By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the September 2020 issue of EAA Sport Aviation magazine.
Over the past months, we’ve all learned to deal with one form of turbulence or another. If there is one thing that makes flying uncomfortable, it’s turbulence. Like driving a car over a bumpy dirt road, it simply jostles the bones and sucks some of the joy out of the experience. Even light to moderate turbulence can easily make passengers nervous or nauseous. Other times, turbulence can be downright dangerous. But just as with other forms of turbulence in life, if we watch for the signs and take the right steps, we can usually avoid the worst of turbulent conditions.
Signs in the Sky
Sometimes we have pretty clear visual signs of turbulence. We all know that clouds with extensive vertical development signify updrafts, which typically signals turbulent conditions, especially as they evolve into thunderstorms. Even a layer of scattered, puffy cumulus clouds tells us that the sky will not likely offer a smooth transit. But there are other signs of turbulent times ahead. Virga signifies rapidly descending columns of air that spell turbulence. Rotor clouds, which form beneath the crests of mountain waves, are definite signs of serious trouble to avoid by a wide margin. When these conditions exist or are forecast, we might consider changing our route, time of flight, or altitude to minimize our exposure to turbulence. One tip for flying in turbulence is to get our feet doing more of the work. Instead of wrestling with the yoke, we can use our feet to lift whichever wing drops.
Imagine a Rocky Streambed
While turbulence in cruise flight can be an issue, what’s often more problematic is the mechanical turbulence that occurs when strong winds blow over uneven and obstacle-strewn terrain. Negotiating an approach and landing in such conditions can put us in a high-risk situation. One way we can identify areas of potential turbulence in such situations is to visualize the wind like water flowing over a rocky streambed — rising as it encounters obstacles forming an updraft, tumbling like a breaking wave as it passes over a steep drop, and swirling like a whirlpool as it flows around, and accelerating as it passes between obstacles.
Our best bet is to eyeball the situation carefully, paying attention to windsock indications and the obstacles on the ground that can create mechanical turbulence. Where surface winds encounter rising terrain, we might expect updrafts. Where winds encounter downward sloping terrain, we might expect downdrafts. Either case could result in wind shear. Where wind passes between terrain obstacles, we can expect it to accelerate. Downwind of any obstacles, we might encounter turbulence and wind shear. With this in mind, we might need to alter our approach path and touchdown point to avoid the hazards. Also, consider that gust fronts from nearby thunderstorms and microbursts can spell disaster as gust fronts push madly across the terrain. Even if the terrain is smooth, the low-level wind shear can create a dangerous situation for departing and landing aircraft.
Wingtip Tornadoes
One of the more dangerous forms of turbulence occurs when another (typically larger) aircraft is ahead and above our flight path. The issue, of course, is the furiously spinning wake vortices that become exaggerated when an aircraft is heavy, aerodynamically clean, and slow. Wake vortices roll off the wingtips in a counter-rotating fashion. The energy in those invisible wingtip tornadoes can snap us around in a heartbeat, leaving us in a precarious juxtaposition with Mother Earth. Encounters, whether on takeoff, landing, or in cruise, can be the last problem we ever face, so it only makes sense to avoid the danger. Remember that helicopters also produce wake vortices that act similarly in flight to those of fixed-wing aircraft.
Time and distance are the keys to avoiding wake turbulence. On takeoff, wait for at least two minutes after a large aircraft departs. Rotate before the aircraft’s rotation point and climb above its flight path. On landing, shoot for a two-minute interval before landing after a large aircraft. Use a steeper glide path on the approach and touch down beyond the preceding aircraft’s touchdown point. Remember that a light crosswind (1 to 5 knots or so) can cause a wake vortex to drift back over and linger on the runway (or parallel runway), creating catastrophic conditions. In the air, the wake turbulence from a large aircraft typically descends at a few hundred feet per minute and will sink to an altitude 500 to 1,000 feet below the flight path. We should adjust our flight path and altitude to avoid the danger.
A Matter of Speed
Another key to safely negotiating turbulence is knowing and using the right airspeed. Clearly, we don’t want to be flying within the yellow airspeed arc when approaching potential turbulence. In fact, any time we suspect turbulence is a good time to reduce our speed to maneuvering speed.
Maneuvering speed (VA) is not shown on the airspeed indicator. However, it is typically placarded somewhere in the cockpit. What we need to understand is that the published maneuvering speed corresponds to the maximum gross weight condition. Unless we’re at max gross, we should be using a speed slower than the advertised VA speed. We can calculate that speed in a couple of different ways. One way is to multiply the max gross VA times the square root of the ratio of actual weight over max gross weight. It’s easy enough to do unless you’re flying in turbulence or can’t easily calculate square roots in your head, which could easily include most of us. Another way to estimate changes in VA is by percentages, which might be easier. Figure out the percentage below gross weight, divide by two, and then reduce maneuvering speed by that percentage. For example, if our airplane has a max gross weight of 3,000 pounds and is loaded to 2,400 pounds, it is 20 percent under gross weight. That means our VA will be 10 percent under the VA for max gross weight. Easy, right?
Easier still, and what I like to do, is to calculate maneuvering speed for the airplane for a weight corresponding to nothing but me, my flight bag, and minimum fuel on board. I write that down on a card that also has other important performance data on it that I can keep in my shirt pocket for easy reference. Since I calculate my weight and balance before the flight, I can easily figure out what it will be at the beginning and toward the end of the flight and can interpolate accordingly where VA will be.
Some Reassuring Words
While we might be comfortable flying in turbulent conditions, we should keep in mind that it can be a scary proposition for our passengers. We need to reassure them that our aircraft are designed for such conditions and, although it might be uncomfortable, we are not at risk of breaking apart in flight. And just in case the bumpiness is too much for them to handle, keep those air vents open wide and show them where the airsick bags are stowed.
None of us really like turbulence, but it is often a fact of flight and a fact of life. In both cases, if we read the signs and apply the right strategies, we can avoid the worst of the bumps and remain unshaken.
Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman. For more from Robert, look for his Stick and Rudder column each month in EAA Sport Aviation.