There are repeated instances of pilots incorrectly executing “Adjust Vertical Speed” RAs. In some cases, this wrong reaction to this actually led to the deterioration of spacing between the aircraft.
What are the facts?
TCAS II is designed to generate an “Adjust Vertical Speed” RA instead of a stronger “Climb” or “Descend” RA, whenever possible.
The objective is to solve a predicted risk of collision by a reduction of the current vertical speed, either in climb or in descent, while maximising compatibility with the ATC clearance. The reduction is associated with four different values: 0, 500, 1000 or 2000 ft/min.
RAs could occur when aircraft are in close proximity and the vertical speed of closure exceeds 1500 ft/min;
This type of RA is mainly issued when an aircraft is climbing or descending to level-off 1000 ft from another aircraft. It reinforces the controller’s clearance and helps to ensure successful level-off at the cleared flight level.
“Adjust Vertical Speed” RAs are the most frequent RAs triggered by TCAS II.
What are the potential solutions?
It is essential that all RAs are followed accurately. Aircraft Operators - Remind flight crews that:
It is necessary to observe carefully the RA display when manoeuvring, bearing in mind that an "Adjust Vertical Speed" RA always requires a reduction of the vertical speed.
All RAs are followed accurately, even if there is a discrepancy between RA and ATC clearance or instruction.
Your attention is required
Note the subject and investigate the relevance in your own operational environment;
In order to track the issue and develop sound recommendations it is requested that reports concerning this type of incidents are sent by Aircraft Operators and ANSPs to EUROCONTROL ACAS Monitoring Group.
Discussions are underway between EUROCONTROL, FAA, and industry to determine whether modification to the Adjust Vertical Speed RA (and associated aural alert) is feasible and necessary.