Cumulonimbus and aviation

Numerous accidents have occurred in the vicinity of thunderstorms. It is often said that the turbulence can be extreme enough inside a cumulonimbus to tear an aircraft into pieces. However, this kind of accident is relatively rare. Moreover, the turbulence under a thunderstorm can be non-existent and is usually no more than moderate. Actually, most thunderstorm-related crashes occur due to a stall close to the ground when the pilot gets caught by surprise by a thunderstorm-induced wind shift. Moreover, aircraft damage caused by thunderstorms is rarely in the form of structural failure due to turbulence but is typically less severe and the consequence of secondary effects of thunderstorms (e.g., denting by hail or paint removal by high-speed flight in torrential rain).

Thus, cumulonimbus are known to be extremely dangerous to air traffic, and it is recommended to avoid them as much as possible. Cumulonimbus can be extremely insidious, and an inattentive pilot can end up in a very dangerous situation while flying in apparently very calm air.

While there is a gradation with respect to thunderstorm severity, there is little quantitative difference between a significant shower generated by a cumulus congestus and a small thunderstorm with a few thunderclaps associated with a small cumulonimbus. For this reason, a glider pilot could exploit the rising air under a thunderstorm without recognising the situation - thinking instead that the rising air was due to a more benign variety of cumulus. However, forecasting thunderstorm severity is an inexact science; in numerous occasions, pilots got trapped by underestimating the severity of a thunderstorm that suddenly strengthened.

Even large airliners avoid crossing the path of a cumulonimbus. Two dangerous effects of cumulonimbus have been put forward to explain the crash of flight AF447 that sank into sea on the 31st of May 2009 about 600 kilometres (370 mi) northeast of Brazil. It encountered a mesoscale convective system in the intertropical convergence zone (known by sailors as the "doldrums"), where cumulonimbus rise to more than 15 kilometres (49,000 ft) in altitude. However, the aircraft did not disintegrate in flight. A different hypothesis was put forward and later confirmed: accumulation of ice on the aircraft's pitot tubes.

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Wikipedia