Cockpit Confidential by Patrick Smith Page B

to one side. Winglets ( see winglets ) also are a factor. One of the ways these devices increase aerodynamic efficiency is by mitigating the severity of wingtip vortices. Thus a winglet-equipped plane tends to produce a more docile wake than a similarly sized plane without them.
    Despite all the safeguards, at one time or another, every pilot has had a run-in with wake, be it the short bump-and-roll of a dying vortex or a full-force wrestling match. Such an encounter might last only a few seconds, but they can be memorable. For me, it happened in Philadelphia in 1994.
    Ours was a long, lazy, straight-in approach to runway 27R from the east, our nineteen-seater packed to the gills. Traffic was light, the radio mostly quiet. At five miles out, we were cleared to land. The traffic we’d been following, a 757, had already cleared the runway and was taxiing toward the terminal. We’d been given our extra ATC spacing buffer, and just to be safe, we were keeping a tad high on the glide path. Our checklists were complete, and everything was normal.
    At around 200 feet, only seconds from touchdown, with the approach light stanchions below and the fat white stripes of the threshold just ahead, came a quick and unusual nudge—as if we’d struck a pothole. Then, less than a second later, came the rest of it. Almost instantaneously, our 16,000-pound aircraft was up on one wing, in a 45-degree right bank.
    It was the first officer’s leg to fly, but suddenly there were four hands on the yokes, turning to the left as hard as we could. Even with full opposite aileron—something never used in normal commercial flying—the ship kept rolling to the right. There we were, hanging sideways in the sky; everything in our power was telling the plane to go one way, and it insisted on going the other. A feeling of helplessness, of lack of control, is part and parcel of nervous flyer psychology. It’s an especially bad day when the pilots are experiencing the same uncertainty.
    Then, as suddenly as it started, the madness stopped. In less than five seconds, before either of us could utter so much as an expletive, the plane came to its senses and rolled level.
Sometimes when a plane is landing, I see a long trail of mist coming from the wingtip. What is this?
    As air flows around a wing at high velocity, its temperature and pressure change. If humidity levels are high enough, this causes the cores of the wingtip vortices described in the previous question to condense and become visible, writhing behind the plane like gray, vaporous snakes. Moisture will condense around other spots too, such as the flap fairings and engine attachment pylons. You’ll witness what appears to be a stream of white smoke pouring from the top of an engine during takeoff. This is water vapor caused by invisible currents around the pylon. Other times, the area just above the surface of the wing will suddenly flash into a white puff of localized cloud. Again, this is condensation brought on by the right combo of humidity, temperature, and pressure.
What is windshear?
    One of those buzzwords that scare the crap out of people, winds-hear is a sudden change in the direction and/or velocity of the wind. Although garden-variety shears are extremely common and almost never dangerous, encountering a powerful shear during takeoff or landing, when airplanes operate very close to their minimum allowable speeds, can be dangerous. Remember that a plane’s airspeed takes into account any existing headwind. If that velocity suddenly disappears or shifts to another direction, those knots are lost. Shears can happen vertically, horizontally, or both, as in the case of a micro-burst preceding a thunderstorm. Microbursts are intense, localized, downward-flowing columns of air spawned by storm fronts. As the air mass descends, it disperses outward in different directions.
    Windshear got a lot of press in the 1970s and 1980s, when relatively little was known