Distribution of Trailing Vortices — Page 89, Lesson 106

I want to walk you through the distribution of trailing vortices — what happens to those two powerful swirls of air behind an aircraft once they're generated. This is the part of wake turbulence that matters for separation standards, so let's get the numbers and behaviour exactly right. First, the basic geometry. The two trailing vortices typically remain separated by about three quarters of the aircraft's wingspan. So if you picture a large jet with, say, a 60-metre wingspan, the two vortex cores sit roughly 45 metres apart. That spacing is a direct function of the wingspan — it's not a fixed distance, it scales with the aircraft. Now, what do they do in still air? They tend to drift slowly downwards, and then they level off. That levelling-off happens usually between 500 and 1000 feet below the flight path of the aircraft. So the vortices sink, but they don't keep sinking forever — they stabilise in that band roughly half a thousand to a thousand feet beneath where the aircraft flew. And here's a striking number: behind a large aircraft, the trailing vortices can extend as much as nine nautical miles. That's the length of the wake you're dealing with — a very long, very real hazard trail behind a heavy aeroplane. Now let's look at what happens when the generating aircraft is close to the ground. Figure 5.19 shows this. If the aircraft is within 1000 feet of the ground, the two vortices will "touch down" — they actually reach the surface — and then they move outwards at about 5 knots from the track of the generating aircraft. And they do this at a height approximately equal to half the aircraft's wingspan. So the vortex cores roll along the ground, spreading sideways away from the flight path, at that 5-knot rate, sitting at a height of about half a wingspan above the surface. Now, that's in still air. But what if there's a crosswind? If the surface wind is light and steady, the wake vortex system — the one "in contact" with the ground — will drift with the wind. Figure 5.20 shows the possible effect. Here's the key interaction: you have the vortex's own outward movement of 5 knots, and you have the wind. On the downwind side, the two add together — 5 knots of wind plus 5 knots of outward drift gives you 10 knots of drift. On the upwind side, they oppose each other — 5 knots of wind minus 5 knots of outward drift gives you zero drift. So one vortex stays almost stationary over the ground while the other races away at 10 knots. That asymmetry is exactly why, with parallel runways, wake turbulence from an aircraft operating on one runway can be a potential hazard to aircraft operating from the other — the vortex can sit there, barely moving, right in the path of traffic on the adjacent runway. Finally, the decay process. What kills these vortices? Atmospheric turbulence has the greatest influence on the decay of wake vortices. And the relationship is direct: the stronger the wind — meaning the stronger the atmospheric turbulence — the quicker the decay. So turbulent air breaks the vortices apart faster; smooth, still air lets them persist longer, which is precisely when they're most dangerous. So to tie it together: separated by three quarters of a wingspan, sinking and levelling off 500 to 1000 feet below, stretching up to nine nautical miles behind a large aircraft, touching down and spreading at 5 knots when within 1000 feet of the ground, drifting with a light crosswind with that 5-plus-or-minus-5 knot behaviour, and decaying fastest in strong atmospheric turbulence. That's the full picture of how trailing vortices distribute themselves.