Thunderstorm Penetration

Author: Karsten Shein

So, the worst has happened and in your focus on maintaining your approach vector, you didn’t see the gap between convective cells close ahead of you. Now you are surrounded by lightning, turbulence, and a wall of water cascading across your windscreen. You’ve accidentally penetrated a thunderstorm. Your training reminds you to slow to turbulent air penetration speed, tighten your safety belts, turn on all anti-ice measures, disengage autopilot, and turn up the cockpit lights. After ensuring that you’ve got plenty of air below you, you’ve also given up on trying to maintain a fixed altitude.

But now you have a choice to make. There are two schools of thought on how to extricate yourself. The first is to do a 180 and fly back the way you came. The rationale for this move is that you know you weren’t in the storm a few seconds ago, so it must be the shortest way back out. The second is to keep flying straight and level until you punch through, reasoning that thunderstorms are not that wide and it is best to minimize stress loading. Unfortunately, neither of these is guaranteed to be successful. Thunderstorms are dynamic, ever-evolving phenomena that are often unique and can apply aircraft-destroying forces without warning.

When I learned to fly, I was taught the 180 approach. There are two significant shortcomings with this approach. The first is that when you turn, you reduce lift and increase wing loading. In clear air this may not be a problem, but in a thunderstorm, those changes can bring you much closer to a loss of positive control. All it may take to send you into a spin or buckle your main spar is for your wingtip to encounter a strong shear eddy. The second problem with doing a 180 is that you don’t actually know what is behind you or on either side of you until you can point the radar in those directions. Do you know what part of the storm you entered? Do you know where you are in relationship to the downdraft core or a massive hail shaft? Do you know how conditions have changed behind you in the 15 seconds or so it took you to recognize that you entered the cell? The answer to these questions is, in most cases, no. A turn in any direction could run you right into the worst parts of the storm, and if you are traveling at even a leisurely 120 kt, you’ve already gone half a mile into the storm. Assuming this placed you in the middle of the storm cell, a turn at standard rate would take you one full minute and then you’d need another 15 seconds to fly out, whereas maintaining your course would keep you in the cell for just another 15 seconds.

I lean toward the straight on approach (and so does the FAA in AC 00-24C). The reason is threefold:

• First, by maintaining straight and level flight, you maximize your aircraft’s ability to distribute loading, while at the same time not diminishing lift or thrust. This gives you the best chance at surviving an encounter with the vicious shear that can develop inside the storm.
• Second, if you have onboard radar, you can see in one general direction – forward. By continuing your heading, you have more information about what you may be flying into than if you were to try a different heading. This information can also be helpful in making minor heading adjustments to stay out of the worst echoes or the radar voids that often hold hail shafts. Inside a storm, do not rely on linked surface radar feeds as accurate, as this imagery may be several minutes out of date.
• Lastly, as mentioned before, most of the time you have little idea about the true dimensions of the storm, which way it may be moving, or how it has changed since you entered it. But storm cells are usually no more than a few miles in diameter. Even at a slowed turbulence speed, chances are good that maintaining your current course will extricate you from the storm in less time than it takes to turn and retreat.

If however, your radar is painting a wall of hurt ahead of you, the logical alternative is to go with the 180. There is always a chance that conditions behind you remain benign. Of course, the forces inside the storm may be too severe for any option to be successful, which is why thunderstorm avoidance – even at the cost of delaying or diverting a flight – is really the best option.

Karsten Shein, PhD, is a climatologist with the US National Oceanic and Atmospheric Administration (NOAA), and TrainingPort.net’s Subject Matter Expert for Meteorology.