Progressive implementation of Wake-Independent Departure and Arrival Operations (WIDAO) at Paris CDG airport

Paris Charles de Gaulle (CDG) airport operates four runways organised in two Closely Spaced Parallel Runway (CSPR) pairs. Typically an external runway from each pair is used for landing and an internal runway for take-off. For mainly environmental reasons, the external runways are shorter than the internal runways. The consequence of this is an offset of 600m in West operations between the two runway thresholds and of 900m in East operations (see figure 1).

Figure 1 – Example of runway threshold offset at Paris CDG

The landing touch-down points on the external runways are therefore located down-stream of the runway threshold of the internal (departure) runway. Because wake turbulence (WT) is generated by an aircraft until it touches the ground, there is a risk of wake turbulence of a landing aircraft being transported by the wind to the departure runway. If a departing aircraft is lined-up near the runway threshold, its rotation point could be in close proximity to this turbulence (see figure 2.a). The consequence of a wake encounter in these circumstances could be for the departing aircraft to experience a sudden roll, control difficulties or structural damage, depending on the strength of the wake encountered. The associated risk can be particularly severe in such close proximity to the ground. The same kind of wake turbulence risk is also expected for landing aircraft if an aircraft is departing on the CSPR at the same time (see figure 2.b).
Figure 2 – Wake turbulence hazard on CSPR
The ICAO minimum runway separation recommendation (760m for independent segregated operations for non-staggered runways and even more for staggered CSPR runways like in Paris CDG Airport) is not made with direct reference to wake turbulence issues. Nonetheless, many operators (including the French air navigation service provider, DSNA), apply similar wake turbulence separations to CSPR to those that are defined for in-trail aircraft by ICAO.

In order to ensure the proper application of these wake turbulence separations, DSNA has imposed time constraints on departing aircraft lining-up at runway entry points close to the threshold, if an arriving aircraft is landing at the same time on the parallel runway. These constraints had a negative impact on departure capacity by limiting the lining-up strategy and also on taxiway congestion by imposing the use of non-constrained runway entries in close proximity to the CDG terminals.

It was suggested by DSNA that such restrictions placed on CSPR could be overly and unnecessarily conservative and that the relaxation of these constraints could have a positive impact on runway capacity. The relaxation of these constraints was expected to improve departure queue management, reduce taxiway congestion and simplify complex procedures.

In 2007, DSNA requested EUROCONTROL support to prepare a local safety case to investigate whether independent segregated mode operations on CSPR could be performed with acceptable levels of safety. The Wake-Independent Departure and Arrival Operations (WIDAO) project was started in March 2007 and an agenda was defined together with the French regulator for the progressive relaxation of constraints in 3 steps.

In 2007 and 2008, an extensive data collection campaign was conducted in order to gather the evidence required for the safety case. The EUROCONTROL LIDAR (Light Detection and Ranging) system was deployed and used for collecting more than 6,000 tracks of Heavy wake vortex and 25,000 tracks of Medium wake vortex (see figure 3).

Figure 3 - EUROCONTROL LIDAR data collection
In parallel, radar track post-processing algorithms were developed and used by DSNA to collect more than 80,000 aircraft rolling distances from ground radar and more than 75,000 landing separation distances in approach from airborne radar.

Based on the EUROCONTROL data analysis, two Preliminary Evaluations of the Impact on Safety (“Evaluation Préliminaire d’Impact sur la Sécurité d’un Dispositif de Circulation Aérienne”, EPIS-CA) were introduced and were endorsed by the French regulator in November 2008 and April 2009. These preliminary evaluations led to the implementation of the first two sets of constraint relaxation for Medium departures.

Because of the range of airspace to be covered for collecting the required data for supporting the third set of constraint relaxation for Heavy departures, no LIDAR measurement strategy could be found. A decision was made to base the safety assessment of this third step on the use of wake vortex prediction software (WAKE 4D). 8.74 million WAKE 4D runs (Figure 4) were required for covering all Heavy aircraft departure trajectories derived from the radar data and all potential weather conditions observed by Météo France for more than 20 years at CDG.
Figure 4 – Example of a WAKE 4D simulation run
EUROCONTROL and DSNA are now working together on the finalisation of a full local safety case in line with the EUROCONTROL Safety Assessment Methodology and combining the results of this safety assessment and the details of all previous analysis conducted for the two previous improvement steps. Delivery of this safety case to the French regulator is planned for spring 2010.


  Acrobat WIDAO presentation at Global Wake conference, October 2009, EUROCONTROL Brussels


Vincent Treve
  Last validation: 31/03/2010