Wake turbulence avoidance techniques

This month’s article deals with the problem of wake turbulence. A particularly dangerous phenomenon, wake turbulence is extremely common and all pilots need to be cautious when operating near aircraft that are producing a wake.
Specifically, a large aircraft producing lift sufficient for flight will create wake turbulence sufficient to upset a small aircraft. Since most of us fly light aircraft, operations near a large aircraft’s wake should be avoided when possible. Many accidents and incidents have occurred when a light aircraft was upset by the wake of a larger aircraft. In fact, several incidents have been reported where medium size jet aircraft were upset by the wake of larger jets such as the Boeing 757. These occurrences, for the most part, could have been avoided if some basic precautions had been taken. Although wake turbulence is dangerous if encountered at any time during flight, there are two phases of flight in which a wake turbulence encounter is most dangerous: takeoff and landing.
It is important to understand exactly what causes wake turbulence in order to be best prepared to avoid it. Specifically, as the wing of an aircraft produces lift, a pressure differential is created between the upper and lower surfaces of the wing. This differential causes a rolling of the air upwards from the high-pressure area underneath the wing towards the lower pressure area above the wing. This "rolling" process is what is referred to as a wing-tip vortex. While the vortex is not dangerous to the aircraft creating it, the vortex trails behind the aircraft and can create a hazard for aircraft in its path. These areas of rolling air, or vortices, will extend for quite some distance behind an aircraft and the strength of the vortex will determine how far behind the aircraft it might be encountered. Several factors play a part in determining the strength of the vortices such as aircraft weight and how much light the aircraft is producing.
Production of wing-tip vortices is synonymous with the existence of wake turbulence. Therefore, as mentioned earlier, an airborne aircraft is producing wake turbulence. Since wake turbulence is usually invisible to the naked eye, knowledge of the preceding aircraft’s position is critical to the safe operation of your aircraft. If a large aircraft is going to takeoff of your runway prior to your departure, note the position of the large aircraft’s liftoff if possible. If you can determine where it broke ground, plan your liftoff to occur prior to the large aircraft’s liftoff point. Climbout should be at an angle exceeding the previous aircraft’s angle so as not to pass through their wake. If this is not possible, climb on the upwind side of their flight path to minimize the possibility of a wake encounter. Likewise, if you are to land behind a large aircraft, plan to touchdown beyond that aircraft’s point of touchdown. It is also wise to make your approach path to the runway slightly above that of the preceding aircraft.
While airborne wake turbulence encounters are not as dangerous because sufficient altitude usually exists for recovery, they can none the less make for a wild ride. A five-mile minimum distance and flight path above a large aircraft ahead will minimize the chances of an encounter. If you should fly into the preceding aircraft’s wake inadvertently, be prepared to recover from an unusual attitude, possibly even an inverted attitude.
Earlier I said that wake turbulence is usually invisible. However, there are a couple of telltale signs that wake turbulence is present. At landing fields where dust or grit is present on the runway, the location of the swirling wing vortices can usually be seen clearly. In cases where they are visible, it’s a great idea to avoid flying anywhere near them. Since the wake will not be visible in most cases, it is wise to consider the wind conditions and how they will affect the longevity of wake turbulence near the ground. Specifically, calm winds will allow a steady dissipation of the vortices to each side of the runway. A crosswind can create a particularly dangerous situation in that the upwind vortice may remain near or over the runway for an extended period of time. In this case, delaying landing or picking an alternate runway is wise. Parallel runways have unique problems in that vortices can drift to the adjacent runway if the strong crosswinds exist, so when picking an alternate runway, a crossing runway is usually a better choice. If in doubt, delaying a takeoff or landing for three to five minutes is usually sufficient to minimize the risk of flying through someone else’s wake.
Wake turbulence is a real problem all over the world, and your job is to avoid it if at all possible. If you take the time to carefully plan departures and arrivals to avoid following large aircraft the risk of a wake turbulence encounter will be minimized.