Editorials

May 7, 2014

Turbulence: Everything You Need to Know, Part 1

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By: Patrick Smith
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Pilots will always stay clear of cumulonimbus buildups like these.
Pilots will always stay clear of cumulonimbus buildups like these.
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 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.

So that I’m not accused of sugar-coating, 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 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 as well. People get so accustomed to the sign dinging on and off, seemingly without reason, that they ignore it altogether.

 

This article was originally published on AskThePilot.com  and is used here with the author’s permission. Patrick Smith is an airline pilot, author, and host of AskThePilot.com. His new book is COCKPIT CONFIDENTIAL: Everything You Need to Know About Air Travel. He lives in Somerville, Massachusetts. 

Part 2 of this article will be published on NYCA.com next Wednesday. 

 

 


  • Mabel

    Most turbulence feels like a bumpy road. I’ve not experienced the severe type (thank goodness). It’s reassuring to know it’s not that worrisome from an aviation standpoint.

  • Keith Carter

    BOAC Flight 911

  • Pierre153

    If all this turbulence doesn’t affect a plane, why did the flight over the Rockaways after 9/11 supposedly lose its tail because of turbulence from the flight in front of it, as reported?

    • http://nycaviation.com nycaviation

      That crash was caused not by the wake turbulence itself, but by what is debated to be either or both poor recovery inputs by the First Officer, and a design flaw in the Airbus A300 (which no longer flies passengers in the United States).

  • Will Maynard

    Great article, especially for those who don’t fly often.

    That being said, once you’ve experienced extreme turbulence, it’s difficult to remember that it most likely won’t bring the plane down. I used to fly as a passenger ~250 hours per year; the cabin was practically a second home to me.

    During a quick, 1-hour flight, in which a nasty patch of CAT shook us like a baby with a rattle for 10 minutes, I developed a minor fear of flying. After being slammed into the side of the plane, the passenger to my right, the seat in front of me, luggage avalanches, and feeling like we were in freefall every few seconds, I don’t think there’s any way I can be totally at peace in a plane now. 10 minutes of that felt like a month.

    Of course, I’ll never be deterred from flying where I need / want to go. I still travel regularly – I just sweat and my heart rate jumps significantly at the slightest bump now. I figure that life’s too short to avoid seeing the world, after all.