Impossible to see and difficult to predict, air turbulence is a real hazard for skydivers of any experience level. Thankfully, most encounters with turbulence under canopy occur high enough above the ground that they result in nothing more than a light bump. But occasionally turbulence close to the ground can lead to a scare or even injuries from a hard landing.
Pilots have plenty of tools available for predicting turbulence, but skydivers generally don’t have to worry much about the types of turbulent conditions that affect aircraft. A wind shear at 2,500 feet—a condition that is rough for an airplane pilot flying at high speeds—usually causes a skydiver flying a canopy at about 15 mph to just feel a slight bump for a second or two. Turbulence close to the ground is the skydiver’s enemy. Since you can’t see turbulence, your best defense is to understand how it occurs and avoid flying your parachute where you’re likely to find it. Turbulence occurs:
- Downwind of trees, buildings and other tall obstacles.
- Above areas where two different surfaces are next to each other, such as grass next to asphalt.
- Behind the spinning propellers of airplanes running on the ground.
- Behind other parachutes.
After crossing over trees, tall buildings or other structures, jumpers under canopy may find that smooth winds suddenly turn chaotic. The stronger the wind and the higher the obstacles, the worse the effect will be. Skydiver’s Information Manual Section 4, Category C, provides general guidelines regarding turbulence. You can expect to feel the effects of turbulence at a distance as far as 10 to 20 times the height of the obstacle that the wind is blowing across. So wind coming across 50-foot-tall trees might cause turbulence as far as 500 to 1,000 feet downwind of the trees. Assess your landing area and the direction of the wind to help choose the best area to land. Some drop zones have known problem areas when it is windy, so ask the drop zone manager or staff whether there are spots you need to avoid when the wind is blowing.
Turbulence also results when the sun heats two dissimilar surfaces to different temperatures and causes columns of air to rotate upward. In some cases, dust devils—essentially mini-tornadoes—form. The type of turbulence resulting from uneven heating occurs even when winds are calm, so keep that in mind when you are planning your landing pattern.
Also watch for turbulence if a running aircraft is loading jumpers or taxiing beside the landing area. In these cases, turbulence will be behind and traveling downwind from the airplane. Of course, it’s never a good idea to land anywhere near a running airplane just from the standpoint of avoiding the propeller, the boarding jumpers and other hazards. But if for some reason you are unable to select a clear, open landing area away from the running aircraft, keep in mind that the area directly behind and downwind of it could be turbulent.
One of the most common causes of a jumper experiencing turbulence at a low altitude is also one of the least obvious: flying behind and above another parachute. As a canopy descends through the air, its wake will trail behind it and above it. The farther behind the jumper you are, the higher its wake will be. Though skies are often congested at pattern altitudes and you may need to fly relatively close to other jumpers, try to avoid the turbulent air above and behind them. Passing through another canopy’s wake is a recipe for canopy collapse.
Turbulence can cause problems for even the most experienced canopy pilot. It might be invisible, but that doesn’t mean you need to be blindsided. Knowing the causes of turbulence and where it lurks can help you avoid it.
—Jim Crouch | D-16979
USPA Director of Safety & Training