# Why do wing tip vortices tend to be most dangerous when there is a light quartering tailwind?

I understand how wing tip vortices be effected under a headwind or crosswind. However, what are the differences in effect between tailwind and light quartering tailwind?

Why is it most dangerous when there is a light quartering tailwind?

According to the FAA's Pilot and Air Traffic Controller Guide to Wake Turbulence (section 2.4.5) it's because:

• Light: in light winds the vortices stay around longer; if the wind is greater than 5 knots then the vortices move out of the way quickly and break up
• Quartering: crosswinds blow the vortices across the aircraft's flight path, which means that if you're landing next then one vortex might still be over the runway. With no crosswind at all, the vortices will dissipate on each side of the runway, out of your way.
• Tailwind: the vortices are blown in the same direction as the aircraft, which means that the vortices from a landing aircraft will be blown down the runway rather than away from it. Again, that's an issue for the next aircraft landing.

They have a handy diagram to illustrate the vortex paths from an aircraft at 0, 3 and 6 knots. You can see that a light, 3-knot crosswind causes one vortex to still be in the flightpath after 20 seconds, whereas with 0 or 6 knots of wind the vortex is out of the flightpath completely. You can't see tailwind on this diagram, of course, but the principle is the same: it blows the vortex down the runway (flightpath) rather than away from it.

The wing tip vortices cause significant problems for (small) aircraft taking off after a large aircraft. the FAA Advisory circular 90-23G deals with this.

As an aircraft takes off, the the vortices sink behind the aircraft and move laterally over the ground.

In case of no crosswind, the vortices simply move laterally away from the runway at low speed.

Assume that the aircraft is landing after another one. However, if there is a tailwind, the trailing vortices of the preceding aircraft can be moved into the final approach of the landing aircraft.

In case there is a crosswind, the lateral movement of the upwind vortex is decreased while that of the downwind vortex is increased. This means that the upwind vortex remains in the runway longer than it would be.

Thus a quartering tailwind presents the worst of both worlds: It move the upwind vortex right in the path of the incoming aircraft and keeps it there longer than usual. The situation is similar for takeoffs.

Source: www.gleim.com

This is the reason it is considered to be the most dangerous situation.

Very simply:

• A strong tailwind would break up the vortices more quickly.
• Quartering, as in not straight on, means the upwind vortex is blown into the path of the next aircraft.