They used to call it “prop wash”: A turbulent wake that is present behind any aircraft in flight. Much of what we now know of wake turbulence was learned from tests that NASA conducted from the late ’60s through the early ’80s. The aerodynamic result of an aircraft moving through air can be powerful enough to roll airliners. A turbofan or turbojet aircraft ingesting a vortex can suffer compressor stall. If you ever hear the wake or vortex, it will sound like anything from a soft whistle to the shriek of an artillery shell passing overhead.
If you accept an approach to visually follow a preceding aircraft, it’s your job to stay a safe distance behind that aircraft and avoid the wake turbulence created by the wingtip vortices, or else that wake… could be your own. Do not decrease that separation, unless you can stay well clear. But what’s “well clear”? How do wakes behave? And are there warning signs?
DEFINING THE WAKE
Imagine two horizontal tornadoes, trailing behind the wingtips of any flying aircraft — whenever an aircraft is generating lift. The vortex cores are generated by the high-pressure air beneath the wing swirling up and around the tip to meet the low-pressure air above the wing. Peak tangential speeds of up to 300 feet per second have been recorded in these funnels — I originally said “tornadoes” for a reason — three hundred feet per second, on the Fujita tornado scale, is the upper end of an F3 tornado (“Exterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests and farmland flattened.”) Fewer than 10% of tornadoes get that strong — just imagine what it could do to your airplane.
VISUALIZING THE WAKE
As “seen” from behind an aircraft, the left vortex rotates clockwise, and the right-hand vortex rotates counter-clockwise. An easy way to remember this is that they rotate like they “want to” roll towards each other (but they don’t). An aircraft arriving or departing too closely behind another aircraft can be subjected to rolling moments exceeding the roll-control authority of even aerobatic aircraft. (Roll control would only be effective if the wingspan and ailerons were outside the core rotational flow, which you can’t count on.) But wait, it gets better. Helicopters generate downwash, as well as vortices that have similar characteristics. Helicopter vortices are much stronger than an airplane’s (one of similar weight), are strongest at slower speeds (20 to 50 knots), and counter-intuitively, two-bladed rotor systems produce the strongest ones.
Although most of the energy is within a few feet of the vortex’s core (the more intense the vortex, the larger the core), the field of influence from each wingtip can be 100 feet in diameter; typically it’s 25 to 50 feet. They are fully formed by a distance of two wingspans behind a large aircraft, and tend to remain about three-quarters of a wingspan apart. The size, speed, and duration of the vortexes depend solely upon aircraft weight, speed, and wing configuration. Heavy, clean, and slow is the formula that produces the strongest and longest-lasting vortices. The vortices typically descend 500 to 900 feet below an aircraft, where they level off and drift with the wind, until they dissipate (a process hastened by turbulence).
Vortexes dissipate as a function of time, not distance, although there is some correlation. In the flight levels, a three-minute vortex could cover 25 miles and, at terminal area speeds, up to nine miles. Once they near the ground (100 feet or so), they move outward at from two to five knots in calm winds. Obviously there will be a net movement from the sum of this and any ambient wind: the notorious light, quartering tailwind can cause one to loiter over the runway. Though there are rare cases of immediate structural damage, the real threat from a wake turbulence encounter is structural damage… from the sudden, induced, uncommanded, and extreme roll rates, that is (with yaw elements, as well)— especially when there is little altitude for recovery.
WARNING SIGNS: I’ve used that word “core” several times. There’s a high-speed core, and a larger, slower “shell.’ At shallow intercept angles, you will hit the shell, and experience an uncommanded wing rocking, which will be (relative to a core encounter) fairly gentle. If you feel that, get the heck out — especially if you’re low! (Which way is out? Fortunately, you’ll know.) Be advised — if you intercept a wake by more than a few degrees, you will not enjoy this ‘subtle’ warning.
INSIDE INFORMATION: Wake turbulence almost always occurs in VMC conditions. There are no reports of wake turbulence upsets from aircraft operating under IFR separation standards. If you’re in communication with ATC and wake turbulence might be a factor, they’ll tell you. (You’ll hear the preceding aircraft’s position, altitude, and direction, followed by the words “Caution: wake turbulence.’)
NASA’s studies indicated that the strongest rolling moments occur with intercept angles between two and five degrees. Fortunately, these angles should also offer you the ‘shell warning’ described above. Aside from that, the well-known rules for avoiding wake turbulence are to
- Fly at or above the preceding aircraft’s flight path,
- or 1000 or more feet below it
- and on the upwind side.
The greatest potential for an incident is when a smaller aircraft turns base to final behind a larger aircraft on a straight-in approach.
Land and complete your rollout well after the point where the preceding aircraft’s nose wheel touches down, and take off well before that aircraft’s nose wheel lifts off, diverting your upwind, upwind! Important: That’s true also if it’s on a parallel runway separated by less than 2500 feet. Time is the other means of avoiding it. Wait at least two (better, three) minutes for takeoff behind a larger airplane, in addition to the above spatial avoidance measures. (Further guidance for these situations as well as crossing runways is in the AIM, sections 7-3-1 to 7-3-9, and also in Advisory Circular 90-23F, dated 2/20/02.) Be most cautious following heavy aircraft that are flying near the low end of their speed envelope — especially on calm days, or where there’s only a light crosswind!
Use the threat of wake turbulence as en excuse to get some advanced maneuver “upset training.’ Worth remembering though, even if you don’t, is that if you do get spun (and I don’t mean a holding pattern):
- Don’t pull back, but push forward to reduce angle of attack.
- Add power if you’re nose-high; chop it if you’re nose-low.
- Step on the sky (at 90 degrees, the rudder is your elevator).
- Roll wings level — and be aware that you may have to continue the roll (through inverted, and back to wings level) to do so. This makes the most sense if you think you can take advantage of the initial, violent roll momentum. While most of us might not have that kind of presence of mind to do that, most of the aircraft we fly do not have the kind of aileron authority necessary to counter the induced roll. You’ll be lucky if you can get it stopped.
Bottom Line: Visualize world peace, fine. But visualize those vortices, too!
Editor’s note: For an interesting photo that captures wake turbulence in action (and some unpleasant arrivals) visit http://home.netvigator.com/~pashford/kaitak_photos_1.html and scroll down to item ‘9. Wake turbulence !!! ………………… What wake turbulence ??????’