A CL65 FLT CREW EXPERIENCES AN NMAC WITH A NAVAJO BENEATH THEM WHEN THEY OVERSHOOT THE ASSIGNED ALT ON APCH TO PHL; PA.

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WE WERE ON A TRIP TO PHL. THE CAPT WAS THE PF. THE FO WAS THE PNF. THE FLT WAS DELAYED 1 HR DUE TO FLOW INTO PHL. WHILE ON THE DUPONT 4 ARR; AFTER TERRI INTXN; WE WERE GIVEN VECTORS; ALT CLRNC; AS WELL AS A 10 KT SPD RESTR. BOTH PLTS CONFIRMED THE ALT. TO THE BEST OF OUR RECOLLECTION IT WAS 6000 FT; THE IN-RANGE RADIO CALL WAS MADE AT APPROX 15 MINS PRIOR TO ARR. AT THE 10000 FT POINT; THE PNF BEGAN TO DO THE IN-RANGE BELOW 10000 FT CHKLIST; AND A SECOND CHK OF THE ATIS; DUE TO THE DETERIORATING WX. WHEN THE PNF RETURNED TO FREQ; THE PNF CONFIRMED; 'NO CHANGES' AND THE PNF SAID 'CONFIRM 6000 FT.' THE PF ANSWERED; '6000 FT.' WHILE DSNDING THROUGH APPROX 7000 FT; ATC ISSUED TA FOR TFC; NAVAJO AT 12 O'CLOCK POS. THE TFC COULD NOT BE SEEN AS WE WERE BTWN IMC LAYERS. THE TFC WAS IDENTED ON TCASII. AS THE TFC PASSED BELOW THE ACFT; THE TCASII GAVE AN AMBER 'CAUTION' ALERT. THE PF THEN ROLLED THE DSCNT RATE TO POSITIVE CLB RATE. THE ACFT WAS AT ABOUT 6700 FT AT THIS TIME AND DURING THE TRANSITION PROCESS; THE ACFT REACHED AN ALT OF APPROX 6500 FT. THE TCASII GAVE AN RA 'RED' ALERT. CONSECUTIVELY WITH THE PF REMOVING THE AUTOPLT AND INITIATING AN IMMEDIATE CLB; THE TCASII GAVE AN AURAL ALERT; 'CLB.' AT THAT TIME ATC SAID; 'THE ASSIGNED ALT WAS 7000 FT; CLB IMMEDIATELY.' THE PNF THEN RESPONDED; '7000 FT.' UPON REACHING 7000 FT; ATC GAVE US A DSCNT TO 6000 FT AND VECTORS FOR THE ILS RWY 27R APCH. AFTER THE APCH AND LNDG WE WERE TOLD TO COPY THE PHONE NUMBER AND CALL THE TWR.