Weather

Even the weakest thunderstorm brings gusty winds, and stronger storms sometimes blow down trees. Maybe you’ve never seen hail falling from a thunderstorm, but you’ve seen photos of baseball-size balls of ice on the ground and fear what these could do to an airplane’s windshield. It’s easy to imagine the dangers of flying into a thunderstorm.

You’re wise to be wary of thunderstorms. Even a small thunderstorm could be worse than you might think for an airplane. In fact, a growing cumulus cloud that’s on its way to becoming a thunderstorm can be hazardous.

Not yet a thunderstorm. By definition, a thunderstorm is a cumulus cloud that produces lightning, which creates thunder. Thunderstorms begin life as cumulus congestus clouds (also known as towering cumulus clouds), like those in the illustration on page 41. A rapidly growing cumulus cloud has a hard-edged, cauliflower-like appearance, like the area marked 1. In this part of the cloud fast updrafts are lifting supercooled drops of water so quickly they haven’t frozen.

The smooth area marked by 2 is where the updrafts have slowed and supercooled water has turned to larger ice crystals, which don’t reflect as much light as numerous tiny water drops. Scientists call this glaciation. It’s a sign that the cloud is on the way to becoming a thunderstorm, if it hasn’t already done so. As a cloud becomes more glaciated, the danger of strong, dangerous downdrafts increases.

While scientists are still working out the details of how areas of positive and negative electrical charges form in clouds, they know that strong updrafts and downdrafts with a mixture of ice and supercooled water are needed. The take-home lesson: Assume that by flying into a cloud like the one in Figure 1, it could be like flying into a thunderstorm—or a cloud with turbulence as bad as a thunderstorm’s.

In the heart of a thunderstorm. A mature thunderstorm at the height of its power is a maelstrom of supercooled water that turns to ice when it hits your airplane, hailstones (chunks of ice that can be bigger than baseballs), large raindrops, and violent updrafts and downdrafts that could shake your airplane with turbulence meeting the definitions of severe and extreme:

• Severe turbulence causes large, abrupt changes in altitude and/or attitude; large variations in indicated airspeed; aircraft may be momentarily out of control.
• Extreme turbulence means the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.

When lightning is most dangerous. Lightning is a flash of electricity between positive and negative areas of a cloud, from one cloud to another, or from a cloud to the ground. When a lightning flash—typically carrying a current of 100,000 amperes—hits an aircraft, the current travels through the aluminum skin (or the conducting material embedded in the skin of a composite aircraft) and back into the air. Lightning strikes often cause minor damage, such as burn marks on the skin, broken lights, or a shattered radome. The lightning doesn’t shock anyone or compromise aircraft safety. Since the 1960s aircraft have been built to meet strict safety standards so that lightning doesn’t cause sparks in fuel tanks or lines, or damage electrical and electronic systems.

Lightning’s biggest threat to a general aviation pilot is during the preflight inspection. If lightning hits your airplane on the ground while you’re outside or even close to it, the current could go through you on its way to the ground. You should follow the general safety rule for anyone who is outside: “If you hear it (thunder), fear it. If you see it (lighting), flee it.” The first rumble of distant thunder or first glimpse of far-away lightning should be the signal to take shelter in a metal vehicle with the doors closed (your airplane is fine) or a sturdy building—not a shelter with open sides.

Airlines generally follow this rule by stopping outside ground operations such as refueling or baggage handling when thunderstorms are near an airport.

If you blunder into a thunderstorm. Pilots with experience flying in stormy weather, such as those who
fly into hurricanes, say you should forget about trying to hold altitude—let the airplane ride up and down in the updrafts and downdrafts—and concentrate on maintaining a level attitude while keeping the airspeed below maneuvering speed (VA).

Since keeping the wings level is important, you shouldn’t try to turn around to escape the storm, as you might if you flew into a calm cloud. When you are sure you have control, contact ATC, who might be able to offer slight course changes that could get you out of the storm quicker.

ATC might be able to guide you out of a thunderstorm, but don’t count on it. A controller’s first duty is to separate instrument flight rules (IFR) aircraft from other IFR aircraft. Also, a controller is using a radar screen designed to track aircraft, not show weather details, and different screens show different amounts of weather information.

To see how relying on ATC to guide you through a “hole” in a line of thunderstorms can turn out, take the Air Safety Institute’s online course, “Weatherwise: Thunderstorms and ATC”; pay special attention to the “accident” on page 3.

Cleared to penetrate thunderstorms. Since 1947 a few pilots have flown into thunderstorms to collect research data, and this work will continue. The first in-air thunderstorm researchers were World War II pilots who made a total of 1,362 thunderstorm penetrations in 16 Northrop P-61 Black Widow night fighters for the government’s Thunderstorm Project in 1947 and 1948 in Florida and Ohio (see "Hurricane Hunters," page24).

Hail and lightning hit airplanes several times, but none crashed. They measured updrafts and downdrafts by allowing their airplanes to go up or down with the air currents instead of trying to hold fixed altitudes. This is still the recommended action for any pilot who blunders into a thunderstorm.

From 1972 through 2003 the Institute of Atmospheric Sciences at the South Dakota School of Mines and Technology operated a North American T-28, which had been beefed up and armored to protect it against hail. Various pilots flew it into thunderstorms nearly every year.

A retired A-10 Warthog, which is now being prepared, will begin flying thunderstorm research projects for the Naval Postgraduate School and the National Science Foundation in the fall of 2013 or early 2014. As you might suspect, only pilots with flight time in A-10s will be considered for the job.