Wx Watch: Ill Winds

Riding waves, shears, rotors, bumps, and jumps

March 1, 1999

Wind shear and turbulence are as aggravating as they are potentially dangerous. Turbulent atmospheric motions occur frequently enough that pilots need to be savvy about where and why they can happen, and know what to do when they strike. Here's a brief rundown of some of the dodgiest situations you could encounter.

Gusting, shifting winds during final approach and landing. This sort of mayhem is common after a cold frontal passage, in the proximity of deep low pressure centers, near and within thunderstorms, and in downslope wind conditions (which we'll discuss shortly) in the western United States. The task here is to maintain a higher-than-normal final approach airspeed so that you can maintain a safe margin above the stall when shear-induced airspeed fluctuations occur. The rule of thumb is to add one-half the gust value to your normal 1.3-Vso final approach speed. So, if the winds are 10 knots, gusting to 20, add five knots to your normal speed.

As for crosswinds and shifting winds, be prepared to dust off your best crosswind landing technique. One time-honored method of performing a crosswind touchdown calls for the pilot to lower the upwind wing (to prevent the airplane from drifting sideways) using the ailerons, while at the same time applying opposite rudder pressure (to keep the airplane pointed straight down the runway centerline). The challenge is to know just how much of each pressure to apply, and when to apply them. In gusty crosswind conditions, you'll have to move quickly to compensate for shifting wind speeds and vectors. If you don't, you could join the plentiful ranks of those who've bent a propeller, crumpled a wing, or collapsed the landing gear during a botched crosswind landing.

Wind shear on approach. Wind shear is an abrupt change in wind speed or direction within an air mass. The shear takes place at the boundaries where the winds meet, causing turbulent eddies to occur. It can accompany frontal passages, crop up whenever there are strong surface winds, and are sure byproducts of thunderstorm and other convective activity. An airplane moving at cruise speed through an air mass experiencing wind shear is likely to act as though it's in turbulence — a jolt or two as it passes through the shear's boundary zones. But on approach, at slower airspeeds and reduced power settings, wind shear can be deadly. The airplane can descend from an air mass moving in one direction to another with winds running in an opposing direction. One minute the airplane is on a stabilized approach profile. The next, the bottom drops out as airspeed plunges and dangerous sink rates develop. The airspeed plunge is a direct result of a sudden loss of headwind component. The sink is a result of the corresponding loss of lift and effective increase in the tailwind component. At low altitudes, this decreasing-headwind type of wind shear can cause a crash short of the runway. That's especially true of turbine-powered airplanes. Their engines can take precious time to spool up and generate enough airflow over the wings to arrest the descent and climb away. With propeller-driven, piston-powered airplanes, power and thrust responses to throttle commands are much quicker.

Increasing-headwind shears can put you high on the approach path and tempt you to dive to the runway threshold.

In decreasing-headwind shears, the pilot's response should be to apply power, and perform a go-around or missed approach if the airplane is so low that the situation warrants it. In increasing-headwind shears, a power adjustment is also in order — a decrease, if the touchdown can be made on the first third of the runway, or an increase to climb power if an overshoot appears likely.

Mountain waves. Expect downwind wavelike activity any time stable air flows over high terrain. Mountain waves are more pronounced when the winds aloft cross ridges and ranges at a perpendicular angle. Sometimes, mountain waves are marked by altocumulus standing lenticular clouds (abbreviated as ACSL in aviation weather reports and forecasts). Other times, Kelvin-Helmholtz clouds can identify shearing waves aloft; these clouds look like a sequence of breaking ocean waves. Still other times, waves take place in clear air. When flying in docile waves, you'll have mainly smooth conditions. But you'll notice airspeed increases and decreases if you maintain altitude (or are on autopilot), or uncommanded climbs and descents if you don't. The answer here is to ride out the waves instead of radically climbing or diving, and to inform ATC should you be unable to hold your altitude excursions to acceptable levels. If you're flying on an IFR flight plan, you can try asking ATC for an altitude block. This will give you clearance to go up or down within a certain altitude range-suggested by you.

Severe turbulence can lurk in and around mountain waves, so the best avoidance strategy is to keep as much of a margin of altitude as you can above the waves — if you can see them.

Lee rotors. Rotors-elongated rolling movements that many have likened to horizontal tornadoes — are perhaps the most violent of the dangerous shearing winds a pilot can confront. They form in high-wind conditions off the lee slopes of elevated terrain, and are often invisible. A pilot who ventures into a rotor can expect to roll inverted. A rotor is suspected to have caused the 1991 crash of a United Airlines Boeing 737 on approach to the Colorado Springs airport. "Suspected," because there was no visible proof of the existence of a rotor, but conditions were ripe for their occurrence.

Sometimes, a rotor can signal its presence with a rotor cloud. These usually look like horizontal, roughly cylindrically shaped clouds. Some can be short, some quite long. Some appear in consecutive bands. Some form beneath high-altitude lenticular standing wave clouds. Some have been observed with companion helical clouds that spiral around the central cloud. It goes without saying that if you see rotor clouds, avoid them — and the areas downwind of them. When no rotor clouds are apparent, it's best to arm yourself with any and all weather information that could hint at rotor activity. This includes any mention of particularly strong (greater than 40 knots) winds at ridge altitude, and the combination of stable air at altitude and very turbulent conditions below. Climb to altitudes above the ridge line quickly, cruise at altitudes well above the ridge line, and be ready to take corrective action immediately if landing at airports in areas susceptible to rotor activity. Aerobatic experience, or a course in upset training, could prove vital to the successful outcome of a rotor encounter.

Clear air turbulence (CAT). CAT typically occurs at high (25,000 feet plus) altitudes, and it's usually associated with jet stream activity. Expect CAT when there's a strong core of jet stream winds at the southeast quadrant of a trough aloft. That's where winds can hit 150 knots or higher. Turbulence occurs because air accelerates as it enters the cores of strongest winds and decelerates as it leaves, creating shear zones and eddy currents. It's called clear air turbulence because the air at those altitudes is usually cloud-free. Another spot where CAT lurks is near cutoff lows aloft, just to the north of areas of low pressure encircled by height contours. You can find jet cores and cutoff lows on 500- (approximately 18,000 feet), 300- (approximately 30,000 feet), 250- (approximately 34,000 feet), and 200-millibar (approximately 39,000 feet) constant pressure charts. Expect a probability of CAT if the winds at those altitudes vary by more than six knots per thousand feet for a particular vertical cross-section of the atmosphere, or there is a variation in wind speed of 40 knots or more over a 150-nm horizontal distance. CAT can cause severe turbulence, so it's important to slow the airplane to an appropriate maneuvering or turbulent air penetration speed as soon as it's encountered. (Of course, the same holds true for any other type of turbulence.) Hopefully, your weather briefings will have mentioned any CAT, and you'll have learned about it from pireps passed along to flight watch.

We all tend to think of the early spring months as having the bumpiest and windiest flying weather of the entire year. You know, March winds and all that. But the truth is that all of the mayhem that we've been discussing here can happen any time of year, depending on the region in which you are flying and the weather systems at work. Deep down, we all know that. But this is the time of year when a lot of pilots resume a more regular flying schedule, so the bumps, shears, and waves hit many of us after a pause of many weeks. Those first shots of bad air can be heart-stopping eye-openers, so don't say I didn't warn you.


E-mail the author at tom.horne@aopa.org.