figure 5-11 — Under the Thunderstorm
Underneath
If you are unable to circumnavigate the thunder-storms in your area and the ceiling capabilities
of your aircraft will not permit an over-the-top flight, you should consider flying below the base
of the cloud. The speed of downdrafts usually decreases closer to the surface (Unless a
microburst is present!). Therefore, an altitude should be selected which will keep you as far away
from the cloud base as possible and still enable you to maintain adequate terrain clearance. Here
you can use the ⅓ rule which specifies selecting an altitude ⅓ the distance from the surface to the
base of the cloud (Figure 5-11). This procedure is not recommended for areas of mountainous
terrain. Below the storm, expect a low ceiling, poor visibility, and moderate turbulence. Perhaps
the most dangerous threat to flight below a thunderstorm is the downburst, or microburst, which
can be deadly to the unsuspecting pilot.
Penetration
Mission urgency or fuel state dictates whether thunderstorm penetration is required when
avoidance is not possible. The lower in the storm the penetration is made, the less the chance of
encountering hail, structural icing, or being struck by lightning. Therefore, another version of the
⅓ rule applies: penetrate through the lower ⅓ of the storm, since most hazards are more severe in
top ⅔ of the cell (Figure 5-12). However, with the strong updrafts and downdrafts, adequate
terrain clearance should also be considered in the selection of a penetration level. When crossing
a line of thunderstorms (a squall line for example), attempt to determine the orientation of the
line and penetrate the line at right angles (Figure 5-13). During the penetration of a
thunderstorm, do not attempt to turn back once you are inside the storm. Remember that singlecell
thunderstorms are only about one to five miles in diameter, and turning around will only
increase your time in the storm. Turning around can also result in a pilot becoming disoriented
and flying in the storm for a considerably longer period of time than continuing directly through
the storm in the first place. With no other information to make a decision, a penetration altitude
between 4000 and 6000 feet AGL should be adequate.
Version 3.2/Dec 08 5-11