ACR-MLG ALT DEVIATION OVERSHOT DURING CLIMB CAUSING LESS THAN STANDARD SEPARATION WITH ACR-MLG ON SAME DEP PARALLEL TRACK.

Narrative

DURING ENRTE CLB; CENTER ASKED FOR MAX FORWARD SPD AND SLOW CLB DUE TO ACFT AHEAD AND AT HIGHER ALT. THE ADVISED PLAN WAS TO UNDERRUN SLOWER/HIGHER TFC. WE WERE ASSIGNED SEVERAL STEP CLBS AS THE OTHER ACFT WAS BEING WORKED UP HIGHER ALSO. WE WERE ASSIGNED 24000' WHILE FL260; THEN TO FL280. INITIALLY WE WERE ASSIGNED FL260. I CLBED UP TO THIS LEVEL AND LEVELLED OFF. NEXT; CENTER ASSIGNED US FL270 AND SAID OUR TFC WAS AT 2-3 O'CLOCK AT FL280. THE TFC HAD BEEN VISIBLE FOR THE PAST 5-6 MINS. AS WE PASSED FL270; BOTH CAPT AND I LOOKED AT THE ALT ALERT IT INDICATED 27800'. WE TRIED TO ASKED CENTER TO VERIFY OUR ALT BUT FREQ WAS BEING USED BTWN CENTER AND ANOTHER ACFT. CAPT ASKED ME WHAT ALT I THOUGHT HAD BEEN ASSIGNED. I SAID; 'I HEARD FL280.' THE OTHER PLANE WAS AT 3:30-4 O'CLOCK NOW. CENTER BREAKS IN ON FREQ AND ADVISED US TO DSND TO FL270 IMMEDIATELY. WE DID. THE FACT I COULD SEE THE TFC PROBABLY LOWERED MY SENSE OF ALT AWARENESS. THE FACT I HEARD FL280 MADE ME THINK ALT ALERT SHOULD BE AT FL280 VS FL270. OUR PROX MAY HAVE RUNG CAPT'S ALARM BELL; BUT WITH OUR VISUAL AND INSTRUMENT CUES; IT DIDN'T SEEM TOO CLOSE TO QUALIFY AS A NEAR MISS FROM OUR VANTAGE POINT. SUPPLEMENTAL INFO FROM ACN 92257: I REALIZED I HAD NOT SET THE ALT ALERT CORRECTLY (IT READ 27800') AND ASKED THE F/O WHAT ALT WE WERE CLBING TO. HE SAID '280.'