Wake turbulence can be dangerous at any altitude or phase of flight. Know how to visualize its location – and what to do if you find yourself in an upset.
There’s an Alert Area over my home airport on the northeast side of Wichita, Kansas. There are no restrictions to civilian IFR or VFR flight in an Alert Area; they exist, according to definition, “to inform nonparticipating pilots of areas that contain a high volume of pilot training operations, or an unusual type of aeronautical activity that they might not otherwise expect to encounter. Pilots are advised to be particularly alert when flying in these areas.” Alert Area A-683 warns pilots about extensive visual traffic at McConnell Air Force Base, most notably, KC-135R heavy tankers at this, one of the U.S. Air Force’s so-called “supertanker bases.”
Every time I look up and see a tanker fly in McConnell’s visual pattern that take it almost directly over the airport where I’m based, or I sight a KC-135 on upwind, downwind or base leg when I’m in the air, I think not only about traffic avoidance, but also about avoiding that airplane’s very significant wake turbulence as well.
Some pilots might not realize that when you are in visual meteorological conditions you are responsible not only for avoiding a collision with other airplanes, you are also required to maneuver to avoid encountering that airplane’s wake. If you are IFR in instrument meteorological conditions controllers will provide separation from airplanes and the expected location of their wake turbulence. But even if you’re IFR in VMC, or ATC advises you of traffic and you report it the traffic sight, you assume responsibility for avoiding the airplane and its wake turbulence, too.
Most instruction and guidance about wake turbulence avoidance focuses on avoiding wakes during takeoff and landing. Certainly, that’s where you’re most likely to have a wake turbulence encounter, because you are sharing a very small airspace with other airplanes (and the air they disturb). Comparatively little time is spent on training teaching and reviewing how to avoid wake turbulence encounters away from the runway. Yet as I said earlier, it’s our responsibility to avoid wake turbulence anywhere it may exist in visual conditions.
A Canadair Challenger 604 was cruising at FL340 over the Gulf of Oman in January 2017. An Emirates Airways Airbus A380, en route from Dubai to Sydney, Australia, passed overhead at FL350, 1,000 feet higher than the business jet. The crew of the Challenger was quoted by FlightServicesBureau.org as saying: “A short time later (one to two minutes) the aircraft encountered wake turbulence sending the aircraft into an uncontrolled roll, turning the aircraft around at least three times (possibly even five times). Both engines flamed out [and] the aircraft lost about 10,000 feet [before the crew] was able to recover the aircraft, restart the engines and divert to Muscat.
The aircraft received damage beyond repair due to the G-forces [encountered], and was written off.”
Anyone who has ever flown a 360-degree, level steep turn and hit a bump at the end knows even airplanes as small as a Cessna 150 leave a wake of disturbed air behind them. When the airplane trailing a wake is bigger than the one you’re flying, its wake turbulence may be strong enough to upset your airplane – or worse.
So how do wake vortices behave? What strategies can we use to avoid them, in climb, cruise and descent as well as during takeoff and landing?
Wake Turbulence Advisory
FAA Advisory Circular (AC) 90-23G provides specific information about wake turbulence formation and behavior, as well as suggesting avoidance techniques. Let’s summarize and comment on the highlights of this guidance.
Lift is generated by pressure differential above and below the wing surfaces…the lowest pressure over the upper wing surface and the highest pressure under the wing. This pressure differential results in swirling air masses trailing downstream of the wingtip. The wake consists of two counter-rotating cylindrical vortices (AC-90-23G figure 1). The strength of the vortex is governed by the weight, speed, wing shape and wingspan of the generating aircraft. The extension or retraction of flaps, slats or other wing configuring devices will change the vortex characteristics of an aircraft. However, most significantly the vortex strength increases with an increase in aircraft operating weight or decrease in aircraft speed. The greatest vortex strength occurs when the generating aircraft is heavy, slow and clean (flaps and slats retracted) since the turbulence from a “dirty” aircraft configuration hastens wake decay.
Flying around Wichita with a KC-135 in the McConnell pattern? Until it turns final, the tanker is fairly heavy, flying fairly slow, and has its flaps and other wing devices retracted, generating its greatest vortex strength, according to the FAA. AC 90-23G continues:
Air density is also a factor in wake strength. Even though the speeds are higher in cruise at high altitude, the reduced air density may result in wake strength comparable to that in the terminal area. In addition, for a given separation distance, the higher speeds in cruise result in less time for the wake to decay before being encountered by another aircraft.
Since both a traffic pattern encounter and an en route, high-altitude confrontation may result in significant vortex strength, we need to know how wake turbulence behaves after it leave the airplane’s wingtips.
Where Does It Go?
The horizontal tornadoes that roll off an airplane’s wingtips when lift is generated have been found to exhibit predictable behavior:
The vortex circulation is outward, upward, and around the wing tips when viewed from either ahead or behind the aircraft. The vortices remain spaced slightly less than the distance of the generating airplane’s wingspan apart, drifting with the wind. Vortices sink at a rate of several hundred feet per minute (fpm), slowing their descent and diminishing in strength with time and distance behind the wake-generating aircraft. Atmospheric turbulence hastens decay.
The worst-case atmospheric conditions are light winds, low atmospheric turbulence and a stable atmosphere. En route in these conditions, vortices can descend more than 1,000 feet. In rare cases, wake turbulence can rise in an updraft or when it bounces off the top of a strong inversion layer.
If the air is indeed stable, the wake turbulence behind a heavy airplane will take as much as 5 miles to make the 1,000-foot descent. The vortices will then linger at that lower altitude, drifting downwind, until the turbulence eventually dissipates. AC-90-23G Figure 5 shows some of the behavior of wingtip vortices.
Caution: Wake Turbulence
Air traffic controllers will provide a “Caution: Wake turbulence” advisory call any time they feel wake turbulence will be an adverse factor for your airplane. This is normally a one-time advisory – the controller only tells you once, and you’re expected to heed the warning thereafter.
We usually expect to hear the wake turbulence advisory when following a larger airplane for takeoff or landing; they’re not commonly heard away from the immediate airport area. That’s because, as stated at the beginning of this article, if a pilot is in visual conditions it’s that pilot’s responsibility to visually detect other aircraft and to then predict and avoid the location of its vortices as well.
A few notes from the FAA about wake turbulence avoidance:
Whether or not a warning or information has been given, ATC expects the pilot to adjust aircraft operations and flightpath as necessary to preclude wake encounters.
When any doubt exists about maintaining safe separation distances between aircraft to avoid wake turbulence, pilots should ask ATC for updates on separation distance and groundspeed.
If a larger aircraft is observed above on the same track (meeting or overtaking), adjust your position laterally, preferably upwind.
Fly at or above the preceding aircraft’s flightpath, altering course as necessary, to avoid the area behind and below the generating aircraft.
The proper technique for recovering from a strong wake turbulence encounter may seem counterintuitive. It also differs from what was taught about upset recovery for many years. For some time, pilots were encouraged to learn to command a roll in the direction of turbulence-induced roll, to turn an upset into a full, 360-degree roll to recover upright, most sources now recommend against rolling recover because most airplane will lose a substantial amount of altitude in the process (consider the Canadair Challenger experience).
Instead, most experts recommend letting the airplane enter an unusual attitude during the encounter, and then recovering from that attitude. AC90-23G states:
It may be better to allow the aircraft to transition through the wake and then recover from any resultant unusual attitude, rather than aggressively trying to control the aircraft during the wake encounter. If an autopilot is engaged and remains engaged, it may be better to allow the autopilot to recover from the wake vortex encounter rather than disconnecting the autopilot and using manual control inputs. However, be prepared to assume manual control of the aircraft if the autopilot disengages.
Prior experience or training that emphasizes use of rudder input as a means to maneuver in roll may not apply to all aircraft operations. Using the rudder to counter roll rate during a roll upset may lead to an undesirable aircraft response. Large, aggressive control reversals can lead to loads that can exceed the structural design limits.
Specialized upset recovery training in aerobatic airplanes is still a superb idea for all pilots. If you attend training intended to give you the skills needed to best recover from a strong wake turbulence encounter, expect it to emphasis unusual attitude recovery, and not commanding a roll with the vortex until you are again upright, as your best response to an upset.
Back Under the Pattern
When I’m departing or inbound to my home airport and see a KC-135R in the McConnell Air Force Base pattern above me, or any time I am operating in the vicinity of a larger airplane away from the immediate runway environment, I consciously visualize a pair of wingtip-vortex tornadoes extending behind the airplane, gradually drifting down to about 1,000 feet below the airplane 2 to 5 miles behind it and hovering there, drifting with the prevailing wind. When I see the traffic, I also see its wake so I can see and avoid the entire turbulence complex, not just the airplane that generates it.