Date: 201307
Local time of day: 1801-2400
Location: LAS.Airport, NV
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B LAS
ATC / advisory: Tower LAS
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Communication Breakdown; Distraction; Workload; Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: ATC Issue All Types; Conflict NMAC
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Airport; Environment - Non Weather Related; Procedure
Primary problem: Ambiguous

Narrative: A few miles inside KADDY on the TYSSN 3 Arrival; got cleared direct to PRINO for the expected visual to 25L. Soon thereafter; another Controller came on the radio and said change of plans and gave us heading 340 and plan on RNAV visual 19L. About 20 miles from field; cleared direct TRROP RNAV visual 19R behind a 757. Spacing was a little tight; the 757 was widening out his visual approach; and the RNAV routing was taking us in tighter. So Approach cleared us to widen it out to ensure spacing with the 757 and cleared us a visual to 19R. Radio traffic was congested; and the controllers sounded somewhat stressed. After going to Tower; the Controller said any one who heard her voice to switch to 119.9; which we did. I was manually flying the aircraft and slowed down and configured to ensure adequate spacing and followed the 757 path to turn final about two miles north of the stratosphere. Started a gradual descent down to 3;500 FT MSL and configured to slow. I noticed on TCAS an aircraft above us by 1;000 FT descending with us. As I observed; it slowly got closer; and I asked the Captain to query Tower if they were aware of the traffic descending above us (which by this time was causing a TCAS Traffic Advisory). They told us it was (another carrier) who was maintaining visual separation with us. I looked up out of my right window and saw the traffic; and wondered how they could see us since I was almost looking straight up at them. While continuing on base; the other carrier continued to descend into us and we received a TCAS RA to descend. By this time I was at 3;500 FT MSL (about 1;500 FT AGL). I followed the RA command while the Captain informed Tower. I was very concerned about descending quickly so close to the ground with a lot of tall buildings nearby; but did so carefully. The Captain informed Tower of the RA and they asked us our intentions. The Captain said we were complying with the RA command and planned to still proceed to the field if possible. We couldn't go-around at the time because that would have taken us into the other aircraft and would not have complied with the RA. What was frustrating was as I descended to 1;000 FT AGL the conflicting traffic continued to descend with us and came within 400 FT vertical. The Captain said he saw them come within 300 FT vertical on his TCAS. As I leveled at 1;000 FT AGL and cleared outside to ensure we didn't overfly final and get any closer to the buildings; the TCAS RA was resolved. By this time I was also rolling out on final north of the stratosphere and proceeded inbound to 19R. The Captain told me to look left out his window and I was somewhat shocked to see the other carrier co-altitude and abeam us on final to 19L. We were concerned about the horizontal displacement; but after assessing the situation and seeing aircraft airborne departing Runway 25; felt it was safe and more prudent to land the aircraft than to do a go-around. After landing; Ground Control relayed a phone number for the Captain to call Tower; so they could get more information on the RA. As I replay the event; I keep thinking we followed the clearance given us; ensured proper spacing with the 757 gave Tower a heads up of the impending traffic conflict; properly followed the TCAS RA while staying away from terrain/obstacles; and safely landed the plane. While we thought of performing a go-around; we initially couldn't because of the traffic conflict; and once on final felt it was safer and more prudent to land the plane. Extremely unsettling circumstances; I don't ever recall hearing this other carrier on frequency. A few questions: Why did the other carrier not also receive a TCAS RA; and if they did; why didn't they follow it but continue to descend with us? Did they truly have us in sight the whole time? I saw them only that one time as they flew over us until I rolled out on final. Why did Tower clear them the visual to 19L and us to 19R when the conflict could have been avoided if they'd been sent to 19R and us to 19L? I thought as a crew we handled all the threats well and avoided a potentially catastrophic outcome.


Second narrative: ATC never advised us of the traffic and our flight paths would cross. As the pilot flying flew the RA; I verified the traffic on the map display and noticed the aircraft was just off our tail and 300 FT above us continuing to descend on top of us. I looked behind me an saw the aircraft descending and now parallel and overtaking us as he was lined up for 19L. We were still receiving an RA and at no time did the other aircraft initiate a go-around or seem like they were compiling with an RA on their side. We continued to comply with the RA until below the PAPI and 1;000 FT AGL; the RA ceased and we leveled to regain the PAPI and continued the approach.


Synopsis: An air carrier crew descending to LAS 19R took evasive action responding to a TCAS RA and descended to 1;000 FT AGL evading an air carrier aircraft to 19L which was within 300 FT; but did not take evasive action.