Calm Surface Winds Can Be Deceiving

This time of year, beware of wind shear.

A few weeks ago, I was sitting in my car alongside the north end of Teterboro Airport in New Jersey listening in on the tower frequency. Several of the pilots on the ILS to Runway 19 were asking for wind checks, and the tower's answer was consistent -- 240 degrees at seven knots. No big deal, right? But a look at the airliners passing overhead at around 3,000 feet on final for Runway 22 at Newark gave a clue as to why there was so much concern about the surface wind. The big boys were crabbed at what looked to me like a 15- to 20-degree angle to the right of their actual track. Maybe more. Then I clicked on my Garmin 396 to check winds aloft on XM Weather. Sure enough, even as low as 3,000 feet, the early winter wind was howling out of the northwest at better than 50 knots. With almost calm conditions on the ground, that sounded like a recipe for significant wind shear. But strangely, this time, no one at Teterboro was reporting any such excitement on final.

It is not unusual at Teterboro to hear the tower controller asking every third arrival or so whether they experienced wind shear on final. The reported gain or loss of airspeed is then announced to each arriving flight, as in: "15- to 20-knot loss of airspeed reported by all types at 1,200 feet." When I asked a Falcon jet pilot based at Teterboro what he expects this time of year when wind conditions vary to that extreme, he said, "You've really got to be concerned about wind shear."

But sometimes, the decrease in wind vector is not abrupt; rather it's a steep but steady decrease all the way down. I once flew in the jumpseat of one of Corning Glass' Dornier 328s on final to Newark when the wind was blowing at 50 knots or better at 3,000, but virtually calm at the surface. The pilot was sweating the approach, and it required full concentration to manage the constantly changing heading, rudder, airspeed and power as the crosswind component steadily decreased all the way down final.

That experience also reminded me of my early training in stub-wing Grummans at TewMac Airport in Massachusetts (closed in 1997, sad to say). Its 2,600-foot main Runway 3-21 had a stand of tall pines that shielded the northern two-thirds or so of its length from the prevailing northwest wind -- especially strong in winter. When landing on Runway 3, the effect was of requiring a significant crab to the left all the way down final to compensate for the stiff crosswind -- which would then virtually disappear just as the time came to flare for landing. That, added to TewMac's notoriously narrow runway, made for interesting arrivals, especially in icy conditions. Even with nosewheel airplanes, we learned early on what the rudder was for.

So when your onboard weather is showing strong winds aloft, but the tower or ATIS is reporting calm conditions at the surface, be prepared for a steep drop-off somewhere in between. It might come all at once in a strong wind shear, or it might decrease steadily all the way down. Either way, it's a good time to be on your toes -- literally.