Wake Turbulence — Page 192, Lesson 284

I want to walk you through wake turbulence — one of the most critical hazards you'll manage as a professional pilot. Let's start with the definition. The term wake turbulence describes the effect of rotating air masses generated behind the wing tips of jet aircraft. These rotating air masses are called wake vortices, and they're present behind all aircraft, but they're particularly severe when generated by large aircraft. Now, why should you care? Wake vortices are most hazardous to aircraft with a small wingspan during take-off, initial climb, final approach, and the landing phase. So if you're flying a smaller aircraft behind a heavy one, or if you're the heavy aircraft and a smaller one is following you, this matters a lot. What determines the strength of these vortices? The characteristics of the vortex are determined by four factors: the aircraft's gross weight, the wingspan, airspeed, and attitude. The greatest turbulence is produced by heavy aircraft flying slowly in a clean configuration — that means no flaps or gear extended to disturb the airflow. So picture a heavy jet on approach, clean and slow — that's the worst-case generator. Over time, the effects disperse. The localized effect of the vortex spreads and loses intensity. Practically speaking, we can think of the vortex patterns from an aircraft as two counter-rotating cylindrical air masses trailing from the aircraft. They rotate in opposite directions — one clockwise, one anticlockwise, depending on which wing tip they come from. Here's the geometry. Typically, the two vortices are separated by about three-quarters of the aircraft's wingspan. In still air, they tend to drift slowly downwards and either level off — usually not more than 1000 feet below the flight path of the aircraft — or, on approaching the ground, they move tangentially at about 300 feet per second from the track of the aircraft, again in still air. That sideways speed then decays to an average of 5 knots. When does wake vortex generation start and stop? It begins when the nose wheel lifts off the runway on take-off, and it ceases when the nose wheel touches down again on landing. So from rotation to touchdown, the vortices are being produced. Now, crosswind changes the picture. In a crosswind situation, the outward path of the upwind vortex will be opposed by the crosswind, while the downwind vortex will be assisted in its outward dispersal. Here's a critical number: if the crosswind component is 5 knots, the upwind vortex will remain effectively stationary, virtually underneath the flight path of the aeroplane. That's dangerous because it doesn't drift away — it sits right where the next aircraft might be. Windshear close to the ground can cause the vortices to descend at different rates and even cause one vortex to rise. Atmospheric turbulence and high winds close to the ground cause vortices to decay more quickly. So the conditions that require additional attention are light winds — because the vortices hang around longer. What about helicopters? Wake vortices from helicopters are generated by the down-wash of the main rotor, where they transform into a pair of trailing vortices similar to those of fixed-wing aeroplanes. Evidence suggests that per kilogram of aeroplane mass, helicopter vortices are more intense than those of fixed-wing aeroplanes. So a helicopter of the same weight as a fixed-wing aircraft produces stronger vortices. Now, ICAO defines wake turbulence categories of aeroplanes by maximum take-off mass. There are three categories. Heavy means a maximum take-off mass equal to or greater than 136,000 kilograms. Medium means less than 136,000 kilograms but more than 7,000 kilograms. Light means 7,000 kilograms or less. You can see this in figure 14.1. Separation minima — the minimum distances required between aircraft — are applied by distance for both take-off and landing, and where complex runway arrangements are in use. These minima shall be applied to an aircraft on approach and departure phases of flight when an aircraft is operating behind another aircraft at the same altitude or less than 300 metres — that's 1000 feet — below. So if you're within 1000 feet below the preceding aircraft, wake turbulence separation applies. We'll cover the exact separation distances in a moment, but that's the altitude trigger.