Aircraft wings produce a phenomena whenever they're producing lift called vortices. Vortices vary in size, strength and life depending on three main factors: Weight, airspeed and angle of attack.
As most aircraft land at similar airspeeds and with similar angle of attack, weight is used as the determining factor for separation. Aircraft have been categorized into 4 different wake turbulence categories depending on their Maximum Take-Off Weight (MTOW).
|Wake Turbulence Category||Weight|
|Light||7,000 Kg or less|
|Medium||From 7000 Kg to 136,000 Kg|
|Heavy||More than 136,000 Kg|
Only A380-800 aircraft
These categories are used mainly for leaving enough separation between aircraft during the approach and takeoff phases. It is only required when a lighter aircraft is following a heavier one. The following table shows the "rules" that shall generally be applied during arrival.
|Aircraft in front||Aircraft following||Separation (in nm)|
During departures, a 2 minute separation is applied when a lighter aircraft is departing after a heavier one. It shall be 3 minutes if the second aircraft is departing from an intersection.
Question: You have a light and a heavy aircraft both ready for departure. Which one would you clear first for takeoff? (Answer at the end)
If you look at any approach chart, you will usually see a table with various different numbers, depending on the aircraft category (A, B, C, D or E).
This is another way of differenciating aircraft, but in this case, depending on their approach speed.
|Aircraft Category||Approach Speed|
|A||Up to 90kt|
|B||Greater than 90kt and up to 120kt|
|C||Greater than 120kt and up to 140kt|
|D||Greater than 140kt and up to 165kt|
|E||Greater than 165kt|
Answer: You should clear the light aircraft for takeoff first, as this way it is possible to avoid wake turbulence delay. However, if by doing this you will delay the heavy aircraft in excess, the faster aircraft (usually the heavy) shall be released first.
Content updated: 15. February 2019.