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Proficient Pilot: Turbulent Advice

The right approach to rough air

It is not unusual for passengers to be nervous about turbulence. I usually try to allay their anxiety by explaining that turbulence is much like motor-boating on a choppy lake or driving along a bumpy road. It might be annoying or uncomfortable, I tell them, but it is nothing to fear.

The suggested action in each case is also the same: Slow down to soften the ride. Pilots know that flying safely in threatening turbulence requires slowing to VA, the maneuvering speed. This assures that the airplane will stall before its limit load factor can be exceeded. Such a stall is momentary and protects the structure against damaging loads. It usually is best, however, not to use VA as a target speed. This is because airspeed can fluctuate wildly in turbulence and could be above VA just as a particularly punishing gust assaults the aircraft. Instead, VA should be used as a form of redline when the going gets rough.

In other words, fly below VA but not so slowly that the flight controls become mushy. A practical target in the worst turbulence is about two-thirds of the way between the clean stall speed (the bottom of the green arc) and maneuvering speed.

We are taught that VA decreases as gross weight decreases. This is because stall speed decreases as the aircraft gets lighter. But there is an easier to understand reason for the lower VA. A vertical gust (one that attacks from above or below) is a force that causes the aircraft to accelerate vertically. Such acceleration is expressed in Gs. The greater the acceleration, the greater the G-load.

A given force more easily accelerates a light object than a heavy one. A vertical gust, therefore, imposes more of a load on a lightly loaded airplane than one that is heavily loaded. Since a lightly loaded aircraft is more easily pushed about by gusts, it is logical that such an aircraft must be flown more slowly when encountering potentially dangerous turbulence.

Operating handbooks for many light aircraft, however, provide only one maneuvering speed, the one that applies when the aircraft is at its maximum-allowable weight. VA for lighter weights can be approximated by reducing the published VA by two knots for each 100-pound reduction in gross weight.

Different aircraft react differently to a given gust. The primary reason for this is wing loading: the amount of aircraft weight supported by each square foot of wing area. The greater the wing loading, the more difficult it is for a gust to displace or accelerate the aircraft, and vice versa. This is why the pilot of a Cessna 310, which has a wing loading of 31 pounds per square foot, might report moderate turbulence while the pilot of a Cessna 172, which has a wing loading of only 13 pounds per square foot, would report the same turbulence in more colorful terms. This is also why pilot reports of turbulence are meaningless without knowing the types of aircraft involved.

A pilot encountering turbulence has three options: change altitude, alter course, or ride out the roughness. Fighting turbulence with aggressive movement of the flight controls can make a bad situation worse. This is because maneuvering loads, the Gs created by moving the flight controls, are cumulative. They combine with gust loads to create greater total loads.

There might not be much you can do about lessening turbulence intensity but you can minimize the overall effect by applying gentle control inputs, which admittedly is easier said than done when being tossed about the sky.

Conventional wisdom suggests that maintaining a level attitude results in less strain on the airplane than attempting to maintain a given altitude.

Maintaining altitude while flying through up- and downdrafts is counterproductive. Raising the nose to hold altitude in a downdraft results in losing airspeed, which prolongs the time spent in the downdraft. Instead, maintain attitude, accept the altitude loss, and pass through the downdraft as quickly as possible to minimize its effect.

Lowering the nose to maintain altitude when in an updraft increases airspeed, which reduces the time spent in the updraft. Instead, maintain airspeed to remain in the updraft longer and take advantage of the gain in “free” altitude (which will be available for sacrifice in the subsequent downdraft).

By maintaining attitude and going with the flow instead of against it, a flight is safer, more comfortable, and more efficient (faster).

It is a shame that lightplanes do not have a switch to control turbulence as jetliners do. This switch operates the seat-belt sign, but airline pilots know better. Turning the sign off invariably triggers turbulence and turning it on causes turbulence to subside. It never fails.


Barry Schiff

Barry Schiff

Barry Schiff has been an aviation media consultant and technical advisor for motion pictures for more than 40 years. He is chairman of the AOPA Foundation Legacy Society.

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