The problem of vortices
There are two kinds of dangerous phenomena behind large aircraft. Jet wash and wingtip vortices. These are the most important components of what is commonly referred to as “wake turbulence”. The intensity of this turbulence depends on a number of factors, among them the mass of the aircraft concerned. Jet-wash is simply the rapidly moving air expelled from a jet engine. While it is extremely turbulent, it dissipates quickly in both time and space.
Wingtip vortices on the other hand are much more stable and can remain in the air longer after the passage of an aircraft. Wingtip vortices represent the primary and most dangerous component of wake turbulence.
The hazards of wake turbulence are particularly significant during the landing and take-off phases of flight. Aircraft are in a configuration that creates the strongest vortices while they are also flying at a low speed and low altitude. This leaves little margin for recovery in the event of flying into wake turbulence.
In daily operations the risk of encountering wake turbulence on approach or take-off and initial climb out is mitigated by increasing the spacing between lighter aircraft and a preceding heavier one. The time or distance based minima prescribed to ensure this spacing (the so called wake turbulence separation minima) are static and are based on a worst case assumption of the persistence time of the vortices. While this practice ensures safety, it also reduces the actual throughput of runways below what would otherwise be achievable.
With airports increasingly becoming the bottlenecks in meeting air traffic demand, every measure that can safely increase runway throughput has to be examined and the cost effective ones implemented.
Crosswind to the rescue
As mentioned above, the existing wake turbulence separation minima are static, they prescribe the separation to be maintained between a heavier and a lighter aircraft without regard to the prevailing meteorological conditions, like for instance the cross-wind component on the runway.
The behavior of the vortices follows a pattern that makes their position predictable if the conditions in the air mass in which they were generated are known precisely enough. It is possible to make use of the lateral displacement of the vortices behind the generating aircraft as a result of the lateral movement of the air mass concerned (crosswind). We can determine when it is safe to use conventional, rather than the higher wake turbulence, separation between subsequent aircraft even when the leading aircraft is a heavy.
In effect, use the fact that in some cases, vortices are gone with the wind!
The CREDOS project
The acronym CREDOS stands for “Crosswind-Reduced Separation for Departure Operations”. Originally, it was the name of a European Commission
project in the 6th Framework Program. Its aim was to investigate the possibility of the safe conditional reduction of wake turbulence separation minima for departures.
In time, the name CREDOS was also taken to mean the concept and operational procedures associated with the conditional reduction of wake turbulence separation minima.
CREDOS aims to replace the original, static, and hence often over-conservative, wake turbulence separation minima with lower ones based on actual meteorological conditions, resulting in improved runway throughput.
The CREDOS benefits
The CREDOS benefits arise from a temporary increase in runway throughput. This increase occurs only when a lighter aircraft directly follows a heavier one and the crosswind requirements are met… Only in this case is there a potential for reducing the spacing between that aircraft pair compared to the wake turbulence separation that would otherwise have to be applied. In any case, CREDOS use is not expected to create additional airport slots.
The benefits are at maximum during peak periods or when queuing create delays of departing traffic at the runway. An actual change in the declared runway capacity may only be published if a longer period of stable crosswind conditions is forecast.
In a queuing situation the benefit for aircraft further down the queue can add up to a substantial reduction of waiting time even if the gain experienced by individual aircraft is limited. This translates into improved punctuality and a reduction of environmental impact.
The actual benefits are dependent on traffic composition, the usage of the runway and the SID structure.
What has been explored so far?
The CREDOS project and hence the concept as it now stands is focused on accommodating a single independent runway used for departures only with aircraft on the same Standard Instrument Departure (SID) or different SIDs with the first 4 miles identical and, in both cases, also in a straight line. Neither take-off from an intermediate part of the runway nor super heavy aircraft like the Airbus 380 were considered. These more complicated set-ups will need to be the subject of future work, but there is no reason to believe that benefits would not accrue also in the more complex situations. The same is true for the eventual use of the CREDOS concept for arrivals.
How do you harness the crosswind?
Allow crosswind to blow away the vortices and the problem of wake turbulence is gone… While this is true, one must be sure that the crosswind has actually done its job.
Since visualizing the movement of vortices on and in the vicinity of an aerodrome is not practical, we must resort to an indirect way of ascertaining that they are gone. We know that crosswind transports the vortices. If we are sure of the presence of crosswind that is within the range of prescribed directions and is of sufficient strength, we can safely assume that the vortices are indeed gone… after a certain time.
We can measure wind on the aerodrome but the higher we go or the further we are from the aerodrome itself, the less accurate the measurement will be. Additional sensors can help but costs set a limit to what can reasonably be done.
It is clear that the capability to accurately measure the crosswind conditions (and hence to use the actual presence of crosswind to build confidence in vortices being blown out of the way) limits the volume of airspace in which the reduced spacing can in fact be applied.
Depending on the actually available measuring and forecasting capability, aerodromes using CREDOS will have to designate a volume of airspace around and beyond the runway concerned in which the reduced spacing can be applied. The volume must have assured wind measurements and must contain, to the extent required and with an appropriate buffer, the tracks of departing aircraft.
CREDOS in practice
As mentioned earlier, CREDOS was a project to explore the possibility of reducing or eliminating the throughput reducing effects of wake turbulence based on the fact that crosswind blows wingtip vortices from the path of aircraft taking off behind a heavy.
When actually implemented, the CREDOS system and associated procedures will form a decision support tool for aerodrome controllers with the purpose of enabling the safe, temporary suspension of departure wake turbulence separation minima on an aircraft pair basis with a view to increasing runway throughput.
The automation support in CREDOS will provide controllers with the necessary clues to clear a lighter aircraft to take-off behind a heavier one with minimum spacing without having to deal directly with the complex issue of wake vortex behavior and wind measurements.
The most important elements of the CREDOS user-interface are the strip display and the Take-off Advisory Trigger Line. Of course these elements
have been developed for the simulations and in actual operations different displays may be introduced, depending on local requirements.
The strip display shows the controller whether the suspension of wake turbulence separation between a given aircraft pair is a “GO”. If the conditions are not right, the “GO” indication is replaced by different symbols. These indicate the status of the system and the prevailing conditions.
The Take-off Advisory Trigger Line is shown on the runway controller’s situation display across the planned track of the leading aircraft. If take-off clearance is given to the second aircraft only after the leader has crossed the line, all the spacing criteria will be met. In a simple implementation, the Take-off Advisory Trigger Line is placed at a static position on the display for overall guidance only. In a more sophisticated environment the Take-off Advisory Trigger Line is drawn taking into account the speed of the first aircraft in the pair and hence it is much more accurate and efficient than the static line or a simple time counter. Of course time counters can also be used where these fit better with local procedures and practices.
Is CREDOS safe?
The operational safety of CREDOS is built on two major pillars. On the one hand, the concept and the ways in which it may be implemented has been
subject to a thorough safety analysis with mitigation measures defined for all hazards induced by CREDOS including its interaction with existing procedures and systems.
The other pillar is the way CREDOS will be used in daily operations. Strict procedures will be followed, supported by reliable automation and wind measuring instruments. If conditions (e.g. crosswind) change, suspension of wake turbulence separation will be terminated immediately.
In all circumstances, the pilot in command remains the final authority on whether or not the suspension of wake turbulence separation for his or her flight is acceptable. If it is not, the tower must be informed but there is no requirement to state the reason. There will be no obligation to accept the suspension of wake turbulence separation and no penalty may be applied to an aircraft having indicated its refusal.
The need to build trust
While the controllers in the tower will have the benefit of current information on CREDOS status, the Take-off Advisory Trigger Line, wind and so on, the flight crew has at best a wind direction indicator somewhere off the runway plus whatever they are told via radiotelephony and the ATIS. Obviously, it is not feasible to provide them with full CREDOS information and hence they are being asked to base an important, safety critical operational decision on the trust they have in the skills of the controllers.
You may ask: why is this any different from trusting the controller to safely separate aircraft in the air under instrument meteorological conditions?
In fact, there should be no difference except that in the case of taking off behind a heavier aircraft the flight crew tends to count the elapsed time also themselves, something they rarely do in other circumstances! In other words, they check the correct application of the wake turbulence separation minima.
With CREDOS, the flight crew no longer has this ability.
Extending the otherwise well established trust between flight crew and controller to the take-off spacing case should not be too difficult but the need to work on this must be recognized and the necessary actions undertaken. It is important to keep in mind that the flight crew is not obliged to accept the reduced spacing if they have the slightest doubt about its safety at any given moment.
Does the CREDOS concept have a future?
While the principle of CREDOS is relatively simple, its implementation and safe use requires investment in both training, procedures and equipment. The CREDOS concept recognizes the need for an automated spacing advisory tool and the Take-off Advisory Trigger Line is one example of such a tool. The support for the “GO” indication represents another level of system enhancement that needs to be in place before CREDOS can be used.
The quantifiable benefits of CREDOS depend on a lot of circumstances and will vary from place to place and even from day to day. What is not in question any more is the potential of CREDOS to deliver benefits, even in the limited form the current project has explored. If the additional elements (different SIDs, arrivals, etc.) are also considered, the benefit potential is certainly more substantial.
In terms of cost/benefit, the future of CREDOS will no doubt be impacted by the demand situation and the effectiveness of other methods of increasing airport capacity generally and runway throughput in particular. CREDOS’ relatively modest gains will come into their element when the range of diminishing returns with other solutions is reached, making the CREDOS cost/benefit ratio, in relative terms, more attractive.
The CREDOS project has done the important groundwork on which to build the more complex realizations of the concept. There is still some time and if it is used well, this concept is likely to remain in the airport air traffic management tool-box of the future.
Want to read more?
For a more detailed description of the CREDOS concept arranged in separate sections for pilots and air traffic controllers, go to the CREDOS end-user portal available here. The portal is a product of BluSky Services, commissioned by EUROCONTROL.