Turbulence: Everything You Need to Know

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 flyers. 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. So much 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. They can withstand an extreme amount of stress, and the level of turbulence required to dislodge an engine or cause structural damage is something even the most frequent flyer — or pilot for that matter — won’t experience in a lifetime of traveling. Over the whole history of modern commercial aviation, the number of jetliner crashes caused by turbulence, even indirectly, can be counted on one hand.

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. Passengers might feel the plane “plummeting” or “diving” — words the media can’t get enough of — when in fact it’s hardly moving. 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. Fewer than forty feet of altitude change, either way, is what I saw. Ten or twenty feet, if that, most of the time. Any change in heading—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!”

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. We can also request higher or lower altitudes, or ask for a revised routing. If you feel the plane climbing or descending midflight, good chance it’s because of a report from fellow pilots up ahead. Air traffic control can’t always honor such requests, though, in which case you have little choice but to endure it.

In the worst of it, 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; it’s surprisingly hands-off. 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?”
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.”
Pilot 2: “Do you have any napkins over there?”

Avoiding turbulence is a combination of art and science. We take our cues from weather charts, radar returns, and those real-time reports from other aircraft. Larger carriers have their own meteorology departments, and we get periodic updates from the ground. Often, though, it’s as simple as looking out the window. Some indicators are unmistakable, and relatively easy to avoid. 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 weather is always changing, and predicting the where, when, and how much of turbulence can sometimes be a guessing game. 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 smooth. You just never 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 severe.

One of those severes took place in July 1992, when I was captain on a fifteen-passenger turboprop. It was, of all flights, a twenty-five-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.

Now, it would be unwise of me to sugarcoat this too much, and I concede that powerful turbulence has, on occasion, resulted in damage to aircraft and injury to their occupants. Each year worldwide, about a hundred people, half of them flight attendants, are hurt by turbulence seriously enough to require medical attention — head, neck, shoulder and ankle injuries being the most common. That works out to about fifty passengers. Fifty out of the two billion or so who fly each year. And a majority of them are people who fall or are thrown about because they aren’t belted in when they should be.

The bad news is, that number will probably be going up. If it feels like you’ve been seeing more and more news stories about dramatic turbulence encounters, that’s because you have. This is partly the result of the media’s obsession with anything related to flying, the ease with which scary-looking videos can be shared and spread online, and the fact there are more airplanes flying than ever before. But it’s also true that the skies themselves are getting bumpier. Evidence shows that turbulence is becoming stronger and 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 destabilizes weather patterns and intensifies storms, experiences like the one I had over Maine, and the ones that keep popping up in the news, will become more common.

Because turbulence can be 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. In the rearmost rows, closest to the tail, the knocking and swaying is more pronounced.

As many travelers already know, flight crews in the United States tend to be 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.

 

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There’s also something known as “wake turbulence.”  This is a different phenomenon…

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 an 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 — those upturned fins at the end of the wings — 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.

If you’re interested, it’s possible to stake out a spot near and airport and actually hear wingtip vortices as they drift toward the ground:

You need to be very close to a runway — preferably within a half-mile of the end. The strongest vortices are produced on takeoff, but ideally you want to be on the landing side, as the plane will be nearer (i.e. lower) at an equivalent position from the threshold. A calm day is ideal, as wind will dissipate a vortex before it reaches the ground. About 30 seconds after the jet passes overhead you’ll begin to hear a whooshing, crackling and thundering. It’s a menacing sound unlike anything you’ve heard before. See — or hear — for yourself in this footage captured on my iPhone.

It was taken at the Belle Isle Marsh Reservation, a popular birdwatching spot about a half-mile north of runway 22R at Boston’s Logan International Airport. The plane is a 757. Excuse the atrocious video quality, but the sound is acceptable and that’s the important thing. You begin to hear the vortices at time 0:45, and they continue pretty much to the end. Note the incredible gunshot-like noises at 0:58.

Play loud!

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