Closing the gap

Can wake vortex research lead to increased airport capacity? The answer to that may be apparent within a year or two, but other benefits are far more distant

Some global hub airports have been severely congested for years and have exhausted almost all the physical and procedural methods for increasing runway capacity. Now one of the most congested of those airports, London Heathrow, is being told a little more research into wake vortex behaviour could enable it to handle an additional 59 to 84 take-offs a day, or nearly 26,000 flights a year.

This was the promise made by Andy Harvey of Eurocontrol's experimental centre in Brétigny sur Orge at the WakeNet Europe workshop in February, He was talking about the Eurocontrol-coordinated CREDOS (crosswind-reduced separations for departure operations) programme, but there are additional wake vortex research projects under way - or in the pipeline - that could safely reduce separations between aircraft on airport approach or in-trail en-route traffic, increasing capacity even further.

Runway capacity at many of the world's major hub airports is the ultimate limitation on air transport industry's ability to meet projected passenger demand. Harvey says there are many major airports in Europe today that have capacity to spare, but in a few years will reach their movement limits unless capacity-boosting techniques are made available. He says Frankfurt Main in Germany, already constrained almost as badly as Heathrow, could gain between 23 and 41 additional daily movements, or an annual total of 11,315, by taking advantage of the CREDOS research.

Advanced programme

CREDOS is the furthest advanced of Europe's wake research programmes focused on increasing capacity, as opposed to those studies aimed at improving wake detection, visualisation and warning for safety reasons alone (see box and diagram). The US Federal Aviation Administration, one of the many participants in the WakeNet forum, has been concentrating its research on how best to avoid wake conflicts during simultaneous approaches to closely spaced parallel runways (CSPR). One component of the methodology employed in positioning aircraft landing on CSPR pairs is, when there is a crosswind, to always put the light and medium-weight aircraft on approach to the upwind runway, and the heavies on final for the downwind one that way the less energetic wakes from the lighter aircraft tend to drift across the approach path for the heavies, rather than the converse.

Eurocontrol's Brétigny unit has been heading the CREDOS programme. Harvey says it is conducting a three-year validation study based on observing and modelling wake displacement during take-offs when there is a crosswind. "When it's finished we want those airports that have taken part to start a local implementation project immediately. We are already pushing them to do that," he says. The idea is that, if the wake of an aircraft taking off is being displaced to the left or right of track by a crosswind, the following aircraft does not need to wait as long before it is cleared for take-off.

The local aspect is important because at each airport the topography, weather and relative runway positions all vary. Harvey says the airports chosen for early studies include Paris Charles de Gaulle, Frankfurt Main and London Heathrow, not only because they need the capacity advantages as soon as they can get them, but because they all have high movement rates by large numbers of heavy aircraft, and all have parallel runway operations. Pulsed lidar (laser detection and ranging) wake vortex monitoring systems at these sites is cost-efficient because usable research data will accumulate fastest.

There is a distinct shortage of lidar sensors needed to carry out the research, so Eurocontrol, the FAA and NASA are all sharing their resources, placing the devices where they will produce the best results. The current pulsed lidar systems work by sensing the Doppler shift of particulates in air currents, which is acceptable near the ground where dust and other matter is airborne. In higher, cleaner air other methods will have to be used.

Eurocontrol says it owns one lidar, made by Lockheed Martin's Coherent Technologies, and it is being used at Paris Charles de Gaulle. Between them, the FAA and NASA own four lidars. Meanwhile France's Onera and UK-based Qinetiq are both working on lower-cost lidar technologies using fibre optics, according to the head of Eurocontrol's airport and environment division Paul Wilson. This is important because the Lockheed Martin units cost about $1 million each, he says, and the budget for wake research is limited.

Research co-operation

There has always been transatlantic cooperation on wake research, Wilson says, but now the effort has accelerated. It is only recently that lidar sensors able to measure wake vortex behaviour accurately and reliably have become available. Now, he says, "the research is starting to bear fruit and it will become apparent we can move into an operational implementation phase".

But Wilson warns: "It is vital that whatever is put in place is globally harmonised." One of the problems associated with wake vortex separation standards today, he says, is that there are about 15 local variations based on the fact that some countries or regions group aircraft into different categories based on their weight. Some countries have three categories - heavy, medium and light - and others have five or more categories that define the groups more precisely. FAA wake turbulence programme manager Steve Lang says it is vital pilots should be able to expect the same standards wherever in the world they fly.

"Wake vortex solutions are very complex. There is no easy answer," says Lang. "But now we have identified some near-term solutions we think we can implement within five years. Others will be further ahead. What we hope to do is establish this working group - which is not yet named - to be able to identify the most feasible targets operationally. Then we will decide where the research will go."

Meanwhile, the application of improved knowledge of wake behaviour to increasing airport approach capacity is being researched by Eurocontrol at Brétigny, and also by UK-based air navigation service provider NATS - which provides the terminal area control for all the London airports and the aerodrome control at Heathrow - where next year a trial of time-based separation (TBS) for approaches - as opposed to the distance-based separation in use today - will begin.

The need for TBS is based on the fact that, for aircraft on final approach, a strong headwind means aircraft that are separated by distance have their landing rate dramatically reduced because their ground speed is lower. Wilson says that a 20kt (37km/h) headwind means a 20% reduction in the number of landings that can be carried out within a given time. Separating aircraft by time instead of distance would solve the problem, he says. "There's a great deal of frustration in control towers if you have a 20kt headwind. You still have no flexibility - you have to apply the ICAO wake vortex separation distance. The controllers know there is a better way of doing it."

Also headwinds, like crosswinds, tend to displace aircraft wakes, but in the headwind case they end up below the flightpath of following aircraft rather than being blown to one side. Finally, wake energy dissipates in proportion to the length of time since the aircraft passed a given point in the air, not with distance.

Wilson says that Brétigny has devised a proof-of-concept display that gives controllers a visual indication - in real-time ambient conditions - of the separation distances that equate to an approved time-based spacing. The display, he says, shows a line trailing each aircraft, its length varying to indicate the approved time-based spacing between the pair of aircraft. The controller has to provide the pilot of each following aircraft with instructions that will position the aircraft close to the end of the trailing line.

NATS technical leader research and development Charles Morris says managing TBS will be complex: "There are significant challenges. Individually each challenge looks resolvable, but the problem is that it impacts right across the stakeholder community. Co-ordinating and resolving all the different interests is going to be the biggest challenge." He adds: "I have to be realistic. Controllers today need high-grade skills to manage distance-based separation. TBS will have to be no more difficult to manage." He says there is an alternative to the Brétigny TBS display that uses similar logic, but projects a vector line ahead of each aircraft to indicate the minimum separation by time.

To ideas for avoiding en-route wake turbulence are being studied. The most obvious is an onboard lidar-like device that would be able to "see" wakes and display them for pilot avoidance, but the technology does not exist yet. So the prospective solution involves integrating real-time wind and weather information as it affects each aircraft, combined with aircraft position inputs and a computer model of wake behaviour that can take into account all the ambient circumstances.

If two equipped aircraft could exchange all this information via a datalink such as automatic dependent surveillance - broadcast, pilots could be provided with a forecast position of the wakes of nearby aircraft that would be shown on their aircraft navigation displays.

Turbulent encounter

If the encounter is predicted to be turbulent, four-dimensional manoeuvring information could be provided to the pilot to avoid the predicted wake location (see diagram). The Computer Centre at the Russian Academy of Sciences is one of several units studying the potential of systems like this, and it presented the status of its programme at the WakeNet workshop.

Meanwhile, all aircraft types will soon face a review of the current in-trail and approach separation standards based on the energy in the wakes they produce. In February the FAA, Eurocontrol, the European Joint Aviation Authorities and Airbus met in Brussels to agree a formal structure of the working arrangements" for a reclassification programme.

The case for a general review of wake-related aircraft separation distances has been strengthened by lessons learned during the recent tests carried out on the Airbus A380, says the FAA's Lang, but he insists that reclassification would have been "inevitable". Wilson is optimistic that the framework for a reclassification programme can drawn up by the end of this year.

Determining the cost of a reclassification programme is important, says Wilson, as is the issue of where the funding will come from. He forecasts that reclassification "may not be all that expensive" to achieve. Lang adds: "It might be possible to take a simple cut at reclassification" with further refinements in the future.