You are excited for your first cross-country flight the next morning, but soon later your mood changes looking at the forecast, and the excitement is completely gone. Thunderstorms in the vicinity of the airport you are going to depart from. Are thunderstorms that dangerous? The answer is yes. There are hazards in numerous combinations which makes it impossible to fly through a thunderstorm. The lifespan of a thunderstorm is approximately 1hr, however, a group or thunderstorm can stay for several hours.
Formation of thunderstorm
Thunderstorms don’t form anytime, anywhere. They require 3 main ingredients to develop which are warm moist air, unstable atmosphere, and a lifting force. Thunderstorm forms in 3 stages- cumulus stage, mature stage, and dissipating stage.
FAA Pilot’s Handbook of Aeronautical Knowledge-Chapter 12
- Cumulus stage
Warm air is lighter and tends to lift to a higher altitude. And when this warm air is moist, the water vapors also lift up with the rising warm air. The moisture in the air condensates to water droplets as they rise to result in a huge and high cumulus cloud. The cumulus stage can have strong updrafts reaching up to a speed of 6000 feet per minute.
- Mature stage
Cumulus clouds continue to grow up and the water droplets collide with each other and start growing larger and larger. At a certain point, the rising air is unable to further lift the water droplets due to its rising weight and hence starts falling towards the ground as rainfall. The mature stage consists of strong updrafts and downdrafts, making it a cumulonimbus cloud i.e. a CB cloud. The moving air inside the cloud builds up electric charges and releases it in the form of lightning.
The atmosphere above the tropopause is stable and a thunderstorm requires an unstable atmosphere to grow. So when the CB cloud reaches tropopause, it starts flattening and an anvil top can be seen. This indicates the thunderstorm is in a mature stage.
- Dissipating stage
Dissipating stage is the end of a thunderstorm cell and contains strong downdrafts, along with the rain. The cloud starts losing its shape and slowly starts to look more like a stratus cloud and eventually the storm dies.
How can you avoid thunderstorms?
The best way to avoid thunderstorms is – DON’T FLY. But why avoid them? Thunderstorms can have really severe weather such as heavy rain, extreme turbulence, severe icing, hail, lightning, and even tornadoes which makes it unsafe to fly and nearly impossible in small general aviation aircraft like the Cessna 172 or the Piper Arrow.
- A single cell thunderstorm can be clearly visible to the eye as a huge CB cloud with heavy rains pouring down and lightning.
- Embedded thunderstorms are very difficult to identify as they are hidden between other cumulus clouds. Aircrafts equipped with onboard weather radar or even FIS-B can display the echoes coming from the thunderstorm.
- Always check for aviation forecasts such as METAR/TAF, convective outlooks, and PIREPS before planning your flight.
- A squall line, a line of thunderstorms, is mostly associated with a cold front and can have severe weather.
- When flying around the thunderstorm, it’s ideal to stay at least 20nm away from the thunderstorm, and also avoid flying under the anvil to avoid the possibility of hail falling from the anvil and severe clear air turbulence.
- Do not take off or land facing a thunderstorm.
- Do not fly under thunderstorm
- Suspend the flight till the weather is good to fly.
What if you have no choice rather than flying inside a storm?
You are on your flight and there is no way to avoid thunderstorms or no place to land nearby and the only option is to fly through the storm. In that case, there are a few points to keep in mind.
- Once inside, there is no turning back. Due to associated turbulence, making turns can increase the stress on the aircraft.
- Reduce to the maneuvering speed and maintain wings level attitude, accept speed and altitude deviations.
- Tighten your seatbelts and shoulder harness if installed.
- If possible, enter the storm at an altitude well below the freezing level and switch on all the heats, such as pitot heat, carburetor heat, anti-ice.
- Increase the cabin light to maximum brightness and look at the instruments to avoid temporary blindness due to lightning.
References
https://www.faa.gov/documentlibrary/media/advisory_circular/ac%2000-24c.pdf
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/14_phak_ch12.pdf