Inertia’s Surprising Effect on Wind Shear

Microburst

Suppose you are the captain of a Boeing 747 or Airbus A380. You’ve just received a report of wind shear from the Cessna 152 you’re following on final. Should you expect less, similar, or more wind shear?

Before you answer, be sure to consider inertia.

This one harkens back to my last post on inertia and turbulence. Here’s a no-brainer: a 747 is a lot heavier than a Cessna 152. In fact, a Boeing 747-400 at max gross landing weight is Wind Shearabout 390 times heavier than a C-152 at it’s max gross! That means the big airplane has a whole lot more inertia, making it more resistant to changes in motion.

Remember last time, we got technical and found the somewhat obvious result that heavy airplanes give a much smoother ride through turbulence. But because of inertia, the opposite is true for wind shear! That’s right, the flying pilot on an A380 had better be careful when a lighter airplane reports wind shear (and had better taxi carefully when CRJ’s are around, but that’s another story).

I know it seems counterintuitive at first, but think about it in terms of inertia. When an airplane experiences wind shear, the shear is actually an external force acting upon the aircraft to change it’s motion.

A light plane has less inertia, and is much more likely to be pushed by the wind. The light airplane’s motion relative to the ground  actually changes with the wind shear. Consider this in terms of groundspeed: when showing a groundspeed of 60 knots, the airplane will rapidly accelerate to say, 70 knots groundspeed upon entering a 10 knot wind shear. Pilots are likely to experience this as more of a turbulent bump along with a momentary fluctuation in indicated airspeed.

The adventures of inertia manHeavy metal pilots are in for something completely different. These airplanes have so much mass and inertia that they are barely affected by the external force of wind shear. If the airplane is showing a groundspeed of 200 knots, the groundspeed will show little to no change, even with a large shear of say, 20 knots. Our 747 continues to move at the same rate and direction of travel, but experiences a large and rapid change in indicated airspeed.

Left on it’s own, the airplane will gradually accelerate back to it’s equilibrium speed without crew action, so long as it doesn’t lose too much airspeed or altitude. Fortunately for the flying public, modern airline crews have specific wind shear training and an enormous amount of excess thrust at their disposal, greatly reducing the dangers of wind shear to the flying public.

In writing this article, I don’t intend to give the GA pilot false confidence. Wind shear can be serious business for all airplanes and the best strategy is to avoid it. Yes, your airplane’s lack of inertia will dampen wind shear encounters, but only a massive reserve of excess thrust is going to get you out of severe wind shear encounters — and sometimes that’s not enough.


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About the author
Pat Flannigan is a professional pilot and aviation blogger. He has been flying for fifteen years and is currently working as an airline pilot in the United States.

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