There is a great deal of ignorance about how wake vortices behave, and existing ICAO standards may be inappropriate as a result

All the standards associated with aircraft separation for wake turbulence avoidance are due for a shake-up. The only questions are: how soon will it be done, who will oversee it and who will pay for it?

It would be easy to see the wake vortex trials associated with the Airbus A380's certification as the catalyst for the world's renewed interest in wake separation criteria, but that is not true. First of all, wake vortex measurement has never been conducted as a certification criterion before. It is simply an accident of history that the A380 arrived on the scene just as the laser radar (lidar) technology that enables detailed study of wake vortex behaviour, patterns and decay became sufficiently mature to be effective.

The A380 may be a particularly appropriate first subject for study because it is likely to be the heaviest commercial aircraft that will enter service in large numbers in the next 30 or 40 years. But although the primary motivation for wake turbulence study is the safety of aircraft in trail behind others - whether on approach, departure, or en route - a critical reason for gaining more wake behaviour knowledge in today's increasingly crowded skies is the need to find out whether, because of a historical lack of wake data, the aviation world has been overly cautious in drawing up wake separation minimum distances. If International Civil Aviation Organisation separation standards are found to be over-generous, reducing spacing between aircraft appropriately could safely yield higher movement rates at capacity-choked airports. It will also enable the crews of aircraft on airways or oceanic tracks - particularly where reduced vertical separation minima apply - to adopt intelligent techniques for wake avoidance rather than just staying well clear of other aeroplanes.

Here is a neat illustration of how crude today's ICAO separation standards are: when there is a strong headwind on a landing runway, aircraft approach it at a lower groundspeed, so the time between successive aircraft is increased yet controllers are not allowed to compensate by putting aircraft closer in trail than the ICAO wake separation minimum, despite knowing that wake energy dissipates with time, not distance. So the landing rate drops dramatically. Meanwhile on a day when landing with a tailwind is operationally necessary, the ICAO standards do not - as theoretically they should - require an increase in the distance separation because the time interval between the aircraft has dropped as a result of their higher groundspeed.

It's all very well pointing out this inconsistency, but it is not going to be simple to develop a tool that will enable approach controllers to manage separation by time rather than distance, as it will be essential that this new highly skilled, high-workload task remains at least as easy to perform as it is today.

A wealth of new information about aircraft wakes is there to be unlocked and used for the industry's benefit and that of its customers. Work on extracting it should not be delayed.

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Source: Flight International