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Ep 25 – Wake Turbulence Recategorization

Regs, Podcast | January 15, 2021


Hello aviation professionals, and welcome to Podcast 25: Wake Turbulence Recategorization.

ICAO has recently, as of November 5, 2020, changed the categories of aircraft for wake turbulence purposes. Some say the changes are in direct result of the Challenger upset which occurred in the Arabian Sea behind an A380 in 2017. The crew of the Challenger was on radar and passed 1,000 feet underneath a 380. The aircraft rolled multiple times and both engines flamed out. The crew got one engine restarted and diverted. The aircraft was bent beyond repair. There were many injuries onboard as well.

Wake Turbulence As A Threat

Wake turbulence is a threat anytime the aircraft is airborne and sometimes when it is not such as on the ramp near a helicopter.

Wake turbulence is not only a consideration during takeoff and landing phases (as the Challenger occurrence proves) but the new ICAO wake turbulence categories (or RECAT, as they call) it are designed only to apply in the takeoff and landing phase.

For enroute wake, a consideration is that aircraft navigation systems are so accurate that aircraft pass each other on virtually exactly the same track. I’ve told this story before on a previous podcast, but I was coasting in from Hawaii to North America and another aircraft was catching us 1,000 feet below. They had been slowly gaining on us all night from Hawaii. As they flew underneath us, our radar altimeter became active and gave readings of the distance to the aircraft below until the pass was complete. The width of a narrow body airliner is about 12 feet and our tracks were close enough to activate the radar altimeter.

Old Categories

The old wake categories were (just because you might come into contact with the old categories in some countries):

  • Light: indicates an aircraft type with a maximum certificated take-off mass of 7,000 kg (15,500  lbs) or less
  • Medium: indicates an aircraft type with a maximum certificated take-off mass of less than 136,000 kg (300,000 lbs) but more than 7,000 kg (15,500 lbs).
  • Heavy: refers to an aircraft type with a maximum certificated take-off mass of 136,000 kg (300,000 lbs) or  more.

However, there is or was one exception which is the Super category, which is the Airbus A380-800 which is 560,000 kilograms or approximately 1.2 million pounds.

Notable Occurrences

Here are a few occurrences that have happened recently:

Qantas Flight 32

Qantas Flight 94 which was a 380 departed from Los Angeles International Airport in June of 2018. Less than two hours into the 16-hour flight to Melbourne, Australia, the A380 descended for 10 seconds while flying at 30,000 feet. The incident was a result of Flight 94 flying through the wake turbulence generated by another Qantas A380, Qantas Flight 12, which was flying 1,000 feet above and 23 miles ahead of Flight 94.

At altitude, data suggests that vortices sink at a rate of 300 to 500 feet per minute and stabilize about 500 to 1000 feet below the flight level of the generating aircraft.

2008 Mexico City Learjet Crash

November 2008 saw a Learjet crash in Mexico City killing all onboard and many people on the ground after encountering wake turbulence from a 767. The investigation stated that the plane did not slow down to the required speed as instructed by the air traffic controller. This brought the aircraft closer to the Mexicana 767-300. After the speed reduction clearance was given, the crew waited more than a minute before starting the reduction. Weather at the time was clear and calm which helped maintain the wake vortices.

Lessons Learned

Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach. Light-aircraft pilots must use extreme caution and intercept their final approach path above or well behind the heavier aircraft’s path. We fly lots of visual approaches in clear weather and we are trying to get on the ground quickly for many reasons. Keep your head in the game right until engine shutdown.

Helicopter Wakes

Wake turbulence can be experienced on the ramp by way of helicopters. Helicopter wakes may be of significantly greater strength than those from a fixed-wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds of 20 to 50 knots. Don’t be reliant of the size of the helicopter only for wake generation as the number of main rotor blades the helicopter has affects the size of the wake. A mid-size helicopter with 2 blades will produce wake as strong as that of heavier helicopters because a two-blade main rotor system produces a stronger wake than rotor systems with more blades.

What Are Wake Vortices or Wake Turbulence?

All aircraft generate wake vortices. When an aircraft is flying, there is an increase in pressure below the wing and a depression on the top of the aerofoil. Therefore, at the tip of the wing, there is a differential pressure that triggers the roll of the airflow aft of the wing.

Characteristics of Wake Vortices

What are the characteristics of wake vortices? This is from Airbus.

The active part of a vortex has a very small radius, not more than a few meters. However, there is a lot of energy due to the high rotation speed of the air.

Descent rate: In calm air, a wake vortex descends slowly. As an order of magnitude, in cruise, it could be 1000 ft below and behind the generating aircraft at a range of around 15 NM. EASA says this could be as far as 25 NM behind a heavy aircraft. In the approach phase, the descent is usually limited to around 700 ft. However, depending on weather conditions, the descent rate may vary significantly and may even be very small. One of the key factors affecting this descent is the variation of the temperature with the altitude. A temperature inversion limits the rate of descent.

Decay rate: One important parameter of a wake vortex is the decay of its strength over time. The decay rate varies slightly from one aircraft type to another. In calm air, due to low external interference, decay rate is low.

Ground effect: When the aircraft is close to the ground, less than a wingspan, the two vortices tend to drift out from the runway centre line, each towards its own side, at a speed of around 2 to 3 kt. It is this phenomenon, when associated with a light crosswind component that tends to “hold” the upwind vortex roughly on the centreline of the runway, whilst the downwind” vortex moves away from the runway but perhaps towards a parallel runway.

Due to this phenomenon, the decay is much faster in ground effect.

Aircraft weight: Wake vortex strength increases with the weight of the aircraft. This is why the previous ICAO aircraft classification is based on the MTOW. Research has shown that other parameters are also important. More about that in a minute.

Weather conditions: The weather conditions play a major role in wake vortex development and decay. If there is moderate turbulence, a vortex will dissipate very quickly says Airbus. Strong winds are associated with turbulence and this will also contribute to a rapid vortex decay.

Calm weather creates the most critical situation as the vortex strength decreases slowly and the vortex effect may be felt far behind the generating aircraft.

ICAO says: “The lift that the aircraft’s wing is designed to produce directly affects the intensity and lifespan of the generated vortex. Therefore, the new separation minima is based upon wake vortex categories of the preceding and the following aircraft which, in turn, are derived from the maximum takeoff weight …but research flight testing also found that aircraft speed and wingspan also affect the strength of the wake generated and also the following aircraft’s reaction to the wake.”

As a result, aircraft are placed into six wake vortex categories, common for departure and arrival separation.

Due to increasing traffic levels, outside of a pandemic, and congested airports, regulation changes were planned and implemented, with the aim of increasing airport capacity.

What Is RECAT?

Wake Turbulence Recategorization (or Wake RECAT) is the safe decrease in separation standards between certain aircraft. There were 3 phases, but the last phase became effective November 5, 2020.

The Airbus 380 changed the wake turbulence game when it was introduced, and this resulted in many hours of flight testing. What was also found is that certain aircraft pairs have too much distance between them and could be safely reduced thereby increasing airport capacity. This is the RECAT program.

Using that information, aircraft were re-assigned to one of six new categories (A through F) which were derived by redefining the transition weight between the old categories, adding a Super category and splitting each of the Medium and Heavy categories into two new ones. The resulting categorization is as follows:

  • CAT A – “Super Heavy”
  • CAT B – “Upper Heavy”
  • CAT C – “Lower Heavy”
  • CAT D – “Upper Medium”
  • CAT E – “Lower Medium”
  • CAT F – “Light”

So 3 heavy categories, 2 mediums and a light.

Some examples are Category A includes the 380 and the Antonov 124 and 225.

Upper and lower heavy is large airliners such as the 757, A350 and the Dreamliner.

Upper medium includes the G5 and the 737NG series and Airbus 320.

Lower medium includes Gulfstream 4, Challenger 600 series and the 737 classics.

And the light category or Category F includes everything else such as the Lear 60, Citation 525 and 650.

Preceding Aircraft Following Aircraft Minimum Separation

Under the RECAT program, the required separation is much more detailed.

A Lear 60, which was a medium on the old system, behind a heavy aircraft was separated by 5 miles. Now a Lear 60 is designated as F category and will be separated 6, 7 or 8 miles behind heavy aircraft depending upon which specific aircraft type is the leader. The increased airport efficiency comes in where similar aircraft pairs are now flying closer together.

CAT A 3 NM 4 NM 5 NM 5 NM 6 NM 8 NM
CAT B 3 NM 4 NM 4 NM 5 NM 7 NM
CAT C 3 NM 3 NM 4 NM 6 NM
An Empty Field Indicates Minimum Radar Separation

New for the RECATegorization program are time-based separation limits for departure. An example is a Lear 60 behind a G5 is 2 minutes and a Lear 60 again category F behind a 757 is 140 seconds. There are charts out there if you interested.

CAT A 100s 120s 140s 160s 180s
CAT B 100s 120s 140s
CAT C 80s 100s 120s
CAT D 120s
CAT E 100s
CAT F 80s

Let’s Talk Operations

Situational awareness is critical here but, if wake is encountered, it could be a simple as rocking of the wings. Airbus research says to release or guard the controls only and never use the rudders. One jolt may be accompanied by more turbulence so always be repaired to go around. Accidents have happened where the crew encountered a small amount of wake on approach but continued the approach only to lose control close to the ground.

Pilot Procedures

What are some pilot procedures with regards to turbulence?

  • Fly a stable approach. This is important for many reasons as we all know.
  • Larger aircraft in front are probably flying the glide path so smaller aircraft can choose to fly slightly higher to avoid a wake encounter. Take the wind into account.
  • Always have a plan to go around if necessary.
  • Fly assigned speeds as ATC is providing separation and requires those speeds to maintain separation.
  • Watch for the previous aircraft’s touchdown point or liftoff point.
  • Minimize your time on the runway after landing so ATC can maintain separation.
  • For enroute wake use all available resources to maintain a safe distance. Wake can exists 25 miles behind large aircraft. Watch out for crossing traffic. If you are operating in oceanic airspace, use SLOP.

In The News

Let’s changes gears for a moment. In The News is a segment of the podcast where I talk about other happenings in aviation.

Getting a coffee can be great for managing fatigue but it can also expose the aircraft to level of risk that may not be evident.

On 6 February 2019, an Airbus A330 on a scheduled flight from Frankfurt to Cancun, Mexico, was west of Ireland when a cup of coffee was spilt on the left side Avionics Control Panel (ACP). After 20 minutes, the ACP became very hot and emitted an electrical burning smell. When the right side ACP also began to malfunction and the left side unit began to emit smoke, the resulting communications difficulties were such that the Captain could only hear radio communications through the First Officer’s (FO) speaker. The Audio Control Panel on the FO’s side started to melt a button on the panel. With the smell of burning electrical and some smoke present, the crew alternated between using oxygen so one crew member was always using oxygen. The crew diverted to Shannon.

The aircraft manual states that Airbus highly recommends that flight crews put and store all objects in their dedicated area in the cockpit—in other words, cups in the cup holders.

The investigation found that “the size of cups used by this operator on this route made it more difficult to take cups in and out of the cup holder.” So pilots did not generally use the cup holders. The investigation also noted that “a lid properly secured on the top of the cup may have reduced the amount of liquid spilled on the console.”

If you spill a liquid on a panel and nothing happens, this is not a guarantee that no damage has occurred. Flights have had smoke in the cockpit many days after a liquid spill that was not reported. You must report the spill or risk a future flight having to deal with it. I have seen photos of avionics that have had liquids spilled on them and it is amazing they continued to work at all.

That’s it for podcast 25. Have a great day!


Airbus. Operations Wake Vortices.

Skybrary. A332, en-route, North Atlantic, 2019.

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