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Sci Tech
Turbulence of a different kind
ALL AIRCRAFT generate wake turbulence. But heavier the aircraft severe is the turbulence. Wake turbulence describes the `effect of two rotating air masses generated behind the wing tips of large jet aircraft.' The terms `wake vortex' and `wake vortices' are other terms of `wake turbulence,' which describes the nature of air masses. Such wake vortices are two counter-rotating cylindrical air masses trailing from the aircraft, which are generated the moment the aircraft lifts from the ground and ends when it touches down.
Wake Vortex is a by-product of lift and is present behind every aircraft in flight. Lift is generated by the creation of a pressure differential over the aircraft wing surfaces. The pressure differential over the wing (where low pressure occurs) and under the wing (where high pressure occurs) triggers the rollup of the airflow aft of the wing, resulting in rotating air masses trailing down stream of the wing tips. Once the aircraft is airborne, two counter rotating cylindrical vortices are created, which are hazardous to the following aircraft, especially during take off, initial climb, final approach and landing.
Close to ground, the wake vortices tend to drift down and move sideways from the track of the generating aircraft but may rebound upwards as well.
The effects of wake turbulence on an aircraft causes Induced roll; Loss of height; and Structural stress.
Out of these three, induced roll is considered to have most dangerous effect on aircraft. It is especially dangerous during take-off and landing when there is little altitude or speed for recovery. The tests conducted by NASA have shown that the capability of an aircraft to counteract induced roll primarily depends on wingspan and counter control responsiveness.
The strength of the vortex is directly proportional to the wingspan, weight and speed of the aircraft. However, heavier the aircraft, the stronger the wake vortex. In other words, the `strongest vortices are produced by heavy aircraft flying slowly with flaps extended.' More over, the strength of the vortex diminishes with time and distance behind the generating aircraft.
Flight tests have shown that cortices sink at a rate of about 2-2.5 m/sec (400-500 ft min) and tend to level off at about 275 m (900 feet) below the flight path of the generating aircraft.
A vortex circulation is outward, upward and around the wing tips when viewed from either ahead of or behind the aircraft.
`These vortices, remain spaced, about a wing span apart, and drift with the wind at altitudes greater than a wing span from the ground.' When an aircraft encounters wake turbulence, a slight change of altitude and lateral position may provide a turbulent free flight path.
Vortices break up in one of the following three ways.
Turbulent Diffusion: This occurs over a long period of time when each vortex gets enlarged, merges and then dissipates.
Simuous Oscillations: Disturbances along the length of the vortex, become unstable and the vortices touch and link together.
Vortex Break-down: The vortices burst suddenly and the vortex core is widened.
At times a pilot may not even know that he is encountering a wake. It may be one or more jolts with varying severity depending on the direction of encounter and point of encounter (a moderate wake during final stages of approach may be more dangerous than a severe wake encountered during cruise). It also depends on the distance from the generating aircraft and category of the generating aircraft.
A pilot suspecting a wake should get away by using the following guidelines.
Takeoff: Lift off short of large aircraft rotation point.
Climb: Climb above the preceding aircraft's flight path, if you can. Otherwise, a slight deviation with a parallel climb to the preceding aircraft may help avoidance of wake. Crossing behind and below the preceding aircraft shall be avoided at all times.
Crossing or Following: If crossing the flight path of a preceding aircraft is unavoidable, cross above the flight path or at least 1000 feet below (keeping in mind the minimum flight altitude and terrain clearance).
While following, stay above the flight path or at least 1000 ft below.
Approach: Maintain adequate lateral separation with a position on or above the preceding aircraft's flight path. (may not be possible on intermediate or final approach, in which case a longitudinal separation shall be applied as suggested below in `separation minima').
Landing: The touch down point shall be beyond the preceding aircraft's touchdown point, or land well before a departing aircraft's rotation point.
Crossing approaches: During crossing approaches, cross above the flight path of preceding landing aircraft.
Helicopters: They may produce strong vortices when compared to fixed wing aircraft of same weight. However, vortex sensing systems, like Laser Doppler Velocimeter, Monostatic Acoustic Vortex, Sensing System, Anemometer Windline facilitates better understanding of wake vortex and to modify the applicable separation standards thereby increasing the airport capacity. These systems provide information on the presence and strength of the wake vortices and hence it becomes important to both pilots and air traffic controllers.
Capt. N. P. Puri and Ramiah Saravanan
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