One of the greatest dangers to aviation are thunderstorms. When evaluating accidents related with thunderstorms it became apparent that the dangers associated were not always recognized by the aviators and possible evasive action was not carried out leading to disastrous results.
We thought that it was useful to dedicate some space to document and summarize information regarding thunderstorms and the effects on aviation and pilots. We are going to explain some of the characteristics of the phenomena and possible actions on how to avoid these storms and penetration procedures when caught inside.
Dangers to aviation
Thunderstorms represent some of the worst dangers in aviation. The effects they have are most likely to be found in the form of turbulence, downburst, microburst, tornadoes, icing, lighting, hail, heavy rain, surface wind gusts, runway contamination, low status and bad visibility.
Severe turbulence is caused by mixing of up- and downward moving air and can be expected anywhere in and under the Cb, even at distances of 25 NM ahead of and 10000 feet above the shower depending of its intensity.
A huge amount of cold air flows out of a Cb causing a strong downdraught with a cross section of 2 - 5 NM in center and horizontal wind shear and updraught around the center and with severe horizontal wind shear outside of the Cb. The total affected area may have a radius of more than 15 NM. The horizontal wind shear component may be as large as 6 kts per second over a period of 16 seconds, which exceeds the acceleration of any aircraft.
This is like a downburst but on a much smaller scale and with a cross-section of 0.5 - 2 NM. The horizontal wind may average at 45 kts with a maximum of 90 kts. It is very dangerous as the phenomena is concentrated over a small area.
These may develop under a shower visible by dust or moist but they can be invisible too when the air is dry under the convective condensation level. Typical horizontal dimension is several hundreds of meters and windspeeds of over 150 kts are not uncommon. Lifetime may last from several minutes to hours. Smaller types like the waterspout have a radius of 25 to 50 meters and a lifespan of about an hour.
Since the most severe icing occurs in an environment of supercooled water droplets. In a Cb this will be between levels where the temperature is around zero and minus 23° Celsius. Icing may also occur at higher level altitudes, near the top of the Cb where mixed water and ice particles may stick to the cold airframe.
Hail in a Thunderstorm
Hail may be present in temperatures well above zero Celsius, the occurrence at +20° Celsius is quite normal. As hail is formed by the Bergeron Findeisen and coalescence processes, requiring supercooled water droplets, it will typically be present in the lower and medium levels of a Cb, since these droplets are scarce above the -23° Celsius level.
If rain is present in the lower parts of the Cb it may cause engine flame outs and reduced visibility from the cockpit.
This may cause heavy turbulence, severe down draughts and may exceed crosswind limitation of the aircraft. Wind gusts can be present in a range of 25 NM ahead of and 15 NM around a Cb.
The presence of rain, hail or snow may result in a poor braking action. As a Cb is relative short lived phenomenon braking action coefficients are hardly ever measured and actual braking action may be unexpectedly poor.
Aluminum aircraft form a Faraday cage and are relatively safe from a lightning strike. If it does happen then this will result in compass deviations, electronic failures and such. Composite aircraft, fly by wire systems, computerized navigation and the like may make an aircraft more vulnerable to a lightning strike. Lightning will occur near or in (70% of lightning occurs within the Cb).
An electrically charged aircraft (static electricity) can be detected by static VHF disturbances on the radio and or St Elmo's fire, at this point it is more prone to a lightning strike. Charging can also occur when flying in Cirrus clouds near Cb's.
To electrically discharge an aircraft static wicks are used. They are connected to the airframe and release the electrons back to the atmosphere, thus reducing radio interference.
Often seen near a shower in moist areas and affects the ceiling limits. Results in sudden visibility reduction in mountainous terrain making flight below MOCA (Minimum Obstacle Clearance Altitude) impossible without ground violations.
This will occur in heavy precipitation fog or stratus over high ground.