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.
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.