“Cessna 1234A cleared for takeoff, caution wake turbulence from the departing Citation jet.” It’s a common warning at controlled airports where light planes mingle with jets and airliners. Encountering wake turbulence at low altitude immediately after takeoff is a well-known danger that can have fatal results. But wake turbulence is increasingly recognized as a danger to all aircraft, at all levels.
On January 7, 2017 a Challenger 604 business jet passed 1,000 feet below an Airbus 380 super wide body traveling the same airway in the opposite direction over the Arabian Sea. Shortly afterwards the Challenger entered a rapid, un-commanded roll and pitched sharply downwards. Four passengers were thrown about the cabin and injured. The aircraft executed several complete rolls, lost 9,000 feet of altitude, and one engine flamed out. The cockpit crew succeeded in regaining control of the aircraft, re-started the failed engine, declared an emergency and landed safely at Muscat Airport. The cabin attendant, though also injured, administered first aid to the passengers, who were hospitalized upon arrival. The Challenger was determined to have exceeded its design structural limits, and was declared a total loss.
The dangers of wake turbulence are well known. It is caused by the vortexes generated where air under the wing twists upward and outward at the wingtip, producing a clockwise vortex from the left wing and a counterclockwise vortex from the right wing, when viewed from behind. The vortexes descend at approximately 400 feet per minute, and remain approximately one wingspan apart.
The wake turbulence hazard has historically been most associated with small aircraft at low altitudes following larger airline or general aviation jet aircraft. A low altitude wake turbulence encounter can be fatal for a light aircraft, which lacks the control power to cope with the vortex effects of the larger aircraft. However, low altitude wake turbulence is also a hazard to air transport aircraft of similar size. On July 24, 2017, a Westjet Boeing 737-800 departing Los Angeles following an Airbus A321 encountered wake turbulence which caused an un-commanded roll and autopilot disengagement between 500 and 900 feet above the ground.
High altitude en route wake turbulence encounters have also become a more common and recognized hazard, particularly since the introduction of the very large Airbus A380. Wake turbulence from such an aircraft can persist as far as 25 miles behind it. Two recent NASA reports have highlighted the A380’s wake hazards. In March 2017, a westbound CRJ700 that flew under an eastbound A380 experienced wake turbulence which injured a cabin attendant approximately 18 miles behind the A380. And in July 2016, an MD-80 flying at 33,000 feet encountered wake turbulence that caused un-commanded 30 degree roll and autopilot disengagement about 15 miles behind an A380 climbing from 31,000 to 35,000 feet.
Wingtip vortexes typically level off about 900 feet below the altitude flown, close to the 1,000 foot minimum separation currently allowed between en route aircraft flying in opposite directions. An international steering group formed to assess A380 wake turbulence recommended that aircraft should be aware of possible encounters up to 20 miles behind and 1,000 feet below such large wide body aircraft when flying the same airway.
Pilot reactions can aggravate the effects of wake turbulence. An FAA advisory circular points to a history of incidents in which pilot inputs exacerbated situation caused by the encounter. It suggests that if altitude and conditions permit, it may be better to allow the aircraft to transition through the wake, and then recover from the resultant unusual attitude, rather than applying aggressive control inputs during the encounter. Upset training dealing with proper recovery from such conditions is becoming an established part of airline training programs. And the National Aeronautics and Space Administration has sponsored research into new cockpit displays and software designed to give pilots guidance about control inputs and use of power in recovering from upsets.
Although wake turbulence has been a recognized hazard since the early days of aviation, its challenges continue to persist and grow. The good news is that government, the airlines, the industry, and academia have recognized the challenges, and are working collaboratively to address them.