Turbulence penetration: avoiding structural failure

There are many types of turbulence and their occurrence can be quite surprising for most pilots. Most are aware of mechanical turbulence which is prevalent in the lower levels when moderate or strong winds are blowing over obstacles creating tumbling, turbulent vortices of confused air.

Another well known phenomenon is convective turbulence – known as lift and sink by soaring pilots. This occurs in unstable air masses and is exacerbated with steeper lapse rates and often "readable" by looking at cumulus family clouds – except on very low humidity days when their may be no clouds but convective turbulence will still exist. Cumulonimbus clouds and towering cumulus are good indications of strong updrafts and downdrafts and associated wind shears.

Clear air turbulence (CAT) is generally seen most by high flyers who operate in proximity of the jet stream where very strong wind gradients that change with altitude create strong shears. That is, wind speed and direction can be changing greatly over a short distance.

Another phenomenon that has not generally been named historically as a form of turbulence is kind of a mix of all of the above.

Wave turbulence forms downwind of mountainous terrain and is well marked by rotating rotor clouds downwind of the barrier and the lens shaped clouds (Alto Cumulus Lenticularis) that are essentially parallel and downwind from the hill feature. Incidentally, they can form a series of "Chinook" clouds hundreds of miles downwind from the initial set.

There are a couple of unique features about the wave, it actually forms best in stable air and sometimes there are no clouds to warn pilots the conditions are in place for wave. Although portions of the wave (generally around the "lennies") provide smooth air when flying parallel to them, if one crosses a wave at high speed the greatly varying lift or sink is seen as very strong wind sheer – capable of ripping wings off aircraft.

Moreover, the worst turbulence ever experienced by this aviator occurred when a pilot friend intentionally flew his 80 foot wingspan glider into the upwind side of a rotor cloud to obtain strong lift. I thought the wing tips were going to clap together, they were flexing so much, I also thought I was going to get a second chance to look at my breakfast.

The only good part of that experience was I knew the parachute had been recently repacked.

This month I have invited aviation writer extraordinaire, Barry Schiff, back with one of his fun to read articles that provides suggestions for how to fly in turbulence. If you enjoy his excellent work as much as I do, you are invited to visit his website: www.barryschiff.com to see his works.

TURBULENCE TALES
By Barry Schiff

The worst turbulence I ever experienced while flying a jetliner occurred during the early 1970s near Newfoundland in clear air at 39,000 feet on a flight from Rome to New York.

We had begun to encounter light turbulence so the passengers, fortunately, had their seat belts fastened. Without further warning, our Boeing 707 was hammered so violently that it was impossible to prevent bank angles well in excess of 60 degrees.

Without approval from Center, I exercised emergency authority by grabbing and fully retarding a fistful of throttles, deploying the spoilers, and heading downhill as rapidly as possible.

The assault ended almost as suddenly as it had begun after losing 6,000 feet. The cockpit and cabin were in turmoil with loose objects and cups of coffee having been flung everywhere. Thankfully, there were no serious injuries. I learned later that we had begun to cross a tight kink in a narrow, high-speed jet stream.

Today’s forecasting is better and usually anticipates such phenomena giving pilots the opportunity to plan flights along smoother routes.

The worst encounter I’ve had in a light plane occurred in 1959 while flying northbound in a Cessna 310D in California’s Owens Valley, which is east of and parallel to the Sierra Nevada mountains.

The winds-aloft forecast was ideal for the development of a strong mountain wave, but in that prehistoric era I had never heard of such a thing.

The airplane was pounded by powerful, teeth-rattling gusts that threatened to flip the airplane onto its back. The gusts were so erratic and forceful that I literally could not read the instruments. I could tell, though, that our beleaguered Cessna was being shoved skyward even with the throttles closed.

The peak of nearby Mt. Whitney (14,500 feet) was below us. Without supplemental oxygen, the increasing altitude plus the effect of fear-induced rapid breathing made me wonder if I would pass out before the airplane came apart.

I finally reversed course although I am not certain if I made the turn or if the turbulence had spun us around and spit us out.

It speaks well for airplane design that neither event resulted in structural damage.

Light plane pilots know to reduce to manoeuvring speed (VA) during serious turbulence encounters. The idea is to ease structural strain and prevent damaging loads.

A pilot I knew who spent years probing and investigating the innards of thunderstorms in a single-engine airplane — he died of natural causes at a ripe age — passed along some excellent advice. He said to regard VA as a limit, not a target, a maximum speed that should not be exceeded when the going gets rough. This is because airspeed can fluctuate wildly in turbulence, and a pilot may find that he is flying beyond VA just when a powerful gust strikes the airplane, which, of course, can dangerously increase airspeed even further."

He recommended flying below VA but not so slowly that the controls become mushy. In the worst turbulence, try to maintain two-thirds of the way between the clean stall speed (the bottom of the green airspeed arc) and VA.

This could result in some gust-induced stalls, but these are so brief that they are hardly noticeable (except for admonitions from the stall-warning indicator).

Fighting turbulence with rapid control inputs should be avoided because gust loads and manoeuvring loads are cumulative. That is why pilots are advised to maintain attitude not altitude during harsh turbulence encounters. Keep the airplane right side up and don’t worry about altitude unless safety demands otherwise. The idea is to maintain control of the airplane without damaging it.

Every pilot learns the operating limitations of the aircraft he flies. He is obligated to abide by them. One is the maximum-allowable limit load factor. In Normal-category light planes, this usually is 3.8 Gs positive and 1.52 Gs negative. Curiously, though, the only way to abide by these limitations is to observe loads on a G-meter, and few airplanes have them. G-meters are inexpensive, self-contained, and easily installed. I would not own an airplane without one. If you have space on your panel, I urge you get one.

During a recent flight from Central to Southern California, I was over the coastal mountains heading east toward Santa Monica Airport about 20 miles ahead. A very apparent and impressive drift angle had developed indicating the presence of a strong northerly wind. Light-to-moderate turbulence had begun to buffet the airplane, but it was not alarming.

The wind on the surface, however, was reported light and from the east indicating that I would soon descend through a wind shear or gradient. I reduced airspeed accordingly. When downwind of the mountains and descending about 500 fpm over the Malibu shoreline, I was struck by a single hammering gust of such intensity and ferocity that I could not determine if it had attacked from above or below. It literally gave me a neck ache that lasted all day.

It was of sufficient power that I made a mental note to inspect the airplane for wrinkled skin after landing. There was none, but I surely would like to have had a G-meter in the airplane.

After I got home, I called a weather briefer and learned that there had been reports of a 40-knot change in wind speed between 2,000 and 3,000 feet msl. It normally takes only a 4-knot change per 1,000 feet to induce turbulence.

That was a monstrous shear.