Wake turbulence forms when an aircraft makes lift, especially near the wingtips where high pressure air below the wing curls around into low pressure air above it. These rotating tubes of air are called wingtip vortices, and they can roll a smaller following aircraft unexpectedly. Aviation groups aircraft by weight because heavier aircraft usually create stronger wake vortices.
Safe spacing behind big jets gives the wake time and distance to sink, drift, and weaken.
Understanding Aviation: Wake Turbulence Categories
Wake is most troublesome when an aircraft is flying slowly, cleanly, and close to its maximum lift demand. This often happens just after takeoff and during approach. Flaps, gear, and aircraft configuration affect the airflow, but a slow, heavy aircraft still needs a large lifting force.
That is why controllers and pilots pay special attention near runways, where aircraft use similar paths at low height. A following aircraft may meet disturbed air before there is enough room to recover.
The danger is not simply a rough ride. One vortex can produce a sudden rolling force that exceeds the smaller aircraft's available roll control.
The location of the wake matters as much as the size of the leading aircraft. Behind an aircraft in level flight, the vortices tend to move downward and outward from the flight path. Near the ground, their motion changes because the surface interrupts the airflow.
A vortex may move sideways along the runway area or linger near a departure path. Wind can carry it into places that appear clear.
A light crosswind is sometimes more troublesome than a strong one because it can keep one vortex near the runway centerline while moving the other away. Pilots must not assume that a visible flight track is a safe track.
Air traffic control uses distance or time separation to manage this risk, but the exact rule depends on the operation. Radar arrivals can be spaced in nautical miles because controllers can monitor position and speed. At airports without that support, a controller may use time after departure or landing.
Extra spacing is often needed when a small aircraft follows a much larger one, when the runway paths cross, or when weather makes wake movement uncertain. Pilots can receive a wake turbulence caution, yet that warning does not remove their responsibility to fly carefully.
If conditions permit, a pilot following a larger aircraft on approach aims to stay above its approach path and touch down beyond its touchdown point. On departure, the safer choice is often to lift off before the larger aircraft's rotation point and climb above its path.
Students should separate aircraft category from actual wake behavior. Categories give controllers a practical starting point, not a perfect prediction. A relatively light aircraft can create a strong wake if it is slow and heavily loaded.
A large aircraft can leave a weaker wake when it is faster or using less lift. Air density, wind, altitude, wing design, and pilot actions all change the result. The basic spacing calculation uses time equals distance divided by speed, but ground speed matters because wind changes how quickly the following aircraft reaches the same air.
When studying an incident or a runway procedure, note the leader's size, its phase of flight, the follower's path, the wind direction, and the available height for recovery. Those details explain why wake turbulence rules are conservative.
Key Facts
- Wake strength increases with aircraft weight because lift must approximately equal weight in steady flight: L ≈ W.
- A useful vortex strength relationship is circulation Γ is proportional to W/(rho V b), where W is weight, rho is air density, V is speed, and b is wingspan.
- FAA wake categories include Small at 41,000 lb or less, Large above 41,000 lb up to 300,000 lb, Heavy above 300,000 lb, and Super for the largest aircraft such as the Airbus A380.
- Representative radar spacing behind a Heavy aircraft is about 4 to 6 nautical miles, with smaller followers usually needing more distance.
- Wake vortices usually sink below the flight path and can descend at about 300 to 500 ft/min before weakening.
- Basic timing formula for spacing is t = d/v, where t is time, d is separation distance, and v is the following aircraft ground speed.
Vocabulary
- Wake turbulence
- Disturbed, rotating air left behind an aircraft that can affect another aircraft flying through it.
- Wingtip vortex
- A spiral of air that forms near a wingtip because air moves from high pressure below the wing toward low pressure above the wing.
- Wake category
- A grouping of aircraft, usually based mainly on maximum takeoff weight, used to set safe spacing rules.
- Nautical mile
- A distance unit used in aviation and navigation equal to about 1.852 kilometers.
- Separation minimum
- The smallest allowed distance or time gap between aircraft under specified conditions.
Common Mistakes to Avoid
- Thinking wake turbulence only happens behind jet engines is wrong because the strongest hazard usually comes from lift generated by the wings, not from engine exhaust.
- Using the same spacing behind every aircraft is wrong because a light airplane behind a heavy jet is much more vulnerable than a heavy jet behind a small airplane.
- Assuming wake stays exactly on the runway centerline is wrong because wind can push vortices sideways and make one vortex linger over a runway.
- Following below the flight path of a heavier aircraft is unsafe because wake vortices tend to sink, so staying above the preceding aircraft path and landing beyond its touchdown point helps reduce exposure.
Practice Questions
- 1 A small aircraft is assigned 5 nautical miles of spacing behind a heavy jet. If the small aircraft is flying at 120 knots, how many minutes of spacing does this represent?
- 2 A wake vortex sinks at 400 ft/min. How far downward does it sink in 90 seconds?
- 3 A light airplane is landing behind a heavy jet on the same runway with a light crosswind from the left. Explain where the pilot should expect the wake to move and why spacing or flight path choice matters.