The actual taxi-out time of a flight is the time elapsed between the off-block time of this flight and its take-off time.
The unimpeded taxi-out time is the taxi-out time in non congested conditions at airports. The unimpeded taxi-out time is used in the calculation of the additional taxi-out time.
This technical note describes the methodology used in order to calculate the unimpeded taxi-out time.
The computation of the unimpeded taxi-out time is based on the statistical analysis of flights in non congested conditions.
In order to determine whether a flight is unimpeded or not, its congestion level is computed as the number of preceding movements (arrivals and departures) that occur between the off-block of the flight up to its own take-off.
The graphic here below illustrates the evolution of the actual taxi-out time as a function of the congestion level. In this picture, the range between the first quartile (bottom bar of each rectangle), third quartile (top bar) and the median (medium line) is depicted for each level of congestion.
As shown in this figure, as soon as a certain level of congestion is reached (at the inflection point of the curve), the actual taxi-out time increases linearly with the level of congestion (red line passing through the origin). Beyond that inflection point, the actual taxi-out time is directly proportional to the number of movements ahead. At a low level of congestion (before the inflection point), the actual taxi-out time tends to be constant (horizontal red line). This constant actual taxi-out time is considered to be the unimpeded taxi-out time required by any flight to taxi out and take off. The intersection between the two red lines corresponds to the saturation level.
The greater the level of congestion compared to the saturation level, the greater the likelihood of a flight’s being delayed due to traffic ahead. Therefore, to ensure that a flight is unimpeded, its congestion level needs to be sufficiently small compared to the saturation level. However, it needs to be ensured that the flight sample is big enough to produce a robust estimate of unimpeded ASMA time. Considering a flight as unimpeded if its congestion level is smaller than 50% of the saturation level has proved to be a good compromise between this two conflicting facts.
Night flights may be subject to special procedures that are not representative of daytime operations. Therefore flights between 6:00 and 22:00 only are used for the calculation of unimpeded taxi-out time.
A different unimpeded taxi-out time is determined for each combination:
Although unimpeded taxi-out time is relatively static in time, regular checks are made to ensure that it remains representative of the operations for the airport under consideration. In case a change of unimpeded taxi-out time is detected, the causes of that change (new procedure implementation, change in use of infrastructure) are investigated. When required, new unimpeded times are calculated. However, because this method is based on statistical analysis of the actual taxi-out time, a period of several months after the change is required before new robust unimpeded taxi-out times can be established.
Based on this definition of taxi-out time (from off-block time to take-off time), taxi-out time includes:
The following input data are required in order to calculate the actual taxi-out time:
|ADEP||Departure airport code using the standard ICAO 4-letter code as defined in ICAO Doc. 7910.||Airport data|
|AOBT||Actual off-block time means the actual date and time the aircraft has vacated the parking position (pushed back or on its own power).||Airport data|
|ATOT||Actual take off time means the actual date and time that an aircraft has taken off from the runway (wheels-up).||Airport data|
|DRWY||‘Departure runway designator’ means the ICAO designator of the runway used for take-off (e.g. 10L).||Airport data|
|DSTND||The designator of the departure stand, i.e. the last parking position where the aircraft was parked before departing from the airport.||Airport data|
Actual taxi-out time depends on the distance between the departure stand and the departure runway. Because of the great number of possible stand-runway combinations, stands are grouped at some airports.
Unimpeded taxi-out time is calculated in five steps:
Input data quality check
For each departing flight of the traffic sample
Determination of the saturation level
Computation of the saturation level ():
Identification of the unimpeded flights
For each group (departure stand(s); departure runway):
Selection of flights with a
Computation of the unimpeded taxi-out time
For each group (departure stand(s); departure runway):
The following checks are performed to test the validity of the results:
The percentage of flights with valid data and for which it was possible to define an unimpeded reference time should be at least 90%.
Additional taxi-out time for unimpeded flights should have a distribution centered around zero and with a standard deviation below 3 minutes.
In order to be able to consolidate in a single graph for a given airport with different groups (departure stand and departure runway) and different saturation levels the data has to be normalised.
For this reason a congestion index is defined as
By construction, a value of 100 always corresponds to saturation independent of the departure stand or runway used.
The additional time as a function of the congestion index should:
Additional taxi-out time for unimpeded flights should remain close to zero. A change in the unimpeded taxi-out time might indicate a change in operational procedures or use of infrastructure during the reference period under consideration. In such a case, new unimpeded taxi-out times will be calculated in order to consider the impact of these changes.
Every month the following checks are carried out for each of the airports:
If one of these conditions is not met, new unimpeded taxi-out times are calculated.