Many of us who operate small general aviation airplanes out of busier airspace are familiar with the phrase, “caution wake turbulence” issued by the controller. As a pilot wake turbulence is an important hazard to be aware of and something that every aircraft (no matter how large or how small) produces. But to be able to fully understand it, we need to start with this question: how does an airplane form wake turbulence?
Airplanes form wake turbulence as a result of lift. The higher-pressure air on the bottom of the wing flows out and around the tip of the wing to the top of the wing where there is lower pressure. This swirling air is called “wingtip vortices” and is the primary component of wake turbulence.
Knowing how wake turbulence forms and what conditions make it more prevalent will make you a safer pilot. It’s an unseen hazard in the sky that doesn’t have to be a mystery if you understand a little more about how it works.
How does an airplane form wake turbulence?
To understand wake turbulence, we have to understand the basic physics of flight. We are all aware of the 4 primary forces acting on an Airplane: thrust, drag, lift, and weight. Newton’s laws of motion tell us that for every action there is equal and opposite reaction.
For example, when a 2500 lb Skyhawk is at rest the wheels are exerting a force of 2500 lbs to the ground and, in return, the ground is exerting that same force back to the wheels.
So as an airplane travels down the runway and begins to produce lift the weight is transferred from the wheels to the wings and the surrounding air must now take on the responsibility of “holding” the airplane. We know this is possible by the faster, lower pressure, air on top of the wing and the slower, higher pressure, air on the bottom of the wing.
Put simply, the air pushes up on the bottom of the wing and pulls up on top of the wing to allow the airplane to fly. However, not all of the air on the bottom of the wing pushes directly up on the wing.
Some of the higher-pressure air on the bottom of the wing flows out and around the tip of the wing to the top of the wing. This swirling air is called “wingtip vortices” and is the primary component of wake turbulence. This spiraling air, when viewed from behind the aircraft, moves outward, upward, and around the wing tips. This creates strong turbulence behind and below the airplane as the wake tends to sink below the flight path. So, any time lift is produced, wake turbulence is always a byproduct. However, there are times that it is more prevalent than others.
When is wake turbulence most powerful?
Wake turbulence is most powerful when the airplane is heavy, slow, and at a high angle of attack. When an airplane is heavy it needs more lift so the wake is greater. An airplane that has a high angle of attack has a greater pressure difference above and below the wing which increases the strength of the wingtip vortices. When an airplane is slow and/or in the clean configuration it takes greater angle of attack to remain aloft, thus again increasing the strength of the vortices.
In particular, large heavy aircraft tend to create greater turbulence than lighter aircraft. So much so that the FAA gives heavier aircraft different weight designations such as “heavy” to indicate this phenomenon. See our article on “Why are some airplanes called ‘heavy’?”
Wake Turbulence Avoidance
Now that we know how an Airplane forms wake turbulence and what it is, let’s discuss why this is a hazard. When flying a light airplane it is especially important to watch out for the wake turbulence created by the larger airplanes in our vicinity.
These larger airplanes weigh more so more lift is required and they will create a larger disturbance in the air. This disturbance can be so great that it can upset the attitude of the smaller aircraft and even cause structural damage.
If you have ever watched a boat and a Jet Ski cruise on the lake you know that it is generally more important for the Jet Ski to avoid the larger boat’s wake than the other way around. Just like airplanes the larger, heavier boat displaces more water thus creating a larger wake.
Wake turbulence is produced anytime an airplane is creating lift so it should be taken into account no matter what phase of flight you are in, but when flying small general aviation aircraft it is most critical to watch for those instances that happen closest to the ground, i.e takeoff and landing. We will discuss each scenario separately below.
Wake Turbulence Avoidance During Takeoff and Landing
When taking off behind a larger aircraft it is important to take note of the point at which that aircraft lifted off and for you to be airborne before that. This is because the wake turbulence will sink behind the airplane and you will want to try and remain above the wake path.
When taking off behind an arriving larger aircraft it is important to rotate after the point that they touched down. In order to maximize your runway length, waiting for the wake to dissipate is a good alternative.
When landing behind a larger aircraft it is important to take note of the point at which that aircraft touched down and try to remain above that aircrafts flight path and land beyond their point.
When landing behind a departing larger aircraft it is important to touchdown before the rotation point of that aircraft.
Wake Turbulence Avoidance in Cruise Flight
During cruise flight, aircraft are typically traveling at a faster speed which requires a smaller angle of attack to achieve level flight, so the wake is reduced. Typically at altitude smaller GA aircraft and larger commercial jets have much more space between them compared to takeoff and landing so this allows the turbulence to dissipate before it is ever an issue. Also, when you are at altitude during cruise this normally allows for more altitude to recover if wake turbulence is encountered.
Wake Turbulence Dissipation
Just like any disturbance in the air, wake turbulence will dissipate with time no matter where it occurs and having a strong wind will assist in breaking up the turbulence quicker. This is important to understand especially when preparing for takeoff or landing.
If there is a strong crosswind the wake will be pushed across the width of the runway and it may be beneficial to place your flight path on the upwind side of the larger airplane. If the wind happens to be strong and straight down the runway the turbulence will dissipate and move past your location much faster.
Related Questions
Are there regulations on wake turbulence?
Ultimately you are responsible for your aircraft and must do what is necessary to maximize the safety of your flight. But to assist us in assuring a safe flight the FAA has created “weight turbulence categories” based on the weight of the aircraft. This category sets separation minima for takeoff, landing, and in route. Although these minima have been established with research and flight data it is important to know when and who is responsible enforcing these separation guidelines.
VFR Flight
When flying VFR and receiving radar vectors the controller is responsible for giving wake turbulence advisories, but when flying VFR not being vectored by ATC the PIC must see and avoid any potential wake turbulence situations.
When departing a controlled environment the tower controller is responsible for issuing a time delay for the departing aircraft, based on the FAA guidance discusses earlier. However this delay may be waived at the pilot’s discretion. When arriving to an airport you may be advised of wake turbulence but it is your responsibility to decide how to avoid it.
IFR Flight
When on an IFR flight plan in IMC, ATC will assume responsibility for wake turbulence avoidance. When an IFR aircraft is maintaining visual separation or has accepted a visual approach it is their responsibility to avoid potential wake turbulence.
As with a VFR departure the controller will issue a time delay but this may be waived at the pilot’s request.
Do helicopters produce wake turbulence?
We cannot ignore our vertical lift friends; they also create lift and therefore create wake turbulence. As with fixed wing aircraft a helicopter in forward flight creates a wake that resembles wingtip vortices.
Even when a helicopter hovers the main rotors create a strong downwash that can extend up to 3 rotor diameters from the helicopter. The downwash impacts the ground and moves outward, upward, around and away from the main rotor(s) in all directions.
Anytime you see a helicopter in the area be aware and treat it as if you are avoiding wake turbulence created from a larger fixed wing aircraft.
Summary
In conclusion wake turbulence is a product of lift and the heavier the aircraft the greater the wake turbulence. Wake turbulence is also greater when an aircraft is at a higher angle of attack, i.e slow and in the clean configuration.
Wingtip vortices are the primary component of wake turbulence and move outward, upward and around the wingtips. The wake created by aircraft will drift below the airplane’s flight path and can be an extreme hazard for smaller airplanes landing or taking off behind the airplane.
It is important to try and remain above the flight path of the larger aircraft and adjust your flight path to the upwind side of the aircraft you are following. As with any flight you are responsible for the safety of your aircraft and its occupants so you must do what is necessary to avoid any circumstances that could put you in potential danger. For more information on wake turbulence the FAA has created an advisory circular, AC 90-23, that may be referenced.