Wx Watch: Dropping in on Bumps

In September 1996 I wrote about the relative merits of cruising at high and low altitude (" Wx Watch: Choosing Where to Cruise," September 1996 Pilot). Apparently the topic hit a nerve. Mail flowed in, expressing support for one option or another. One member summed up the crux of the dilemma by saying that flying high is nice because of the generally smoother air, but that flying low has advantages in that it offers better views of the passing scenery.

I'm sticking to my guns. For anything other than short trips I say it's better to fly at higher altitudes — unless brutal headwinds, icing, or other weather hazards are factors up high.

Turbulence Reporting Criteria

(per AIM 7-1-21)

Light — Slight erratic changes in altitude and/or attitude

Moderate — Greater intensity than light. Changes in altitude and/or attitude, but aircraft remains in positive control at all times.

Severe — Large abrupt changes in altitude and/or attitude. Usually large variations in indicated airspeed. Aircraft may be momentarily out of control.

Extreme — Aircraft is violently tossed about and is practically impossible to control. May cause structural damage.

A fact remains. One way or another, you'll still have to descend for a landing. In the process, there is a strong likelihood that you'll encounter turbulence at lower altitudes.

Low-level turbulence can be produced by several mechanisms, the most common of which are:

• Surface convection. The sun heats up the earth, causing thermals and the resulting turbulence.
• Frontal passages. Any front can cause turbulence, but fast-moving cold fronts create the worst low-level turbulence. If the air following a cold front is significantly colder and/or less humid than the air preceding it, the turbulence can persist for days after the passage.
• Orographic features. Mountains and hills can set up turbulence as low-level winds flow across and around their ridges. (The same thing can happen when wind encounters buildings, hangars, and other large obstacles.) The analogy here is that of water flowing in a boulder-strewn creek. Severe lee wave turbulence and violent rotors at altitudes several times the height of a ridge can sometimes be signaled by standing lenticular clouds.
• Inversions. Often marked by the top of a haze layer or a deck of stratus or cumulus clouds, inversions mark zones where temperatures momentarily rise with altitude. The normal lapse rate doesn't prevail — and, as a result, airplanes flying through an inversion's instability can encounter widespread areas of wind shear.
• Warm fronts. The largest-scale example of an inversion would be a warm front — warm air riding up over cold. Here, there's the potential for shear-induced turbulence, icing in clouds, and freezing rain as any precipitation falls into the colder air below.
• Land/sea interactions. These include seabreeze fronts, as well as the effects of differential heating along any shoreline.
• Upper-level instability. Lows and troughs aloft impart lifting motions in the atmosphere below, creating instability and turbulence. Surface lows can be intensified by upper lows and troughs, making them more aggressive than they might otherwise look on a weather chart.

Before leaving your idyllic perch on high, it's well worth planning ahead. How do you know if it's turbulent below? Obviously, any of the above features should serve as fair warnings. A good preflight weather briefing should have clued you in, but an inflight update from Flight Watch on 122.0 MHz could let you in on the latest information — assuming that other pilots have been diligent in issuing pireps. Sometimes, merely listening in on 122.0 will be enough to give you a fairly good picture of what to expect along your route of flight.

Listening in on AWOS and ATIS broadcasts as you fly can also be helpful. If surface winds are strong, gusty, or shifting in direction, that's reason enough to expect turbulence below.

Speed management will be critical during the descent. To be brutally brief, the objective is to slow the airplane to an airspeed at or below its published maneuvering speed (V _a) for the airplane's actual weight at the time. Flying at the correct V _a is the only way to avoid overstressing the airplane, and it bears emphasizing that V _a is a variable value.

Some manufacturers provide more detailed information about their airplanes' maneuvering speeds than others. Many older (before 1976) pilot operating handbooks, for example, publish a single figure — the airplane's V _a at maximum gross weight. That's fine, as far as it goes. But if an under-gross-weight airplane flying at its gross-weight V _a encounters a bone-jarring gust, it could easily surpass its limit load factor, and airframe damage or destruction could occur. For this reason, some manufacturers post placards giving the V _a values for gross weight and lesser weights. V _a decreases as a function of the square root of the airplane's weight decrease.

For normal-category airplanes, limit load factors are plus 3.8 Gs and minus 1.52 Gs. Flying at or below the V _a for a given weight means that any use of full, abrupt control travel will result in the airplane's stalling before limit loads are reached. So if you are fighting to keep the airplane upright and flying at the proper V _a, in theory you shouldn't have to worry about permanently deforming the wings or suffering an airframe failure. I say "in theory" because airframes age and fatigue, we may not know if someone overstressed the airplane and thereby weakened the airframe on a preceding flight, and truly severe turbulence may cause structural problems beyond the pilot's control.

The issue of maximum gust penetration speed (VB) is closely related to maneuvering speed. In calculating gust penetration airspeeds, manufacturers assume that the airplane endures sudden, sharp-edged gusts. The suddenness of the gusts doesn't give the pilot enough time to react with corrective inputs, so the control surfaces are not fully deflected — as opposed to the full-deflection assumption in the V _a calculations. But the ultimate effect — increased load factors — are the same. In fact, an airplane's gust envelope may even allow it to exceed limit loads briefly because the twisting moments present in a condition of full elevator (or stabilator) deflection are not present when flying through gusts. This means that stresses on the horizontal stabilizer or stabilator can be less in gusts.

The FAA used to require manufacturers to develop a gust envelope that takes into consideration the effects of instantaneous gusts of 15- and 30-feet-per-second (fps) intensity. The 15 fps value translates into a vertical shear worth about 9 knots, which would fit the light to moderate turbulence category. A 30-fps gust is approximately 18 knots of shear and equates to moderate to severe turbulence — but not the kinds of really huge loads encountered inside thunderstorms and other convective or rotor clouds.

The latest requirements are for vertical gusts of 25 and 50 fps, with a gust alleviation factor included. [For those who stay awake nights thinking about the gust envelope, consult FAR Part 23.333(c) and FAR 23.341(a) and (b) for detailed information.]

In general, gust penetration speeds run lower than V _as. You can consult your pilot operating handbook for more information on any recommended gust penetration speeds. But it probably won't do you any good. For some unexplained reason, VBs are not mentioned in piston-powered general aviation airplane pilot operating handbooks. It's really an unforgivable omission. Turbine-powered airplanes certified under FAR Part 25 will, however, publish VBs for various weights.

Absent VB information, flying a proper V _a is a useful strategy in turbulence. Just remember that V _a is not necessarily the airplane's gust penetration speed.

Any lengthy discussion of maneuvering speed, the maneuvering envelope, or the gust envelope can take you into some pretty rarefied territory. Perhaps the best understandable discussion of these subjects can be found in Part 1, Chapter 11 of William K. Kershner's The Advanced Pilot's Flight Manual, Sixth Edition, published by Iowa State University Press.

Knowing that turbulence awaits us on the trip down, and convinced of the need to slow down, the pilot must come up with a target speed and a strategy for slowing down. In no case should the airspeed be allowed to stray out the top end of the green arc (VNO) on the airspeed indicator. When facing severe turbulence, figure on trying to hold an airspeed 1.7 times the clean, power-off stall speed at gross weight as a rough rule of thumb for all weight conditions. This will turn out to be a close approximation of the airplane's VB. Of course, any manufacturer recommendations regarding VB should serve as gospel on this matter.

Now for an example. The Beech A36 Bonanza's published V _a is 141 KIAS, but that's at the airplane's maximum gross weight of 3,650 pounds. Its clean, power-off stall speed is 68 KIAS. Applying the rule of thumb, we see that in severe turbulence pilots should try to hold 68 times 1.7, or 116 KIAS, to be safe. This will assure that the airplane will stall rather than overstress at lower-than-gross weights.

Descending and preserving lower airspeeds at the same time can be a contradictory exercise. The tendency — no, the much-anticipated expectation — is to pick up airspeed in a descent, the product of lowering the nose and heading downhill. You can go down or slow down, but you can't do both at the same time.

But this is the task when descending in turbulence. The first step is to make a power reduction (preferably slowly in piston-powered airplanes; as quickly as you want in a turbine) and bleed off airspeed in level flight. When airspeed drops below VLE or VFE, consider lowering the landing gear or flaps as a means of further slowing the airplane. Airplanes with speed brakes or spoilers have the luxury of still another way of slowing down. The idea is to achieve a steady flight condition, in a stabilized descent, at the right airspeed before entering the turbulence.

Once in the bumps, things can become tense. Turbulence can be uncomfortable at best, terrifying at worst. Do your best to fly the target airspeed, and focus on holding the wings level. Try to keep descent rates at no more than 500 fpm and be assertive with ATC if they demand a "slam dunk" arrival profile. Let ATC know that you're dealing with turbulence and ask for as early a descent as you can negotiate.

Make small corrections when trying to compensate for rolling and pitching motions. Large control inputs are an invitation to overcontrolling, upsets, and overspeeds. By all means, hand fly the airplane. This is certainly no time to use an autopilot. It's not designed to handle really rough air, and newer models certified under the latest applicable portions of FAR Part 23 (Part 23.1329, to be exact) will disengage themselves automatically if they start to command large control force inputs.

Hopefully, the turbulence will be transitory and you'll be able to complete your approach and landing in less rowdy air. (Hopefully, you could have avoided severe turbulence in the first place by carrying out an alternate plan of action). But, alas, the wind shear and gusts that cause low-level turbulence usually have a way of extending all the way to touchdown — and sometimes beyond. It would be a shame to soldier through a descent in turbulence only to botch a crosswind landing.

But that's a subject for a future discussion