Every aircraft generates wake turbulence from the moment the wing produces lift and the aircraft leaves the ground until it lands. This wake is also called wingtip or wake vortices. It forms when an aerofoil generates a pressure difference where the lowest pressure is over the upper surface and the highest pressure under the wing.
Wake turbulence affects all airplanes of all sizes and shapes. We as the pilot in command need to be aware of it because these incidents involves all types especially on an approach to the airport in conditions where the pilot is using visual separation.
As air will always flow from a high pressure area to an area of lower pressure it will move outwards under the wing from the root toward the wingtips where it will curl upwards, thus the wake is formed and it trails behind and slightly below the aircraft from both wingtips.
By the same process described above the air will flow inwards to the wing root on top of the wing. These air flows (inboard above and outboard below) meet at the trailing edge of the wing where they also form vortices. When viewing an aircraft from behind the left vortice will rotate clockwise and the right vortice anticlockwise. Vortices rotate around a core and are a few feet to several meters in diameter, depending on the size, configuration and speed of the aircraft and will spread laterally and descend behind and persist for a few minutes, sometimes longer in certain conditions.
Effects of wake turbulence
These are felt in roll and yaw and are the most dangerous during takeoff and landing where there is not enough altitude to recover. The result of the wake on your aircraft depends, among other things, on the wingspan and distance from the preceding aircraft. Sometimes only felt as rocking like flying through normal turbulence. In severe cases a total loss of control will be the result, recovery from this will depend on pilot skill, altitude, maneuverability and engine power.
Flight tests have shown that if a light aircraft (compared to the one generating the wake) flies into the wake it can experience high roll rates, sinks in excess of 1000 ft/min and if you are not high enough, a tragic result.
Influenced by the weight of the aircraft, speed and angle of attack and wing configuration. The conditions in the atmosphere have their effect how long the wake will persist. A light crosswind (3 to 5 Kts) could result in one wake to remain stationary, for example over the runway, and the other wake will be blown away possibly to a parallel runway.
The wingspan and weight of an aircraft also have a strong influence in the size of the wake. With two aircraft of the same weight but with different wingspan the one with the smaller wingspan needs to generate more lift and the result will be a stronger wake.
There are three categories in which airplanes are divided: light, medium and heavy. This is done so that ATC can apply separation for wake turbulence. The light category are aircraft up to 7000 kg (Cessna 402, Metro 3), medium is up to 136.000 kg (Boeing B757 B737, Dash-8, Beech 1900D) and the heavy category are all over 136.000 kg (Boeing B747 B767 B777, Airbus A340 A380, MD-11). Weights are MTOW or MCTOW.
Air Traffic Control will apply separation distances to IFR aircraft except to VFR following a medium or heavy aircraft or IFR on a visual approach and were the pilot reported the previous traffic in sight and follows and maintains own separation. Controllers will then issue a 'caution wake turbulence' advisory and the pilot is responsible to keep clear.
Separation can be done in distance or in time. During cruise, distance is used and on arrival and departure time is used for separation. Minimum separation distance depends on the weight category of the aircraft:
|Leading Aircraft||Following / Crossing Aircraft||Separation Distance||Separation Time|
|Heavy||4 NM||2 minutes|
|Heavy||Medium||5 NM||2 minutes|
|Light||6 NM||3 minutes|
|Medium||Light||5 NM||3 minutes|
The PIC is responsible for a safe flight. If you feel that the separation for wake turbulence is not adequate, do request a different vector or more separation or even a different altitude. You can also take own responsibility and maintain own separation to keep clear. Be very careful if you do so.
At uncontrolled aerodromes it is easy to forget about wake turbulence. But do remember that even a C-208 (light category) can be operating from the same aerodrome as ultralights and that the wake from a C-208 will be too much to handle by such an aircraft.
Basically there are three things to remember: Do not get too close, do not fly below the leading aircraft flight path and stay alert when light crosswinds exist during take-off and landing.
During take-off and landing, note where the larger aircraft rotates and lands. This is the point where the wake vortices are and make sure that you remain clear by taking off before or landing beyond that point. Also on parallel runways the downwind wake can drift to your runway due to a crosswind. Crossing active runways will have the same problem. If you do take a visual approach behind a larger aircraft fly slightly to the upwind side of the localizer/runway centerline to keep well clear.
If during flight you cross the flight path of a larger aircraft, try to fly above its altitude or if terrain permitting fly an altitude at least 1000 feet lower.
If you do encounter wake turbulence and have enough altitude to try to recover then the POWER-PUSH-ROLL method of Fighter Combat International is advisable. Increase POWER, PUSH to unload the wing until you feel light in the seats. This will reduce the angle of attack. Then ROLL in the direction that will unload the wings or to the horizon. Use rudder to keep the aircraft balanced.
It is an advanced technique and may not work when too low to the ground were there is just not enough room to maneuver.