Wake Turbulence: An Invisible Hazard

Photo: NASA Langley Research Center

Approaches Gone Wrong

In January 1995, the pilots of a Boeing 737 operated by Southwest Airlines reported experiencing an uncommanded roll, first 20 degrees to the left and then 30 degrees to the right, before recovering to straight and level flight. Five people were injured due to the aircraft's abrupt movement. The NTSB determined that the cause of the uncommanded roll was an encounter with wake turbulence after the pilots of the 737 followed an MD-80 on the approach to land.

In June 2006, the pilot of a Piper Saratoga was flying an approach to runway 01L in Kansas City. About two minutes ahead of his flight path, a Boeing 737 was flying a parallel approach. The aircraft was traveling faster than the recommended maneuvering speed, which isn’t normally a problem in smooth air, but can cause structural damage in turbulence. The Piper Saratoga encountered wake turbulence and the aircraft broke up in flight, killing the pilot.

In December 2012, a Citation 550 crashed left of the threshold on runway 17L at Will Rogers World Airport in Oklahoma City after experiencing an uncommanded 60-degree roll to the left. The business jet was following an Airbus A300 on approach to land. Another pilot that witnessed the event described the Citation as being about one minute behind the Airbus on the approach, leading investigators to believe that wake turbulence is to blame.

And then there's this video of a Cirrus being flipped during a landing, just about 30 seconds after the departure of a Blackhawk helicopter.


An Invisible Hazard

These aircraft all share one thing in common: An invisible hazard known as wake turbulence, which can quickly cause a normal airplane to become uncontrollable, and even break into pieces, during flight. It’s a phenomenon that acts a bit like a small, invisible tornado, and, as the accidents above illustrate, can cause severe damage to aircraft.

It’s easy to assume that what we can’t see doesn’t exist, and many pilots enjoy a false sense of security by assuming that air traffic controllers wouldn’t possibly maneuver them into wake turbulence. While it’s probably true that an air traffic controller won’t intentionally direct an aircraft into wake turbulence, it’s also true that controllers are only responsible for wake turbulence avoidance for IFR aircraft. It's the pilot's responsibility to avoid wake turbulence avoidance during VFR flights. In addition, controllers get busy, forget and otherwise make human errors like the rest of us. And it’s also true that wake turbulence patterns can be unpredictable. Pilots need to be on guard and aware of the hazards surrounding wake turbulence. 


What is Wake Turbulence?

Wake turbulence is disrupted air coming off an aircraft's wingtips, and it's a natural result of any aircraft in flight. The turbulence is caused by counter-rotating disturbances of the air that trail off the each of the outboard edges of the wings during flight. The difference between lower-pressure air on top of the wing and the higher-pressure air beneath the wing causes a significant and distinct swirling air mass at the edges of the wings near the wing tips.

When generated by large, heavy aircraft, the force of these wingtip vortices can often exceed the available control forces of a smaller aircraft that crosses its flight path, resulting in a loss of control and potentially an in-flight break up of the smaller aircraft.


Characteristics of Wingtip Vortices

Wingtip vortices tend to flow outward, upward and around the tips of the wings. The strength of each vortex depends on the weight, speed and configuration of the aircraft. The strongest wake turbulence is found behind a heavy, slow and clean aircraft. (Gear and flaps can disrupt and weaken the vortices), and these disturbances have been known to occur at speeds of up to 300 feet per second.

Wake turbulence often occurs during the approach, takeoff and landing phases of flight, but can also 

be encountered in the enroute phase, especially in areas that multiple aircraft frequent, such as over VORs, near fixes and in a holding patterns.

Atmospheric disturbances, normal turbulence due to ground heating, and high winds weaken wingtip vortices, while smooth, calm air allows the vortices to remain in the same place, or nearby, for minutes at a time without diffusing. This is one case in which unstable air and atmospheric turbulence is actually a good thing.


Avoiding Wake Turbulence: Pilot and Controller Teamwork

While air traffic controllers are responsible for wake turbulence separation and avoidance for IFR flights, pilots are ultimately responsible for avoiding wake turbulence in VFR flight environments. Hazardous situations can occur when a pilot cancels his IFR clearance once the runway environment is in sight on a clear day, and visually follows another aircraft during the approach. The controller isn’t required to provide separation standards any longer in this case, and the pilot, focusing his attention on the approach, might not pay attention to the aircraft in front of him.

Pilots should remain aware of the characteristics of wake turbulence and their own recommended separation minimums for following a large aircraft to avoid putting the aircraft in a hazardous situation.

Pilots are encouraged to request minimum separation times from ATC, which can very between three minutes and eight minutes depending on the types of aircraft involved.

In addition, steps can be taken to avoid steering the aircraft directly into the path of the wingtip vortex. For example, a pilot should always fly above the flight path of a larger aircraft, and should avoid flying below or downwind of a large aircraft. And should a controller clear an airplane to land behind a larger jet, the pilot of the smaller airplane should ask for a deviation in order to remain at least three minutes - sometimes longer - behind the larger aircraft.


Rules of Thumb for Pilots

When departing behind a larger airplane, a pilot should rotate at a some point on the runway located before the larger aircraft’s point of rotation.

If a pilot is landing after a larger aircraft departs the same runway, the landing pilot should maneuver to touch down at least three minutes after the departing aircraft left the runway, and should land well before the larger aircraft’s rotation point.

When landing behind a large aircraft, the smaller airplane should touch down beyond the larger aircraft’s touchdown point.

Air traffic controllers can issue a cautionary advisory to VFR aircraft when landing behind a larger airplane by saying “Caution, wake turbulence.” This warning doesn’t require any action on behalf of the pilot, and the pilot is still responsible for maintaining proper separation from the other aircraft. If necessary, a pilot can request time and speed updates regarding a preceding airplane, and should adjust his flight path, either by slowing down to allow for more time to go by, and/or by following the above procedures for landing or departing behind a larger aircraft.


Separation Standards

Aircraft are separated by air traffic control on the basis of their weight classification.

A large aircraft can follow another large aircraft of the same type and weight without much of a hazard, while a light aircraft is separated from a landing heavy aircraft in front of it by eight minutes.

FAA aircraft separation standards require a minimum of four minutes to up to eight minutes, depending on the weight categories involved.


Wake turbulence events like the ones described above, and many others, are mostly preventable. But it’s up to pilots and air traffic controllers to work together to ensure that adequate separation is provided in all cases in order to keep everyone safe and prevent accidents and incidents. 


Source: FAA AC 90-23G