Wake Turbulence: (Ref: AIC P 072/2010) — Page 89, Lesson 104

I want to walk you through wake turbulence, and I'm going to start with the official reference that governs this in the UK — it's AIC P 072/2010. That's an Aeronautical Information Circular, and it's the document that lays down the operational guidance on this hazard. Let's begin with the fundamental picture. Every aircraft, as it flies, sheds trailing wing tip vortices — these are rotating masses of air that stream off the wing tips and extend behind the aircraft for a considerable distance. And I want to stress the word hazard here, because these vortices can present an extreme hazard to any aircraft unfortunate enough to encounter them. The danger is real and quantified: the maximum tangential airspeed in the vortex system — that's the speed of the rotating air itself, measured around the vortex core — may be as high as 90 metres per second, which is 300 feet per second, immediately behind a large aircraft. That's a violent rotational flow, and it can flip or structurally damage a smaller aircraft that flies into it. Now here's the critical operational problem: wake turbulence cannot be detected. There's no instrument on board that shows you where the vortices are. So because you can't see them, it's essential for pilots to be aware of two things — the potential distribution and duration of the trailing vortices, and also the modifications made to the "classic" vortex system by surface wind speed and direction. In other words, you have to know where the vortices are likely to be, how long they'll persist, and how the wind near the ground will move them around. Let's look at when this vortex generation actually happens. Wake vortex generation begins when the nose wheel lifts off the runway on take-off, and it continues until the nose wheel touches down on landing. So the moment the aircraft is rotating and the nose wheel leaves the ground, the vortices start; the moment the nose wheel touches back down on landing, they stop. Wake vortices exist behind every aircraft — and I want to include helicopters in that — whenever they're in flight. But they are most severe when generated by heavy aircraft, and that's the key word: heavy. The greatest danger comes during specific phases of flight — take-off, initial climb, final approach, and landing. Why those phases? Because that's low altitude, where large numbers of aircraft congregate. You've got aircraft climbing out, aircraft descending, all in a confined airspace near the ground. And that's what makes an encounter so dangerous. A wake turbulence encounter is a hazard for two reasons: potential loss of control, and possible structural damage. And if that encounter happens near the ground, there may be insufficient time and/or altitude to recover from an upset. That's the killer scenario — you get rolled or pitched by the vortex, and you simply don't have the height to recover before you hit the ground. Now let's get into the characteristics of these trailing vortices, because they're determined by five factors of the "generating" aircraft — the aircraft that's producing the wake. I want to go through each one carefully. First, gross weight. The higher the weight, the stronger the vortices. A heavy aircraft produces more lift, and more lift means more vigorous trailing vortices. Second, wingspan. The wingspan has an influence upon the proximity of the two trailing vortices — that's the distance between the two counter-rotating vortices that trail from each wing tip. A wider span spreads them further apart. Third, airspeed. The lower the speed, the stronger the vortices. This is a crucial one — a slow, heavy aircraft is the worst generator, because it's producing a lot of lift at low speed, which means intense vortices. Fourth, configuration. Vortex strength is greatest with the aircraft in a "clean" configuration, for a given speed and weight. "Clean" means the flaps and landing gear are retracted. So a clean aircraft, at the same speed and weight, produces stronger vortices than one with flaps extended. And fifth, attitude. The higher the angle of attack, the stronger the vortices. Angle of attack is the angle between the wing's chord line and the relative airflow — the higher it is, the more lift is being generated, and the stronger the vortices become. Now, there's a general rule that ties the hazard to the encounter: the larger the generating aircraft relative to the aircraft encountering the wake turbulence, the greater the hazard. So a small aircraft following a large one is in the most danger. And there's one more important point — a specific contrast. There is evidence that for a given weight and speed, a helicopter produces a stronger vortex than a fixed-wing aircraft. So don't assume a helicopter is harmless just because it's not a large fixed-wing jet — at the same weight and speed, its rotor downwash generates an even more powerful vortex. So the whole picture is this: you can't see the vortices, they're most violent behind heavy, slow, clean, high-angle-of-attack aircraft, they persist from nose-wheel lift-off to nose-wheel touchdown, and they're most dangerous at low altitude where you have no room to recover. That's the foundation of wake turbulence awareness.