TM106101(8/01)
RDR-1600 Pilot’s Guide
51
Advisory Circulars (Cont.)
50
RDR-1600 Pilot’s Guide
TM106101(8/01)
Advisory Circulars (Cont.)
AC 00-24B
1/20/83
(2) As hailstones fall through air whose temperature is above 0°C,
they begin to melt and precipitation may reach the ground as either hail or
rain. Rain at the surface does not mean the absence of hail aloft. You
should anticipate possible hail with any thunderstorm, especially beneath
the anvil of a large cumulonimbus. Hailstones larger than one-half inch in
diameter can significantly damage an aircraft in a few seconds.
f. Low Ceiling and Visibility. Generally, visibility is near zero within a
thunderstorm cloud. Ceiling and visibility also may be restricted in precipi-
tation and dust between the cloud base and the ground. The restrictions
create the same problem as all ceiling and visibility restrictions; but the
hazards are increased many fold when associated with the other thunder-
storm hazards of turbulence, hail, and lightning which make precision
instrument flying virtually impossible.
g. Effect on Altimeters. Pressure usually falls rapidly with the ap-
proach of a thunderstorm, then rises sharply with the onset of the first
gust and arrival of the cold downdraft and heavy rain showers, falling
back to normal as the storm moves on. This cycle of pressure change
may occur in 15 minutes. If the pilot does not receive a corrected altimeter
setting, the altimeter may be more than 100 feet in error.
h. Lightning. A lightning strike can puncture the skin of an aircraft and
can damage communications and electronic navigational equipment.
Lightning has been suspected of igniting fuel vapors causing explosion;
however, serious accidents due to lightning strikes are extremely rare.
Nearby lightning can blind the pilot rendering him momentarily unable to
navigate either by instrument or by visual reference. Nearby lightning can
also induce permanent errors in the magnetic compass. Lightning dis-
charges, even distant ones, can disrupt radio communications on low and
medium frequencies. Though lightning intensity and frequency have no
simple relationship to other storm parameters, severe storms, as a rule,
have a high frequency of lightning.
i.
Engine Water Ingestion.
(1) Turbine engines have a limit on the amount of water they can
ingest. Updrafts are present in many thunderstorms, particularly those in
the developing stages. If the updraft velocity in the thunderstorm
approaches or exceeds the terminal velocity of the falling raindrops, very
high concentrations of water may occur. It is possible that these concen-
trations can be in excess of the quantity of water turbine engines are
designed to ingest. Therefore, severe thunderstorms may contain areas of
high water concentration which could result in flame-out and/or structural
failure of one or more engines.
(2) At the present time, there is no known operational procedure
that can completely eliminate the possibility of engine damage/flameout
AC 00-24B
1/20/83
d. Icing.
(1) Updrafts in a thunderstorm support abundant liquid water with
relatively large droplet sizes; and when carried above the freezing level,
the water becomes supercooled. When the temperature in the upward
current cools to about -15°C, much of the remaining water vapor subli-
mates as ice crystals; and above this level, at lower temperatures, the
amount of supercooled water decreases.
(2) Supercooled water freezes on impact with an aircraft. Clear
icing can occur at any altitude above the freezing level; but at high levels,
icing from smaller droplets may be rime or mixed rime and clear. The
abundance of large, supercooled water droplets makes clear icing very
rapid between 0°C and -15°C and encounters can be frequent in a cluster
of cells. Thunderstorm icing can be extremely hazardous.
Figure 14-1. Cross-Section of a Thunderstorm
e. Hail.
(1) Hail competes with turbulence as the greatest thunderstorm
hazard to aircraft. Supercooled drops above the freezing level begin to
freeze. Once a drop has frozen, other drops latch on and freeze to it, so
the hailstone grows - sometimes into a huge iceball. Large hail occurs
with severe thunderstorms with strong updrafts that have built to great
heights. Eventually, the hailstones fall, possible some distance from the
storm core. Hail may be encountered in clear air several miles from dark
thunderstorm clouds.
