Probability of Wake Turbulence Encounter — Page 89, Lesson 107

Let’s start with the probability of a wake turbulence encounter, because that’s the real-world risk that frames everything else here. Air Traffic Control applies certain separation minima between aircraft. But I want you to understand this clearly: those minima do not guarantee avoidance. ATC-applied separation merely reduces the probability of an encounter to a lower level, and it may minimize the magnitude of the upset if an encounter does occur. So separation is a risk-reduction tool, not a guarantee. Now, when should you be especially careful? Particular care should be exercised when following any substantially heavier aircraft, especially in conditions of light wind. And here’s a striking fact: the majority of serious incidents close to the ground occur when winds are light. So light wind plus a heavy aircraft ahead is your highest-risk scenario near the ground. Now let’s move to wake turbulence avoidance. If you can visualize where the wake vortices are behind a preceding or crossing aircraft, then appropriate flight path control will minimize the probability of an encounter. The key tactic: staying above and/or upwind of a preceding or crossing aircraft will usually keep your aircraft out of the generating aircraft’s wake vortex. So you want to be above the vortex, or upwind of it, or both. But here’s the caution. Deviating from published approach and departure requirements in order to stay above or upwind of the flight path of a preceding aircraft may not be advisable. In other words, don’t bend the published procedures just to dodge a vortex — that deviation can create its own problems. Maintaining proper separation remains the best advice for avoiding a wake turbulence encounter. Now let’s talk about ground effect, because it changes the wake picture dramatically during landing and takeoff. When landing and taking off, the closeness of the wing to the ground prevents full development of the trailing vortices. That’s Figure 5.21. The vortices are much weaker in ground effect. Why does that happen? Because upwash and downwash are reduced. And that reduction causes the effective angle of attack of the wing to increase. Let me unpack that. The downwash behind the wing is what tilts the relative airflow downward, reducing the wing’s effective angle of attack. When the ground suppresses that downwash, the effective angle of attack goes up. So what’s the net result? When an aircraft is “in ground effect,” lift will generally be increased, and induced drag — which we write as CDi — will be decreased. Induced drag is the drag that comes from generating lift, and it’s directly tied to the downwash, so when downwash drops, CDi drops. But there’s more. The reduced downwash will also affect two things: longitudinal stability, because of center of pressure movement, and the pitching moment, because of changes to the effective angle of attack of the tailplane. That’s referenced in Figure 5.23. So ground effect isn’t just about lift and drag — it changes the aircraft’s stability and its pitching behavior too, because the tailplane sees a different effective angle of attack. Let me tie this together. The wake vortices are weaker in ground effect, which is good. But the same mechanism — reduced downwash — increases effective angle of attack, boosts lift, cuts induced drag, and shifts your stability and pitching moment. So when you’re close to the ground on takeoff or landing, you’re flying a slightly different aircraft than you are at altitude. That’s the core of this section: separation reduces risk but doesn’t eliminate it, staying above and upwind is your best visual tactic, and ground effect weakens the vortices while changing your lift, drag, and stability characteristics.