All Shook Up

The flight began innocently enough: an early morning departure from Glenwood Springs to Boulder, Colorado, in late spring. The early departure was intended to avoid the usual turbulence that begins as sunlight fills the valleys of the Continental Divide. Brilliant, snow-capped peaks lined the eastbound route across the Rocky Mountains; the first signs of spring were visible in the lower elevations. Winds aloft were mostly light and variable—a very welcome ingredient for safe and smooth passage across the mountains. When I reached Corona Pass, just west of Boulder, my groundspeed steadily increased from a no-wind 150 knots to about 190, then 200, indicating a high-speed westerly jet along the Continental Divide. That was a very bad sign. Winds like this are nearly always accompanied by moderate or worse turbulence on the downwind side of the range. I tugged my seat belt tighter, reduced power, and slowed to maneuvering speed; I was ready—or so I thought.

Predictably, it began to get rough; then it got real rough. I reduced the power to 12 inches of manifold pressure, not wanting to super-cool the big Continental in my Cessna 185. In dire straits, though, it’s best to "shock cool" an engine—possibly shortening its time between overhauls—than to leave the power up, allowing the speed to increase to dangerous levels and significantly shortening your TBO.

The airspeed indicator looked as if whirling dervishes had suddenly possessed it, jumping from 90 kt to almost 180 and back in seconds. In these conditions, it’s important to understand that airspeed indicators are subject to position error because of the wild pressure fluctuations on the pitot tube and static ports. My mountain bike, strapped loosely to the rear-seat rails, began to flail around, and the handlebars struck the side window with a sharp crack. By any definition, that’s moderate turbulence.

The ride quickly deteriorated well past moderate—and there wasn’t much I could do about it. Occasionally, the airplane was out of control, which defines severe turbulence. For the first time in my nearly 35 years of flying, I was concerned that the airplane would not hold together.

"Relax," I told myself. Patience and calmness are two key ingredients to surviving bouts of turbulence like this.

How I wished for speed brakes—or any other drag item that would help me control my airspeed when it would ramp up 50 or 60 kt above maneuvering speed. Without extendable drag, the salvation in this situation is to let it float, or fly loosely. This is not the time for precision, which can break your airplane into pieces and significantly shorten your flying days. Though you must remain in control, the very concept of control changes when you encounter serious turbulence. Control becomes broader; acceptable tolerances looser. The focus is on keeping the wings and tail surfaces from becoming loaded beyond their structural limits.

There is nothing to be gained by precision altitude and heading adherence here; only an average airspeed, altitude, or attitude is attainable anyway. Speed runup, letting gravity have its way by allowing the nose to point earthward, is one of the precursors to in-flight structural failure. To prevent this, you have to move the controls with small, smooth, slow pressures. Altitude above and below is your friend, because it allows you to control airspeed by climbing or descending without imposing additional G loads on the aircraft.

I climbed through 14,000 feet, then 16,000, finally topping at 17,800. Since I was VFR, I let the altitude vary; if IFR, I would have told center (if I could even key the microphone to talk) that we were experiencing severe turbulence—and let him clear the air for my altitude excursions. If you can’t talk to ATC, take some comfort in the big sky theory: The chances of hitting something in the big sky are not that great. In other words, don’t overload the airplane to avoid traffic that probably isn’t there.

If things get real bad, and you can’t control the airplane, mention the word emergency, and you can do anything you want (FAR 91.3[b]). Don’t worry about the dreaded paperwork afterward—there really isn’t much (often none), and it’s better to be around to file a statement. Save the day first; sweat the small stuff later.

We tend to think about shedding wings or tail feathers in severe shear or turbulence near thunderstorms or mountains, but most airplanes can withstand a lot rougher rides than we think. The reason for most accidents involving encounters with turbulence is loss of control—followed almost always by loss of parts.

Most of our literature on turbulence emphasizes attitude. "Fly attitude, not airspeed" we’re told again and again. That’s certainly good advice any time the airplane is marginally controllable in moderate or greater turbulence—but that’s only part of the picture. That rule overlooks something that is more important: overstressing the airplane.

It’s not improper attitude that breaks airplanes; it’s loads—mostly imposed by pilots.

This seemingly subtle distinction is crucial. Maintaining an approximate attitude is important, but it’s more important not to exceed the airplane’s G-load or airspeed limits.

An easy way to visualize this is to make a lightweight cardboard-box "boat" and place it in a fast-moving stream. When you release it, the boat will happily negotiate the rapids and rocks without much difficulty. But if you hold the cardboard boat against the current, it will probably suffer structural failure, much like an airplane could if you hold it against loads during turbulence.

During certification flight testing, manufacturers don’t generally fly in moderate or severe turbulence; they leave the breaking of wings and tails to test rigs, which are designed to determine exactly when a structure breaks. Most Normal category, FAR Part 23 GA aircraft are certified to between three and four positive Gs, with a 150-percent overload factor. In other words, the Cessna 172 with its 3.8-G load limit can withstand at least 5.7 Gs before things bend or come apart (3.8 G times 150 percent). Utility and Aerobatic certification increases these load factors to between four and six positive Gs with the same 150-percent overload capability.

But our airplanes don’t handle negative G loads nearly as well. Most airplanes can withstand only about half of the positive load factor, or 1.5 to 3 negative Gs. This is potentially more serious and requires a deft touch at the controls while in turbulence. The quickest way to reduce the load limit capability in any airplane is to lower the flaps. With flaps extended, most airplanes have only about half of the G-load capability. Thus, don’t ever extend flaps for airspeed control or drag in serious turbulence.

I have flown through moderate or greater turbulence in aerobatic airplanes with G-meters and even installed a G-meter in my airplane for a while. The G-meter shows that a "bad ride" is perhaps only two to three positive Gs. In fact, some of the most uncomfortable rides are a lot less than two Gs. Very rarely does turbulence subject our aircraft to their load limits. Of course, part of the uncomfortable ride is because of the sudden, unexpected momentary loads—and the fear factor that accompanies all turbulence.

Most of our general aviation airplanes don’t have a gust or turbulence penetration speed; they do, however, have maneuvering speeds, and that is what we fly. It’s the only speed we’ve got for these situations. At maneuvering speed, the airplane is best able to withstand G loads because the wing will theoretically stall before it can be loaded to its limit. Higher speeds increase the risk of failure.

By definition, maneuvering speed refers to the movement of controls. That is a clue. We probably can fly through worse turbulence if we don’t induce loads with the controls. It’s the motion—especially incorrect, sudden, or abrupt motion—that breaks airplanes. The wing will mostly unload itself if we just let it. Remember, too, that the maneuvering speed listed in your pilot’s operating handbook is at gross weight; the target speed lowers when your airplane is lighter. (Maneuvering speeds at different weights are published for some newer airplanes.)

There’s some consolation in knowing that most turbulence encounters are relatively short-lived. If you just relax and ride it out for a few minutes, you will probably get through the worst of it quickly. Of course, when you are inside the jaws of the beast, those few short minutes will seem like hours, if not days.

I talked with Charlie Summers, chief pilot for the South Dakota School of Mines and Technology’s Institute of Atmospheric Sciences, about the research work he’s doing on turbulence. He gathers data by flying an armored North American T–28 through thunderstorms. Read that again. They intentionally fly through thunderstorms, looking for data on turbulence. His experience confirms the belief that flying unloaded is the key to successfully negotiating even moderate or severe turbulence. Though doing this risky work for years, he has rarely seen four Gs on his G-meter, even in the belly of the most formidable beasts.

This isn’t to say that we should seek rough air or disregard the warning signs in sigmets, airmets, or pireps when they are issued. There are atmospheric conditions out there that produce severe or even extreme turbulence, defined by the FAA as being able to cause "structural failure." Fly through that and you are likely to die. But there is far more moderate to severe turbulence that is manageable and flyable out here if we just fly smoothly and keep the wings unloaded.

Turbulence is not something to look forward to; it takes the fun right out of flying. But it needn’t be something to fear, either. There are a few airplanes lost each year to turbulence. But most accidents are due to losing our cool, then losing control—in that order.

My turbulence encounter was enough to make me a real believer in strut-braced Cessnas. I obviously lived, and the reason is because I remained calm and didn’t allow the airplane to become highly G-loaded while riding it out. You might not break any major parts during one of these encounters, but that permanent set in the wing or stabilizer is not pretty. If you encounter severe turbulence, have the airplane inspected thoroughly after the flight by a certified mechanic. Or fly only on smooth Sundays.

DO

1. Relax. Calmness is the key to flying in turbulence.
2. Fly smoothly.
3. Tighten all seat belts.
4. Put your shoulder harness on.
5. Stay close to maneuvering speed.
6. Let the airplane unload itself as much as possible.
7. Extend the gear or speed brakes.
8. Turn the autopilot Off.
9. Tell ATC if you can.
10. Give a pilot report when it’s over.

DON’T

1. Panic.
2. Allow the wing to become G-loaded.
3. Extend the flaps. This halves the wing’s load-limit capability.
4. Make rapid heading or altitude changes.
5. Worry about the paperwork if you "bust" an altitude.