Flight Crew Last 90 Days 237; Flight Crew Type 3000
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Aircraft; Procedure
On the RMBLR Departure out of Atlanta; ATC leveled us off at 8;000 FT MSL. While at 8;000 FT MSL; another aircraft was vectored over us at approximately 8;500 FT. ATC did not call out the traffic to us. We received an RA 'Descend Now' Advisory. I complied with the advisory and started a 2;000 FPM descent into the green arc on the VSI. I descended to approximately 7;600 FT and the Resolution Advisory ceased. We informed ATC of the event and then climbed back up to our assigned altitude. We believe the other aircraft's descent rate caused our system to trigger the RA.
Prior to our departure from ATL; Tower changed our altitude clearance from 10;000 FT to 8;000 FT without explanation. Departure Control also verified that our clearance was 8;000 FT. As we leveled at 8;000 FT; we received first a caution; then a descend command from our TCAS to which the First Officer; as the pilot flying; responded. I called our TCAS directed descent to Departure and they responded with a traffic callout of VFR traffic at 8;500 FT. When the TCAS reported 'Clear of Conflict;' we returned to 8;000 FT.I have never seen 500 FT of separation in level flight not cause a TCAS RA. 500 FT of separation satisfies everything except the TCAS. Controllers should be made aware of this issue; and the TCAS logic that requires an RA in this situation should be re-examined. In the interim; pilots should continue to respond to the RA as we have been trained.
B737 flight crew leveling at 8;000 FT experiences a TCAS RA due to VFR traffic at 8;500 FT and descends.