Thunderstorms pose the greatest threat to aircraft operations. The three key ingredients of instability, lift and moisture combine explosively to produce a thunderstorm. Thunderstorms produce several weather hazards that impact aviation operations which include strong winds, icing, hail, heavy rain, lightning and tornadoes. A cumulonimbus cloud (CB) indicates a mature stage thunderstorm and severe turbulence. When a pilot sees a CB in a Terminal Aerodrome Forecast (TAF), this translates to turbulence and potential trouble. A Terminal Aerodrome Forecast is a concise statement of the expected meteorological conditions at an airport during a period of 24 hours.
The strongest turbulence in a thunderstorm occurs in the zone between the updrafts and downdrafts. Near the ground, there is a low-level area of turbulence which develops as the downdraft spreads out at the surface and can extend outward for many miles from the center of the storm.
How does the FAA address Thunderstorm Flying? Aeronautical Information Manual Chapter 7 states: Above all, remember this: never regard any thunderstorm “lightly”
Even when radar observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy. Following are some Do’s and Don’t of thunderstorm
1. Don’t land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level turbulence could cause loss of control.
2.Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be disastrous.
3. Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated.
4. Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.
5. Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus.
6. Do clear the top of a known or suspected severe thunderstorm by at least 1,000 feet altitude for each 10 knots of wind speed at the cloud top. This should exceed the altitude capability of most aircraft.
7. Do circumnavigate the entire area if the area if the area has 6/10 thunderstorm coverage.
8. Do remember that vivid and frequent lightning indicates the probability of a strong thunderstorm.
9. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar.
There are many guidelines and policies regarding thunderstorms. However, the best policy is Avoidance.