Why Thunderstorms Are So Dangerous for Airliners

airplanethunderstormAs I write this, AirAsia flight 8501 has been missing for less than 24 hours, and in the absence of wreckage its too early to speculate on what happened. But the flight, which took off from Surabaya bound for Singapore, appears to have been traveling through an area of intense thunderstorm activity, so it may be instructive to look at the kind of danger this sort of weather can present to aircraft.

The region around the equator is known to meteorologists as the Intertropical Convergence Zone, or ITC. Here, the heat and moisture of warm ocean waters provides the energy to power tremendous updrafts that produce clusters of thunderstorms called a Mesoscale Convenction Complex. These storms can punch up through the stratosphere up to 50,000 feet, far above the crusing altitude of commercial airliners. From Smartcockpit.com:

A thunderstorm brings together in one place just about every known weather hazard to aviation. A single thunderstorm cell can hold 500 000 tons of water in the form of liquid droplets and ice
crystals. The total amount of heat energy released when that much water is condensed amounts to approximately 3 x 1014 calories. Equated with known energy sources, this falls just below an entrylevel hydrogen bomb. Even a small thunderstorm would have the caloric equivalent of a Hiroshimatypeatomic weapon… The thunderstorm occupies a unique place in the pantheon of aviation meteorology because it is the one weather event that should always be avoided. Why always? Because thunderstorms are killers.

Some of the deadly forces include lighning, airframe icing, large hailstones, extreme turbulence, and downdrafts that can reach speeds in excess of 100 mph. Perhaps the greatest hazard facing a modern airliner, however, is the sheer volume of precipitation that a thunderstorm can put out.

On May 24, 1988, a TACA 737 en route from Belize to New Orleans was descending towards its destination when it blundered through a thunderstorm. At an altitude of just 2000 feet, a deluge of rain and hail doused the flames of its twin turbofans. Unable to regain power, the captain managed through superb airmanship to put the stricken plane down undamaged atop a mile-long levee. Notes superb aviation writer Peter Garrison:

The event was not unique. Nine months earlier, an Air Europe 737 descending through rain and hail over Thessaloniki, Greece, had suffered a double flameout. In that case, the crew managed to restart the engines and land without trouble. In 2002, a Garuda Indonesia 737, also descending among thunderstorms, suffered a double flameout over Java. Its crew ditched the airplane in a river; one person died, and there were a dozen serious injuries.

According to preliminary reports, the pilot of QZ8501 had asked air traffic control for permission to ascend from 32,000 to 38,000 feet in order to evade the weather. Historically, however, attempting to fly over a thunderstorm has proven a dangerous strategy. In 2009, Air France 447 was flying through the upper reaches of a thunderstorm when it hit turbulence and its pitot tubes froze, leading to loss of airspeed indication; in the ensuing confusion the pilot flying lost situational awareness and flew the plane into the ocean.