What if you had a magic wand and by simply waving it, you could create a new runway at your congested airport? Well, Recat which stands for Recategorization of ICAO wake turbulence standards does just that. Not by actually pouring new concrete but by creating additional capacity that, at some airports, creates additional capacity that comes very near to what a new runway would represent.
Since 2 November 2012, ATC at Memphis International Airport in the USA has been streaming inbound and outbound traffic with separation of 3 miles (and on occasion 2.5 miles) instead of the customary 4 miles, representing a jump in capacity that benefits all traffic operating to Memphis. With FedEx being the most intensive user, their feeling of an extra runway having been put down at the airport is not surprising.
But what is Recat and how does it perform its magic?
Recat is one example of how things can be improved without new boxes on the aircraft just by not accepting things as they are. I almost said without investment, but this is not strictly true. There is no free lunch… Recat is the result of 10 years of study and experimentation performed as part of a project led by the FAA and EUROCONTROL. NASA, the DoT Volpe Center, Det Norske Veritas and several other international experts also took part in the effort. This has cost money of course even if not all the funds came from the airports and airlines that will ultimately benefit from the new procedure.
It is pretty early in the air traffic controller and pilot training curriculum that we learn about wake turbulence, its characteristics and dangers. Simply said, the higher the mass a wing has to lift, the stronger the turbulence left in its wake. If a very heavy aircraft is followed by a lighter one, it will get a mighty shake up or even crash. This problem is eliminated in practice by changing the distance inserted between aircraft on approach or taking off. ICAO had developed a system in which aircraft types are categorized according to maximum take-off mass and the separation to be provided depends on whether a heavy is followed by a light or medium or the other way round. If the preceding aircraft is in a heavier category than the aircraft following, the separation is increased by an appropriate amount.
This is not a big deal at airports where the aircraft population is more or less homogenous. If a few heavies turn up, they are handled with the increased separation but otherwise the normal separation is used. There is no significant decrease in capacity. If however the traffic is a mix of all kinds of aircraft, the increased separation becomes the rule of the day and this does mean a serious downward push on throughput. With bigger distances between aircraft, fewer of them can pass through the system in a given amount of time.
The European Commission’s CREDOS project did look at possibilities to eliminate the increased separation in cross-wind conditions. The results were encouraging but experts felt that there was more to the subject than just waiting for nature to clean out the dangerous turbulence.
The original ICAO wake turbulence separation standards were rigid and had been based on just one characteristic of the aircraft, its maximum take-off mass. True, the Boeing 757, which belonged to the medium category, was moved into the heavy club when it was discovered that its wake turbulence characteristics were closer to that of much heavier aircraft, but other than that, the classification system was simple and rather wasteful. It was not by accident that many ANSPs had created their own versions of the ICAO standard, a practice that have often led to discussions between pilots and controllers on the control frequency.
The introduction of more and more new, large aircraft coupled with the trend of more regional aircraft appearing at some airports and the overall airport congestion gave urgency to the need of addressing the wake turbulence issue on an international level. Responding to the widely held expert view that the wake turbulence separation standards as defined by ICAO were outdated, ICAO tasked the FAA and EUROCONTROL to update and harmonize wake turbulence separation standards for all aircraft types.
The methodology developed for the work called for new categories that would be optimized for capacity increase; the minimum safety requirement was that the system must be no worse than before; the hazard metric was to be the wake strength, derived from wake decay data, with this latter obtained from FAA and EUROCONTROL measurements.
The work was divided into three phases. Phase I, the results of which are now being enjoyed in Memphis, can be seen as the first step towards performance based separation minima. The categories move beyond the legacy, purely mass based principle to include other characteristics, like aircraft wing-span, approach speed and roll moment of inertia. Under the new nomenclature, six categories have been defined, identified by letters from A to F, where A is the largest and F is the smallest aircraft. Although the difference between the old and new categories and the associated separation minima is relatively minor, a runway capacity increase of up to 10 % has been realized in tests and lately confirmed in actual operations in Memphis.
10 % may not sound much but 10 % capacity increase with practically no direct investment is extremely attractive. However, Recat is not stopping here.
Phase II of the program is aiming to realize static pair-wise separation standards where the category to be used for a given aircraft changes depending on whether it is a leader or follower.
Phase III will be looking at dynamic pair-wise separation where the aim is to achieve optimum performance based separation and full support for 4D trajectory planning.
Both SESAR and NextGen consider the Recat work as an essential element in achieving their performance goals. If the experience in Memphis is anything to go by, Recat will deliver on its initial promises.
