Editor’s note: Last month’s issue included a cover story on wake turbulence and how we may encounter it even when our training suggests it shouldn’t be a factor in our operations. This article is a companion piece, featuring a deeper dive into wake turbulence characteristics and behavior to help us predict where it is and how to avoid it.
Wake turbulence is a mostly invisible and potentially violent byproduct of lift generation. When an airfoil passes through the atmosphere, a pressure differential is produced, with the lowest pressure over its upper surface and the highest underneath. While creating the lift we need, this pressure differential also causes air to roll up and over the wing tip, where it meets and generates a horizontally aligned, high-energy and invisible vortex—a horizontal tornado, if you will—trailing aft of the wing tip. Every aircraft depending on an airfoil to generate lift, including helicopters, generates these vortices while airborne.
When encountered by another aircraft flying a roughly parallel track, these vortices are associated with induced rolling moments that may be capable of exceeding the control authority of the encountering aircraft. When the second aircraft encounters them roughly at a right angle, turbulence severe enough to cause airframe failure can result. In either instance, wake turbulence is something to be avoided. Knowing its characteristics is key to avoidance.
LATERAL MOVEMENT
Wing tip vortices are not only present over a runway just used by a departing or landing airplane; vortices from the same airplane may also move laterally over to an airport’s adjacent runways. Wing tip vortices, like other things in the atmosphere, are moved by the wind and, because they are invisible, maintaining good situational awareness (SA, the skill of knowing what’s going on around you) means pilots must learn to envision the location of vortices generated by larger aircraft, then modify their flight path accordingly to avoid them.
For example, consider the following scenario: Wind velocity is from 270 degrees at 12 knots. A Boeing 777 has departed Runway 36L. Meanwhile, a Cessna 172 holding short of Runway 36R is cleared for takeoff and cautioned about wake turbulence. Implied in this communication is a warning of the potential dangers of penetrating the wingtip vortices while climbing out. With this warning (and even without it, when practicing good SA), the Cessna pilot should be envisioning the 777’s wing tip vortices and their lateral movement toward Runway 36R. Also, the pilot needs to be asking the following questions: Is this an accident waiting to happen? Is it unsafe to take off? Do I have any options?
Maintaining good SA requires knowing the wind direction and speed; aircraft in the vicinity, as well as their altitude, speed and heading; and the operation of your aircraft. For example, a windspeed of 10 knots causes wing tip vortices to drift at about 1000 feet per minute downwind, according to the FAA’s Pilots Handbook of Aeronautical Knowledge, (PHAK, FAA-H-8083-25B). In addition, a vortex flow field sinks as it moves. In the scenario involving the 777 and the 172, the wind would move the downwind wing tip vortices of the Boeing 777 over Runway 36R. This is an accident waiting to happen; it is unsafe to takeoff until at least three minutes have elapsed, preferably five. Regardless, the vortex flow field of the downwind wing tip vortex must be avoided.
In conjunction with good SA, good risk management compels us to avoid the 777’s vortices, and the Cessna pilot has the option of delaying the takeoff in favor of their natural dissipation. A pilot with low SA who accepts the takeoff clearance risks being rolled inverted by the vortex field immediately after rotation and obviously close to the ground. How do you think that will turn out?
A crosswind of up to five knots can hasten the drift of the downwind vortex toward another runway and result in the upwind vortex remaining in the takeoff or touchdown zone for a period of time.
That scenario involves takeoffs. For landings, a tailwind or crosswind condition can move vortices of a preceding aircraft forward into the touchdown zone; light quartering tailwinds require maximum caution on final approach. Pilots should be alert to large aircraft upwind from their approach and takeoff flight paths.
On May 16, 2019, a Diamond DA62 performing runway lighting calibration operations at the Dubai (UAE) International Airport crashed approximately 3.5 miles from the Runway 30L threshold, killing all four aboard. A Thai Airways Airbus A350-900 flying an approach to Runway 30R was about 3.7 nm ahead of the Diamond twin. Winds were from 020 at five knots. Evidence shows the Diamond was in level flight at about 1000 feet agl and approximately 130 knots when it rolled slightly but was quickly recovered. About seven seconds elapsed and the airplane rolled left until becoming inverted, entering a steep dive. At this writing, the UAE General Civil Aviation Authority has not yet published a final report.
As details of the Diamond accident in Dubai suggest, the onset of wingtip vortex penetration may be insidious and even gentle. Any uncommanded aircraft movements on departure or short final—wing rocking and sudden changes in pitch attitude, for example—may be caused by wingtip vortex penetration. Maintaining high SA is critical for avoiding vortices by visualizing wingtip vortex development of other aircraft taking off and landing in the vicinity of a pilot’s airplane.
For example, if on short final you suspect that vortex penetration is adversely affecting your aircraft, perform a go-around/rejected landing immediately.
AVOIDANCE STRATEGY
The factors involved in the possibility of encountering wake turbulence moved laterally by surface wind during takeoffs and landings need to be better recognized. The following three-step risk management strategy is proposed to identify the hazards, assess the risks associated with those hazards and then mitigate/control the likelihood of encountering the wake vortices. The strategy is derived from the “V” in the FAA’s PAVE risk-management model, i.e., considering the enVironment in which the aircraft is being operated.
Not only does the environment include weather and terrain, but details like runway configuration, airspace and night operations should be used to identify the hazards of wake turbulence. In this strategy, it is used to identify local environmental hazards associated with wing tip vortices. (See Chapter 2 of the PHAK for a detailed review of the PAVE model and the associated risk assessment matrix.)
Step 1. Identify hazards
Pilots should take the time to identify the presence of wing tip vortex hazards in the local environment by asking, “Are wing tip vortices present in the area of my intended takeoff or landing? Where are the wing tip vortices relative to the runway I will be using? What is the wind direction and speed?” If identified, then the risks associated with wing tip vortex hazards need to be assessed.
Step 2. Risk assessment
Risk assessment means establishing the likelihood and severity of risks associated with the identified hazards—wing tip vortices, in this case—by employing the risk assessment matrix. In other words, (1) is the likelihood considered to be “Probable” for encountering wing tip vortices moved by wind over the runway you intend to use and, (2) is the severity of injuries resulting from the wing tip vortices considered to be “Catastrophic” or “Critical”? If the answers to (1) and (2) are “yes,” then the risks are classified as “High.” Risks classified in this category must be mitigated.
Step 3. Risk mitigation
To mitigate/control these particular high risks, the pilot should wait up to three minutes for wake turbulence dissipation prior to departure as detailed in the PHAK, or perhaps as many as five minutes in the case of departing behind a large, heavy airplane. Likewise, if there is any indication or suspicion of encountering vortices/wake turbulence prior to landing, then for risk mitigation the pilot should fly a go-around/rejected landing.
THINK AHEAD
Wing tip vortices moved laterally by the wind over a runway to be used for takeoff or landing, combined with a pilot having low SA, represent a clear and present danger and are a recipe for a fatal loss-of-control accident. Prudent pilots can react proactively to this risk by thinking ahead of the airplane, becoming and staying aware of other aircraft relative to your airplane, and determine the wind direction and speed relative to the runways in use and intended for takeoff or landing.
Learn to envision wing tip vortices generated by departing and landing aircraft. Be smart and safe by employing basic risk-management principles to mitigate encountering potentially hazardous wing tip vortices.
I have experienced wake turbulence twice in my career. Both were surprising and very unexpected.
The first happened when I was flying an approach “under the hood” in a Mooney and a 737 was just touching down at the threshold. I had a stable approach on glideslope and course as I approached the outer marker. The shook violently twice and rolled slightly within a few seconds of other. I had thought that I had struck something. I disconnected the autopilot immediately and landed safely. The autopilot subsequently had to recalibrated. The 737 had been flying a visual approach, did a “slam dunk” with a fast dirty and I paid the price.
The second occurred when landing a Cessna 150. A C130 was doing a maintenance run up on an adjacent upwind taxiway on an angle to the left. The crew notified the tower their activity was concluded and were asked to wait for landing aircraft (me). As I flared, the airplane touched ground and appeared to be picked up by the hand of God and moved about 10 feet to the right.