B757 crew reported responding to an resolution advisory (RA) to descend even though the traffic they...

Narrative:

We were given a descent to FL230 and advised of crossing traffic right to left at FL220. We saw the traffic at approximately 5 miles off the right side. At approximately 2-3 miles we received a TCAS traffic alert (TA) as the aircraft passed off the nose we received a TCAS resolution advisory (RA) to 'adjust vertical speed'. The green box in the vsi was still telling us to descend though the other aircraft was below us and in sight. The flying pilot disconnected the autopilot and followed the RA even though we thought the RA was in error. As the aircraft passed through FL230 the vsi still showed a descent on the vsi green box. At FL227 we leveled off and began a climb back to FL230. We informed ATC of the RA compliance causing the alt deviation. The 'adjust vertical speed' and the vsi green box showing to continue the descent with the other aircraft insight and below us caused confusion to the RA resolution. The system was requiring a reduction in descent rate but the voice command and vsi green box was a cause for the confusion. The new version of TCAS calls for a 'level off' instead of 'adjust vertical speed'. I believe that these factors were a cause for the deviation and confusion.the 2 different TCAS systems have different commands and using both systems can cause confusion. Also; the reason for the RA was unclear since the other aircraft was clear and below. If both systems will remain in use; then the differences will need to emphasize more clearly.

Original NASA ASRS Text

Title: B757 crew reported responding to an Resolution Advisory (RA) to descend even though the traffic they saw was below their altitude. Crew also reported that there were two versions of TCAS that had different commands for the same situation.

Narrative: We were given a descent to FL230 and advised of crossing traffic right to left at FL220. We saw the traffic at approximately 5 miles off the right side. At approximately 2-3 miles we received a TCAS Traffic Alert (TA) as the aircraft passed off the nose we received a TCAS Resolution Advisory (RA) to 'adjust vertical speed'. The green box in the VSI was still telling us to descend though the other aircraft was below us and in sight. The flying pilot disconnected the autopilot and followed the RA even though we thought the RA was in error. As the aircraft passed through FL230 the VSI still showed a descent on the VSI green box. At FL227 we leveled off and began a climb back to FL230. We informed ATC of the RA compliance causing the alt deviation. The 'adjust vertical speed' and the VSI green box showing to continue the descent with the other aircraft insight and below us caused confusion to the RA resolution. The system was requiring a reduction in descent rate but the voice command and VSI green box was a cause for the confusion. The new version of TCAS calls for a 'level off' instead of 'adjust vertical Speed'. I believe that these factors were a cause for the deviation and confusion.The 2 different TCAS systems have different commands and using both systems can cause confusion. Also; the reason for the RA was unclear since the other aircraft was clear and below. If both systems will remain in use; then the differences will need to emphasize more clearly.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1399579
Time
Date	201611
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Traffic Collision Avoidance System (TCAS)
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Conflict Airborne Conflict

B757 crew reported responding to an resolution advisory (RA) to descend even though the traffic they saw was below their altitude. Crew also reported that there were two versions of TCAS that had different commands for the same situation.