ACAS alerts only occur under extreme conditions in
ASAS merging simulations
A new allocation of tasks between controller and
flight crew is envisaged as one possible option to improve air traffic
management and in particular the sequencing of arrival flows (see CoSpace
project). It relies on a set of new spacing instructions where the flight
crew can be tasked by the controller to maintain a given spacing (in time or in
distance) with respect to a designated aircraft. This task allocation, denoted
airborne spacing, is expected to increase controller availability. This could
lead to improved safety, which in turn could enable better quality of service
and, depending on airspace constraints, more capacity. In addition, it is
expected that flight crews would gain in awareness and anticipation by taking
an active part in the management of their situation with respect to a
designated aircraft. The motivation is neither to ‘transfer problems’ nor to
‘give more freedom’ to flight crew, but really to identify a more effective
task distribution beneficial to all parties without modifying responsibility
for separation provision. Airborne spacing assumes airborne surveillance
(ADS-B) along with cockpit automation (ASAS). No significant change to ground
systems is initially required.
Particular airborne spacing applications are currently under investigation to help the air traffic controller sequence aircraft in zones of convergence such as the TMA (Terminal Control Area) and extended TMA surrounding an airport. Two new main kinds of spacing instruction ‘merge behind’ and ‘remain behind’ are being evaluated in air traffic controller and pilot in the loop real-time simulation based experiments. The ‘merge behind’ along with its variant ‘heading then merge behind’ instructions have been devised to assist the air traffic controller create sequences. Similarly the complementary ‘remain behind’ instruction is intended to help air traffic controllers maintain sequences. Past studies have investigated both distance and time based airborne spacing of sequences of aircraft using fast time and real-time experiments. Time based spacing has been found to have potential advantages over distance based spacing.
Figure 1: ‘Merge behind’: lead and trail trajectories for a merge angle of 180° and 60 s of spacing
Before ASAS airborne spacing can be implemented in the ECAC (European Civil Aviation Conference) area, several questions remain to be answered. In particular, the issue of ACAS and ASAS interaction has to be addressed. The IAPA (Implications on ACAS Performances due to ASAS implementation) project conducted a preliminary study of ACAS-ASAS interaction with a rough model of two merging aircraft. Results indicated little interaction expected for nominal conditions but aspects such as turbulent winds, pilot behaviour, closed-loop time based guidance and sensitivity analysis were not addressed ( http://www.eurocontrol.int/acas/ ).
The objective of these CoSpace fast time simulations is to quantify how close
nominal and non-nominal encounters of aircraft performing an airborne spacing
‘merge behind’ application come to triggering TCAS alerts, i.e. a sensitivity
analysis to investigate margins of safety. Using a time-based guidance law, the ‘merge behind’ application was
simulated in fast-time (MATLAB/Simulink) for a range
of merge angles (45°; 90°, 135° and 180°), target spacing times (60 and 90
seconds) and altitudes (6,000 and 11,000 feet) under turbulent wind and extreme
entry conditions. Sixteen merge encounter trajectories were filtered from the
MATLAB simulations. These trajectories were analysed for potential collision alerting conditions using an dedicated ACAS simulator based on TCAS
II version 7 logic to observe how the ‘tau’ behaved with
time, the relation with thresholds and to note the time at which any TAs and/or
RAs occurred.
Results confirm that the TCAS estimated time to go to Closest Point of Approach (CPA), ‘tau’, decreased as merge angle was increased and as target spacing was reduced.
Figure 2: Minimum values for TCAS ‘tau’ as function of merge angles, at 11,000 feet
For 90 s target spacing, ‘tau’ remained at least 18 s above the Traffic Advisory (TA) thresholds, when both aircraft anticipated turns. Even without turn anticipation no TAs occurred before the lead aircraft overflew the merge waypoint. This is consistent with the hypothesis that the airborne spacing ‘merge behind’ application for target spacing over 90 s should not normally induce TCAS alerts when smooth turns are anticipated and aircraft follow their planned tracks.
For 60 s target spacing and merge angles less than 90°, ‘tau’ remained at least 17 s above the TA thresholds, when both aircraft anticipated turns. Even without turn anticipation, no TAs occurred before the lead aircraft overflew the merge waypoint. However, for merge angles above 135°, ‘tau’ margins varied from 11s down to 1s (at 180°). Without turn anticipation several TAs occurred for merge angles above 135°. It should be noted though, that the co-altitude assumption for high merge angles like 180° is very extreme and operationally aircraft would normally be vertically separated.
This analysis based on extreme entry conditions and observing TCAS ‘tau’ behaviour may be useful when designing airspace and procedures for ASAS ‘merge behind’ real-time experiments. Other future research could include a more detailed analysis for sequences of aircraft in descent
For further information contact Dan Ivanescu or Chris Shaw