Turbulence Targets

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

The airplane probably can handle it, but only if you slow down, keep the wings level and accept altitude excursions.

There I was, sliding down from my cruising altitude toward my VFR destination, still 30 or so miles out. It had been a smooth ride, and Otto was following a heading and descending at the selected 400 fpm. I had let the power come up during the descent, along with airspeed. The big Continental in front of me was rumbling along at about 25 squared, still leaned for cruise altitude, and airspeed was well into the indicator’s yellow arc, That’s when it got bumpy. Too bumpy.

The sunny Florida winter day—not unlike spring elsewhere in North America—was great for warming my bones, but also was heating the air and producing updrafts. Looking outside, there was no real clue as to where the bumps would be—in every direction was clear blue sky. I punched off the autopilot, gently added back pressure to level the nose and pulled off several inches of manifold pressure, letting airspeed drop well below the airplane’s published design maneuvering speed, or VA, because I was light. I would be home a few minutes later, thanks to my slower groundspeed, but I’d get home. And I’d be able to use the airplane again.

It’s Everywhere

Although we usually prefer smooth air, turbulence is everywhere, and often unavoidable. It’s present in the usual places: downwind of mountain ranges or obstructions near a runway, when following a large airplane, in and near thunderstorms, where two competing airmasses collide and, as I found out that day over Florida, in clear air at any altitude. Although it can be easy to predict where turbulence can be found, it’s almost impossible to predict its absence.

Turbulence can mean a rough, uncomfortable ride for all aircraft occupants, of course, and injuries in severe cases (usually involving airliners with people up and walking around). While en route, it can translate into greater fatigue for the crew, and creates operational considerations—like gusty, unpredictable conditions for landings and takeoffs—when encountered close to the ground. For a variety of reasons—safety, comfort, efficiency—it should be avoided. Except we usually can’t avoid turbulence. I say “usually,” because there are times and places we can avoid it.

Just east of Albuquerque, N.M., is a well-defined mountain ridge running north/south. If the winds are blowing across that ridge and you cross them within a few thousand feet of their peaks, there will be some enthusiastic bumps, their energy depending on the wind’s strength and your altitude. Meanwhile, those white puffies dotting the flatlands in summer point out two things: Underneath them is rising air, which forms the clouds as it cools and moisture condenses. Fly through or underneath one of them and it’ll be bumpy. Climb above their bases, however, and you’ll usually get a smooth ride for your trouble. Meanwhile, large bodies of water tend to absorb heat energy during the day, by contrast to the land surrounding them. In summer, the land tends to reflect the sun’s energy, creating updrafts. Over the water, however, you might see a downdraft, or simply encounter fewer updrafts during the day. At night, the warmer water can produce its own turbulence.

It’s a V-G diagram depicting an airplane’s flight envelope. From it, we can determine the g-loading the represented airplane will experience when accelerated beyond 1G at various airspeeds. For example, the airplane depicted may suffer structural damage at 200 mph if it encounters conditions leading to a 4G loading. We’re reminded one of the important things to remember about potential damage from G-loading is the airplane’s airspeed. That same 4G loading will result in a stall at 135 mph, instead of airframe damage. One of the takeaways, then, is we should slow down in turbulence. Duh.

The point is many turbulence encounters shouldn’t come as a surprise. Terrain and weather often conspire to create bumps; where and when you’ll encounter them can be obvious, if you know what to look for. All that’s well and good, but how to handle turbulence when it can’t be avoided?

Speed To Fly

The main thing about dealing with turbulence usually is slowing down. Letting down into my Florida destination, I wasn’t thinking about bumps in the clear skies that time of year, and let the speed creep up into the yellow arc. That’s a big no-no when flying turbulence: You never want to be in the airspeed indicator’s yellow arc—if you even have one—in anything except smooth air.

Most airframe manufacturers publish speeds to use in turbulence or rough air. You’ll usually find them in the POH/AFM’s front portion, perhaps in a section dedicated to and defining the aircraft’s various recommended speeds. There usually is a placard on or near the instrument panel listing this airspeed. Typically, the speed given will be VA, design maneuvering speed, at gross weight, which the FAA’s Airplane Flying Handbook, FAA-H-8083-3B, defines as “the maximum speed where full, abrupt control movement can be used without overstressing the airframe.”

A subsequent definition, courtesy of FAA Special Airworthiness Information Bulletin (SAIB) CE-11-17, published in 2011, states, “design maneuvering speed (VA) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane.” (emphasis in the original) 

Another speed is VO, operating maneuvering speed. It’s similar to VA, in that it’s also weight-dependent, but the FAA defines VO as “the maximum speed where, at any given weight, the pilot may apply full control excursion without exceeding the design limit load factor.” “Operating” is used to maintain a distinction from the “design maneuvering speed,” VA. It’s published for newer Part 23 airplanes, like the Cirrus SR 20, and some Cessnas, including the 162 Skycatcher. Cirrus defines it as “the maximum speed at which application of full control movement will not overstress the airplane.”

Maintaining Control

Okay, so there’s no choice: You’re about to penetrate an area of turbulence, perhaps downwind of a mountain peak, or in or near convective activity. What to do? What are the targets you should aim for? 

Best viewed from an aircraft at a substantial distance.

Make sure wing flaps are retracted and remain that way. Extending landing gear may or may not be an option; if you have the choice, putting it down likely will increase drag and limit any tendency to build speed. Of course, we need to set power to what little is necessary to maintain at or below our targeted turbulence penetration speed, VA, VO, or V-whatever-the-manufacturer-recommends, corrected for weight, in level flight. That helps solve the speed issue.

The main things we want to accomplish here is keep the wings level at or below our target airspeed. Happily, nailing one can help maintain the other: If we keep the wings level, the airplane can’t tuck into a steep spiral and build speed. The wings-level task can be assisted by a compliant autopilot, but many are clearly placarded against use in severe or extreme turbulence. Yes, there’s an official definition of that kind of turbulence, but only the pilot can decide if conditions meet it.

Pitch control is the remaining big challenge in turbulence. The natural instinct is to chase the airplane’s turbulence-induced altitude excursions—pull off power in climbs, and add it in descents—but that’s a bad idea. Do your best to maintain a constant attitude, not altitude, perhaps around five degrees nose-up. Accept altitude excursions. If you’re IFR, tell ATC you need a block altitude. If they don’t give it to you, tell them you’re off your altitude anyway. They’ll move people around, even though if it’s that rough, all the smart ones are somewhere else. We would tend to stay off the rudder pedals, using our feet to correct gross excursions, but our primary concern is keeping the rudder centered.

The Softer Kind

Those are some of the keys to dealing with severe and extreme turbulence. But the moderate and light chop kinds deserve some attention, too. Keep in mind, however, that the average run-of-the-mill light chop isn’t that much of a deal, structurally. One of my “normal” landings can produce more G-force than the average summer-afternoon turbulence, but the latter is much less comfortable.

In other words, it’s usually a good idea to get out of light or moderate chop for comfort reasons, but not for structural ones. Yes, repeated exposure to turbulence—hundreds or thousands of hours—can fatigue an airframe, but 30 minutes of light chop won’t hurt anything as long as airspeed is kept under control. Flying in this kind of turbulence can mean a slow slog. So don’t.

Find a different altitude, with smoother air. Get above those white puffies, or top the haze layer. The only time you’ll regret climbing to a smoother altitude is coming back down through the bumps at the destination, when you’ll be tempted to keep your speed up. Bad idea. Pick a different time of day—mornings can be smooth as glass; so can evening and night flying. You also can choose a different route: you don’t always have to go through Albuquerque.

This article originally appeared in the March 2015 issue of Aviation Safety magazine.

For more great content like this, subscribe to Aviation Safety!