Wake Turbulence

study on wingtip vortices clearly illustrates the power of this wake turbulence component.]]

HAZARDS ASSOCIATED WITH WAKE TURBULENCE

Parallel or crossing runways

During takeoff and landing, an aircraft's wake sinks toward the ground and moves laterally away from the runway when the wind is calm. A 3 to 5 knot crosswind will tend to keep the upwind side of the wake in the runway area and may cause the downwind side to drift toward another Runway . Since the wingtip vortices exist at the outer edge of an airplane's wake, this can be dangerous.

At altitude, vortices sink at a rate of 300 to 500 feet per minute and stabilize about 500 to 900 feet below the Flight Level of the generating aircraft.

Helicopter wake

Helicopter s also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds (20 - 50 knots). Some mid-size or executive class helicopters produce wake as strong as that of heavier helicopters. This is because two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades.

HAZARD AVOIDANCE

Wake vortex separation

ICAO mandates separation minima based upon wake vortex categories, which are in turn based upon the Maximum Take Off Mass (MTOM) of the aircraft. The categories are as follows:

Light - MTOM of 7,000kg or less;

Medium - MTOM of greater than 7,000kg, but less than 136,000kg;

Heavy - MTOM of 136,000kg or greater.

There is a variety of separation criteria for take-off, landing and en-route phases of flight based upon these categories. Air Traffic Controllers will sequence aircraft making Instrument Approaches with regard to these minima. Aircraft making a visual approach are advised of the relevant recommended spacing and are expected to maintain their own separation. Common minima are shown below:-

Take-off - an aircraft of a lower wake vortex category must not be allowed to take off less than two minutes behind an aircraft of a higher wake vortex catgory. If the following aircraft does not commence its take off roll from the same point as the proceding aircraft, this is increased to three minutes.

Landing

Staying on or above leader's glide path

Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from Base To Final behind a heavy aircraft flying a straight-in approach. Light aircraft pilots must use extreme caution and intercept their final approach path above or well behind the heavier aircraft's path. When a visual approach following a preceding aircraft is issued and accepted, the pilot is required to establish a safe landing interval behind the aircraft s/he was instructed to follow. The pilot is responsible for wake Turbulence Separation . Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft.

Warning signs

Any uncommanded aircraft movements (i.e., wing rocking) may be caused by wake. This is why maintaining situational awareness is so critical. Ordinary turbulence is not unusual, particularly in the approach phase. A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter. The onset of wake can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake turbulence that they were unable to overcome. Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is vital.

ACCIDENTS/INCIDENTS DUE TO WAKE TURBULENCE

A chartered aircraft with 5 onboard, including In-N-Out Burger's president, Rich Snyder, at John Wayne International Airport on December 15 1993 . The aircraft followed in a Boeing 757 for landing, became caught in its wake turbulence, rolled into a deep descent and crashed.

USAir Flight 427 crashed near Pittsburgh , Pennsylvania in 1994 . This accident was believed to involve wake turbulence.

American Airlines Flight 587 crashed into the Belle Harbor neighborhood of Queens , New York shortly after takeoff from John F. Kennedy International Airport on November 12 , 2001 . This accident was attributed to pilot error in the presence of wake turbulence that resulted in rudder failure and subsequent separation of the vertical stabilizer.

MEASUREMENT

Wake turbulence can be measured using several techniques. A high resolution technique is Doppler Lidar , a solution now commercially available. Techniques using Optics can use the effect of turbulence on Refractive Index ( Optical Turbulence ) to measure the distortion of light that passes through the turbulent area and indicate the strength of that turbulence.

EXTERNAL LINKS

FAA overview

Information About

Wake Turbulence