Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP)
Conflict NMAC
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Airspace Structure; Environment - Non Weather Related
We were with New York Approach descending to 6;000. There was a Twin Commander on frequency asking for a clearance to an area airport. ATC told him to stay below 5;500 for traffic [us] descending to 6;000. ATC advised us of the Twin Commander at one o'clock; five miles and converging. We were descending less than 1;000 FPM. We looked outside but did not see the traffic nor did the traffic show up on TCAS. We were about to level 6;000 and another call from New York Center advised us. She also told the Twin Commander to maintain below 5;500 FT. Suddenly we got a TCAS TA followed by a 'CLIMB; CLIMB' RA. The PIC climbed as the other aircraft showed up on TCAS only one mile from us and about 300 FT below us (which put him at about 5;800-5;900 MSL; not 5;500 as advised by ATC). We climbed to 6;400 when we received a CLEAR OF CONFLICT from TCAS. We advised ATC and she said maintain 6;000; which we did. Note: When we performed the climb we saw the Twin Commander pass underneath us and he would clearly have been within 200 below us had we not climbed. Note also; our altimeter was set to the local as provided by ATC.I feel both the PIC and I did as we have been trained to do. When the TCAS RA sounded we did exactly as we have been trained. My only concern was this aircraft never appeared on TCAS until the very last second (about 1 mile out). I was extremely concerned about not seeing this on the TCAS.
While descending IFR to 6;000 a CE-560XL flight crew responded to TCAS TA and RA alerts due to a conflict with a VFR Aero Commander allegedly at 5;500 MSL while requesting an IFR clearance.