Cockpit Confidential (7 page)

Turbulence: spiller of coffee, jostler of luggage, filler of barf bags, rattler of nerves. But is it a crasher of planes? Judging by the reactions of many airline passengers, one would assume so; turbulence is far and away the number-one concern of anxious passengers. Intuitively, this makes sense. Everybody who steps on a plane is uneasy on some level, and there's no more poignant reminder of flying's innate precariousness than a good walloping at 37,000 feet. It's easy to picture the airplane as a helpless dinghy in a stormy sea. Boats are occasionally swamped, capsized, or dashed into reefs by swells, so the same must hold true for airplanes. Everything about it seems dangerous.

Except that, in all but the rarest circumstances, it's not. For all intents and purposes, a plane cannot be flipped upside-down, thrown into a tailspin, or otherwise flung from the sky by even the mightiest gust or air pocket. Conditions might be annoying and uncomfortable, but the plane is not going to crash. Turbulence is an aggravating nuisance for everybody, including the crew, but it's also, for lack of a better term, normal. From a pilot's perspective it is ordinarily seen as a convenience issue, not a safety issue. When a flight changes altitude in search of smoother conditions, this is by and large in the interest of comfort. The pilots aren't worried about the wings falling off; they're trying to keep their customers relaxed and everybody's coffee where it belongs. Planes themselves are engineered to take a remarkable amount of punishment, and they have to meet stress limits for both positive and negative G-loads. The level of turbulence required to dislodge an engine or bend a wing spar is something even the most frequent flyer—or pilot for that matter—won't experience in a lifetime of traveling.

Altitude, bank, and pitch will change only slightly during turbulence—in the cockpit we see just a twitch on the altimeter—and inherent in the design of airliners is a trait known to pilots as “positive stability.” Should the aircraft be shoved from its position in space, its nature is to return there, on its own. I remember one night, headed to Europe, hitting some unusually rough air about halfway across the Atlantic. It was the kind of turbulence people tell their friends about. It came out of nowhere and lasted several minutes, and was bad enough to knock over carts in the galleys. During the worst of it, to the sound of crashing plates, I recalled an email. A reader had asked me about the displacement of altitude during times like this. How many feet is the plane actually moving up or down, and side to side? I kept a close watch on the altimeter. Fewer than forty feet, either way, is what I saw. Ten or twenty feet, if that, most of the time. Any change in heading—that is, the direction our nose was pointed—was all but undetectable. I imagine some passengers saw it differently, overestimating the roughness by orders of magnitude. “We dropped like 3,000 feet in two seconds!”

At times like this, pilots will slow to a designated “turbulence penetration speed” to ensure high-speed buffet protection (don't ask) and prevent damage to the airframe. This speed is close to normal cruising speed, however, so you probably won't notice the deceleration from your seat. We can also request higher or lower altitudes or ask for a revised routing. You're liable to imagine the pilots in a sweaty lather: the captain barking orders, hands tight on the wheel as the ship lists from one side to another. Nothing could be further from the truth. The crew is not wrestling with the beast so much as merely riding things out. Indeed, one of the worst things a pilot could do during strong turbulence is try to fight it. Some autopilots have a special mode for these situations. Rather than increasing the number of corrective inputs, it does the opposite, desensitizing the system.

Up front, you can imagine a conversation going like this:

 

Pilot 1: “Well, why don't we slow it down?” [
dials a reduced Mach value into the speed control selector
]

Pilot 2: “Ah, man, this is spilling my orange juice all down inside this cup holder.”

Pilot 1: “Let's see if we can get any new reports from those guys up ahead.” [
reaches for the microphone and double-checks the frequency
]

Pilot 2: “Do you have any napkins over there?”

 

There will also be an announcement made to the passengers and a call to the cabin crew to make sure they are belted in. Pilots often request that flight attendants remain in their seats if things look menacing up ahead.

Predicting the where, when, and how much of turbulence is more of an art than a science. We take our cues from weather charts, radar returns, and, most useful of all, real-time reports from other aircraft. Some meteorological indicators are more reliable than others. For example, those burbling, cotton-ball cumulus clouds—particularly the anvil-topped variety that occur in conjunction with thunderstorms—are always a lumpy encounter. Flights over mountain ranges and through certain frontal boundaries will also get the cabin bells dinging, as will transiting a jet stream boundary. But every now and then it's totally unforeseen. When we hit those bumps on the way to Europe that night, what info we had told us not to expect anything worse than mild chop. Later, in an area where stronger turbulence had been forecast, it was perfectly smooth. You just don't know.

When we pass on reports to other crews, turbulence is graded from “light” to “extreme.” The worst encounters entail a postflight inspection by maintenance staff. There are definitions for each degree, but in practice the grades are awarded subjectively.

I've never been through an extreme, but I've had my share of moderates and a sprinkling of severes.

One of those severes took place in July 1992, when I was captain on a fifteen-passenger turboprop. It was, of all flights, a twentyfive-minute run from Boston to Portland, Maine. It had been a hot day, and by early evening, a forest of tightly packed cumulus towers stretched across eastern New England. The formations were short—about 8,000 feet at the tops, and deceptively pretty to look at. As the sun fell, it became one of the most picturesque skyscapes I've ever seen, with buildups in every direction forming a horizon-wide garden of pink coral columns. They were beautiful and, it turned out, quite violent—little volcanoes spewing out invisible updrafts. The pummeling came on with a vengeance until it felt like being stuck in an upside-down avalanche. Even with my shoulder harness pulled snug, I remember holding up a hand to brace myself, afraid my head might hit the ceiling. Minutes later, we landed safely in Portland. No damage, no injuries.

So that I'm not accused of sugarcoating, I concede that powerful turbulence has, on occasion, resulted in damage to aircraft and injury to their occupants. With respect to the latter, these are typically people who fell or were thrown about because they weren't belted in. About sixty people, two-thirds of them flight attendants, are injured by turbulence annually in the United States. That works out to about twenty passengers. Twenty out of the 800 million or so who fly each year in this country.

Anecdotal evidence suggests that turbulence is becoming more prevalent as a byproduct of climate change. Turbulence is a symptom of the weather from which it spawns, and it stands to reason that as global warming intensifies certain patterns, experiences like the one I had over Maine will become more common.

Because turbulence is so unpredictable, I am known to provide annoying, noncommittal answers when asked how best to avoid it.

“Is it better to fly at night than during the day?” Sometimes.

“Should I avoid routes that traverse the Rockies or the Alps?” Hard to say.

“Are small planes more susceptible than larger ones?” It depends.

“They're calling for gusty winds tomorrow. Will it be rough?” Probably, but who knows.

“Where should I sit, in the front of the plane or in the back?”

Ah, now that one I can work with.

While it doesn't make a whole lot of difference, the smoothest place to sit is over the wings, nearest to the plane's centers of lift and gravity. The roughest spot is usually the far aft—the rearmost rows closest to the tail.

As many travelers already know, flight crews in the United States tend to be a lot more twitchy with the seat belt sign than those in other countries. We keep the sign on longer after takeoff, even when the air is smooth, and will switch it on again at the slightest jolt or burble. In some respects, this is another example of American overprotectiveness, but there are legitimate liability concerns. The last thing a captain wants is the FAA breathing down his neck for not having the sign on when somebody breaks an ankle and sues. Unfortunately, there's a cry-wolf aspect to this; people get so accustomed to the sign dinging on and off, seemingly without reason, that they ignore it altogether.

If you can picture the cleaved roil of water that trails behind a boat or ship, you've got the right idea. With aircraft, this effect is exacerbated by a pair of vortices 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 tight, circular flow that trails behind the aircraft like a pronged pair of sideways tornadoes. The vortices are most pronounced when a plane is slow and the wings are working hardest to produce lift. Thus, prime time for encountering them is during approach or departure. As they rotate—at speeds that can top 300 feet per second—they begin to diverge and 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 vortices' whip-like percussions as they drift toward the ground.

As a rule, bigger planes brew up bigger, most virulent wakes, and smaller planes are more vulnerable should they run into one. The worst offender is the Boeing 757. A mid-sized jet, the 757 isn't nearly the size of a 747 or 777, but thanks to a nasty aerodynamic quirk it produces an outsized wake that, according to one study, is the most powerful of any airplane.

To avoid wake upsets, air traffic controllers are required to put extra spacing between large and small planes. For pilots, one technique is to slightly alter the approach or climb gradient, remaining
above
any vortices as they sink. Another trick is to use the wind. Gusts and choppy air will break up vortices or otherwise move them 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.

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.

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.