First, let’s talk about the squally conditions that accompany a line of… — Page 260, Lesson 225

I want to walk you through the hazards associated with thunderstorms, starting with the broader effects and then narrowing into a specific phenomenon called windshear. First, let’s talk about the squally conditions that accompany a line of thunderstorms. When you have a line of thunderstorms, the effects—including severe turbulence and strong, gusty winds—can be felt as much as 40 miles ahead of the actual storm. That’s a significant distance. For that reason, take-offs and landings in these circumstances are hazardous. You don’t want to be near the runway when that kind of weather is approaching. Now, severe turbulence isn’t just inside the cloud. It can also be encountered several thousand feet above the tops of active thunderstorm clouds. This is especially true when the wind speed at that altitude is high—specifically 100 knots or more. So even if you think you’re well above the storm, you can still get hit by severe turbulence. The advice here is clear: it is advisable to avoid flying in these areas, and in particular, do not climb through or near them. Moving on to lightning. A thunderstorm cell must be well developed before lightning first occurs. However, lightning may continue even in the decaying cell—that is, as the storm is dying out. So you cannot rely on lightning as a reliable guide to the degree of turbulence inside a cloud. Just because you see lightning doesn’t mean the turbulence is at its peak, and just because the lightning stops doesn’t mean the turbulence is gone. Now, let’s look at the accident history. Accidents involving loss of control of the aircraft have been caused by flying in and around thunderstorms. In some instances, there was structural failure that probably occurred during the attempt to regain control. So the chain of events is often: you encounter turbulence, you lose control, you try to recover, and the recovery itself can cause structural failure. This brings us to stress requirements. Modern transport aircraft are designed with stress limits set at a level that experience has shown will rarely be reached. But flight research has indicated that in the extreme conditions that may exist within thunderstorms, abnormal pilot-induced loads are added to already high gust-loads. When you add those two together—the natural gust loads from the storm plus the loads you create by maneuvering—the combined stress may exceed the design limits. Here’s the key point about flying technique. In some instances, the correct flying technique is difficult to achieve. The indications are that loss of control, which may follow the use of incorrect techniques, is a more serious hazard than the risk of structural failure due directly to an encounter with turbulence. Why? Because recovery manoeuvres are likely to subject the aircraft to great stresses that may lead to structural failure or serious deformation. So the primary danger isn’t the turbulence itself breaking the airplane—it’s the pilot’s response to the turbulence that can cause the damage. Now, let’s move into a specific hazard: thunderstorm windshear. Accidents have occurred during the take-off, initial climb, and final approach phases of flight. These accidents were probably due in part, if not entirely, to the effect of a rapid variation in wind velocity—and that rapid variation is what we call windshear. For further information, there is a reference to AIC 84/2008 (Pink 150) – ‘Low Level Windshear’, which is a separate document you can consult. So to summarize what we’ve covered: thunderstorms produce hazards up to 40 miles ahead, severe turbulence above the cloud tops especially with winds of 100 knots or more, lightning is not a reliable turbulence indicator, loss of control is a greater risk than direct structural failure, and windshear during take-off, climb, and approach has caused accidents.