It's officially Summer in the northern hemisphere. On June 21, the first day of Summer, we had thunderstorms here in San Jose and elsewhere in Northern California, some with frequent lightning. We even saw one that had a small funnel cloud. While we usually have some minor thunderstorms in the Bay Area at times from Spring to Fall, these conditions were things we see maybe once every 5-10 years. So that's rare here.Some of us pilots stayed on the ground to watch the storm from safety. We also noticed that some were flying around the pattern while thunderstorms with active lightning were within 5 miles of the field. Here in California, we don't often get practice exercising judgment about thunderstorms. But it really is important. While you wouldn't hesitate to drive a car under a thunderstorm, flying a light aircraft near one is a different story. It's a case where we have to know the difference and exercise appropriate judgment. So let's talk about the safety issues behind it.
]]> FAA's Aeronautical Information Manual (AIM) has something to say about flight safety near thunderstorms in sections 7-1-28 and 7-1-29. In general, you want to stay 10 miles away from any thunderstorm and 20 miles away from severe thunderstorms. It can be hard to tell if it's severe, or if it's going to grow into a severe storm before you reach it. So err on the side of caution.
Also, don't fly under the "anvil" (wispy cirrus clouds at the top) of a thunderstorm. The cirrus clouds at the top are always made of ice. You know that storm is moving ice up there. But you don't know if ice is also falling in the form of hail. Under the anvil, there is a risk of hail even outside the visible clouds.
If a thunderstorm is within 10 miles of an airport, these warnings still apply. Every thunderstorm has updrafts and downdrafts, by definition. When the downdrafts hit the ground, the wind spreads out in all directions. Thunderstorms are surrounded by "gust fronts" at the edge of their winds' influence. At an airport, the arrival of a gust front will come as a sudden change in wind direction as the outflow from the storm overruns the airport. That will make things at least difficult if not dangerous if you're in the pattern or on the runway when it arrives.
Once inside the gust front, the local winds at the airport are under the influence of the chaotic pattern of updrafts and downdrafts of the storm. In a light aircraft, the updrafts can suck you up into the storm clouds and the downdrafts can cause uncontrolled loss of altitude, possibly to the ground.
Even airliners have had accidents in thunderstorms. Though since the late 80's, airliners have weather radar and large airports have wind shear detection which together have very effectively mitigated the problem. But we don't have those tools in most small planes or at most small airports. So the prudent advice is to stay out of and away from thunderstorms.
Oh, and lightning too. That's probably the most obvious hazard because it's the most dramatic thing we can see in a thunderstorm. Lightning is a hazard anywhere under the storm's anvil. For all the other reasons mentioned above, you would already want to stay away from there. So just add it to the list.
Fortunately, thunderstorms are usually short-term events. A single thunderstorm cell can grow to maturity and die out well inside of an hour. The atmospheric conditions which started it will likely still be present to make more in the area. It isn't difficult to be alert and be safe. But you have to actually make the decision to do so.
Other useful online resources:
- AOPA Air Safety Foundation - Thunderstorm awareness resources
(includes online training course good toward the FAA Wings program and AOPA/AIG insurance accident forgiveness) - FAA Safety Team - Thunderstorm avoidance tips
- FAA - "Thunderstorms - Don't Flirt 'em, Skirt 'em"
Note about the photo... Another flight instructor and I were waiting out the weather. He went outside to take another look at the storm and then yelled, "Ian! Come quick! Bring your camera!" I got outside and confirmed what he was looking at was a funnel cloud. I got a few pictures before it dissipated.
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