Ask the pilot

There we were, hanging sideways in the sky just a few feet from death. Never before had I seen the ground from such a terrifying perspective. What happened?

By Patrick Smith

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Nov. 10, 2006 | There's nothing exceptional about the approach to runway 27R at Philadelphia International Airport. Least of all on a clear, calm afternoon like the one I remember in 1994. Ours was a long, lazy, straight-in course from the east. We'd come from Boston, our 19-passenger turboprop 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. Our checklists were complete, and everything was perfectly normal.

At approximately 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.

"Get it!" I called out, reaching for the wheel. It was the first officer's leg to fly, but suddenly there were four hands at the yoke, turning it to the left as far as it would go. Even with full opposite aileron -- something seldom used in normal commercial flying -- the ship kept rolling to the right.

A feeling of helplessness, of lack of control, is part and parcel of nervous-flier psychology -- the fear that comes from being at the mercy of two unseen strangers, who you hope are competent, qualified and sober. It's an especially bad day when the pilots are experiencing the same uncertainty. There we were, hanging sideways in the sky just a few feet from death. Everything in our power was telling the plane to go left, and it insisted on going right.

How far did it go? Sixty degrees, or thereabouts. To get a sense of how drastic that is, normal banks are around 15 degrees, and will rarely exceed 20 degrees. Never before had I seen the ground from such a perspective, and it was positively terrifying. We even threw in differential power, instinctively bringing up the right engine to overcome the twist.

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. We evened the asymmetrical power and, just like that, it was over.

"Go around," is what I said next, instructing the copilot to abort the landing, which at this point wasn't a landing at all, and get the hell out of there. He'd already commenced the maneuver on his own. We set target torque and began a climb; we retracted the gear, brought up the flaps, and around we went for another circuit, this time finishing off with a smooth-as-silk touchdown.

What happened -- and we fully knew it -- is that we'd been slammed by the preceding aircraft's wake.

Chances are you've heard the term "wake turbulence" before. If you can picture the cleaved roil of water that trails behind a boat or ship, you've got the right idea. With aircraft, however, wake effect is exacerbated by a pair of vortexes that spin from the wingtips. At the wings' outermost extremities, the higher-pressure air beneath is drawn toward the lower-pressure air on top, resulting in a circular flow that trails behind the aircraft like a pronged pair of sideways tornadoes.

The vortexes are normally invisible, but are occasionally revealed when passing through mist or cloud, as seen in this sensational image. They are most pronounced when a plane is heavy and slow -- that is, when the wing is working hardest to produce lift. Thus, prime time for an encounter is during approach or departure. As the vortexes rotate -- at speeds that can top 300 feet per second -- they begin to diverge, and they sink. If you live near an airport, stake out a spot close to a runway and listen carefully as the planes pass overhead; you can often hear the vortexes' whiplike percussions as they drift toward the ground.

Here's another masterly shot. Those ghostlike whirls show the vortex rotation. Get a wing stuck in that blender, and it's easy to visualize what might happen. The long white streams of condensation show the vortex cores. Those core streams (not to be mistaken for contrails -- the long white patterns left by planes at high altitudes) are a common sight when flying in moisture-laden air, and are sometimes mistaken for jettisoned fuel.

Next page: "Any discussion of wakes is liable to terrorize nervous passengers"