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?

Topics: Air Travel, Ask the Pilot, Business,

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.

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As a rule, bigger planes whip up bigger, more virulent wakes. And as you’d expect, smaller planes are considerably more vulnerable should they run into one. For a wide-body jetliner, wake encounters are rarely serious; for those like our 19-seater, they are a known and carefully avoided hazard. This is one of the reasons certain radio call signs include the suffix “heavy” (as listeners to United Airlines’ Channel 9 audio feature will recognize). It’s a reminder for crews and controllers alike that said flight requires a wider than normal buffer zone. During approaches, non-heavies following a heavy require at least five, and sometimes as many as six miles, of separation. On takeoff, they need two minutes of wait time at the end of the runway. (In the United States, “heavies” are those planes whose maximum takeoff weights exceed 255,000 pounds. Outside the U.S., the International Civil Aviation Organization has its own, marginally heavier “heavy” designation, though the term is not used over the radio.)

When landing behind a heavy jet, pilots of smaller aircraft will, whenever practical, remain slightly above the standard glide path toward the runway — a steeper approach. This keeps you above any vortexes as they sink. It’s nothing drastic, and won’t be noticeable to passengers, but your descent is a degree or two sharper than usual: “half a dot high,” in pilot parlance, referring to the instrument markings used to monitor glide angle during approaches. Another trick is to use the wind. Gusts and choppy air will break up vortexes, or otherwise move them to one side.

Landing in Philadelphia, we had our half a dot. In fact it may have been a full dot. We were especially attuned because we knew the preceding traffic was a Boeing 757. Technically, the twin-jet 757 isn’t a heavy. A midsize jet, it’s barely one-third the heft of a 747, 777 or A340. But what it lacks in weight it makes up for with a nasty aerodynamic quirk, producing an outsize wake rivaling or exceeding that of its larger siblings. A 1990 study by the National Oceanic and Atmospheric Administration pronounced the 757′s vortexes to be the most powerful ever recorded. Does this look like something you’d wanna tangle with — the plane like a storm unto itself? Check out the spin from that left-wing core.

In 1993, a business jet carrying the president of In-N-Out Burger, a popular fast-food chain, crashed at Santa Ana, Calif., killing the executive and four others. They’d been following a 757. A year earlier, in Billings, Mont., a Cessna Citation jet was rolled inverted after the pilots, on a visual approach, failed to maintain adequate distance from a 757 ahead. All eight occupants of the Citation were killed. In response to these and numerous other incidents, both Federal Aviation Administration and ICAO rules call for increased separation limits between 757s and other aircraft. Basically, “heavy” minimums apply, meaning, in most cases, an extra mile of clearance on approach.

Not to be outdone, the new Airbus A380, the largest and heaviest (and ugliest) commercial plane ever built, will require an even greater parcel of sky. For aircraft stuck behind this beastly behemoth, a recently completed three-year study recommends up to 10 miles of separation for landings, and three-minute runway hold times for takeoff. In airspace choreography, greater spacing means longer delays, and potential reductions in airport capacity. This was bad, if not unsurprising, news for the A380 program, already problem plagued and well behind schedule.

Many modern jetliners are fitted with winglets — those small upturned fins out at the tips. These devices increase aerodynamic efficiency — meaning, in turn, economy and range — and one of the ways they do so is by mitigating the severity of wingtip vortexes. Generally, a winglet-equipped plane will produce a more docile wake than a similarly sized plane without them. Although the 757 is no longer in production, winglets are available as a retrofit. The package costs about $250,000 per airplane, not including downtime, and reduces fuel consumption by as much as 5 percent. Reductions in the severity of the plane’s wake are tougher to quantify, but certainly welcome.

Back in Philadelphia that day, we thought we’d done everything right. Plying the busy Northeast corridor, avoiding wakes from 757s was routine. So, what happened?

Beats me. Perhaps the Boeing too, for reasons unknown, had come in slightly high. And it was one of those windless, dead-air kind of days, which would have allowed any vortexes to linger. In any event, nobody was hurt, and not a single barf bag was removed from its pouch. It was, you could say, just one of those things.

Our experience was highly unusual in its severity, but typical in that it lasted merely a few seconds, leaving everybody unscathed. I haven’t raised this topic to scare you, and of the accidents in which wakes are listed as the primary cause, the vast majority have involved small, noncommercial aircraft. One exception was a crash in 1972, when a DC-9 got too close to a Lockheed L-1011. But wide-body jets like the Lockheed were new at the time, and the dangers of wingtip vortexes weren’t fully understood.

“That’s easy for us to say,” says Tom Bunn, former airline captain and creator of the SOAR fearful-flier program. “Any discussion of wakes, however, is liable to terrorize nervous passengers, because it’s hard for them to hold these things in context. I like to keep wake turbulence outside the normal turbulence discussion. It is not a natural phenomenon, it’s man-made.”

Five years ago this month, wake turbulence played a role in the crash of American Airlines Flight 587 in New York City, but exactly how is easily misconstrued. Moments after takeoff from JFK airport, the flight was struck by vortexes from a Japan Airlines 747 ahead. The crew, the first officer in particular, then overreacted, repeatedly commanding full, back-and-forth deflection of the rudder, overstressing the tail and causing it to separate. The overreaction itself was traceable, in part, to design of the Airbus A300′s rudder system, engineered in such a way that pilots could inadvertently summon violent deflections with relatively light inputs. There may also have been a preexisting stress crack in the tail’s composite skeleton — the result of a powerful turbulence encounter years earlier — though this has never been proved. A300 crews have since been retrained.

Today, pilots and air traffic controllers are well versed in the hows and how-nots of wake turbulence. Existing protocols, it would seem, are effective ones, and statistics bear this out. Over the past two decades, the world’s commercial aircraft fleet has doubled, with small jets and turboprops — the types most vulnerable to upset — accounting for almost a third of the total. Yet there has been no corresponding uptick in wake-related accidents. That’s the beauty in a discussion like this: We can marvel at the fury of those horizontal tornadoes, knowing how easy they are to avoid. Knowledge is power, for crews and for anxious passengers.

One final and humorous footnote to what happened in Philadelphia: To make the incident even more memorable than it already was, the crew of that 757 got to watch the whole thing. As we did our little dance on final approach, they had already turned clear of the runway and were taxiing inbound along the parallel taxiway. In other words, the pilots were looking directly at us. They had to have witnessed our hapless fluttering, and probably knew what caused it. Whether they felt bad, or were laughing, is something I’ll never know.

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Do you have questions for Salon’s aviation expert? Send them to AskThePilot and look for answers in a future column.

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