ACR-LGT ON STAR DESCENDED THROUGH THE ALT OF ACR-MLG ON SID; RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative

OUR FLT (ACR IADC) WAS ON THE ACTON 9 ARR TO DFW; ESTABLISHED AT 11000 FT MSL AND 250 KIAS. BTWN FLATO AND CREEK INTXNS; WE WERE HANDED OFF TO AN ARR CTLR WHO INSTRUCTED US TO 'TURN L TO 360 (AND) DSND AND MAINTAIN 6000 FT.' CLRNC WAS ACKNOWLEDGED BY CALL SIGN: 'ACR IADC' AND READ BACK VERBATIM: 'TURN L 360; DEPARTING 10000 FOR 6000.' THE CAPT (FLYING) TURNED TO THE VECTOR HDG AND INITIATED THE DSCNT. PASSING APPROX 10000 FT I OBSERVED MLG Y ONLY MOMENTARILY; APCHING FROM OUR 4 O'CLOCK POS; AND PASSING R TO L DIRECTLY BENEATH OUR ACFT; WHICH APPEARED TO BE WITHIN 1000 FT. WHEN I QUERIED THE CTLR ABOUT MLG'S ALT; THE CTLR STATED THAT HE WAS AT 9000 FT. WHEN TOLD THAT 'WE HAD CLRNC TO DSND TO 6000 FT;' HE TOLD US; 'THAT CLRNC WAS FOR AN ACR DDC.' THERE WAS A DDC ON ARR TO DFW; BUT NOT ON THE SAME FREQ. IT LANDED MINS BEFORE US ON RWY 18R. THE CAPT CALLED THE APCH CTL SUPVR AND LEARNED THAT A CTLR TRNEE WAS HANDLING OUR FLT AND THAT DURING THAT PERIOD; THERE WAS A SHORT LAPSE OF SUPERVISION. CONTRIBUTING FACTORS: HIGH TASK SATURATION IN CERTAIN CTL SECTORS--NOT A GOOD TRNING ENVIRONMENT FOR LESS EXPERIENCED CTLRS. ANOTHER FACTOR MAY BE THE SIMILAR CALL SIGNS; WHICH IS A RELATIVELY NEW PROB SINCE COMPANIES BEGAN MIXING 3 AND 4 DIGIT CALL SIGNS--IT SEEMS TO INVITE ERROR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: DURING A TELEPHONE CALL TO THE TRACON; RPTR'S CAPT WAS TOLD THAT THE NEAR COLLISION WAS CAUSED BY CTLR ERROR. THE INSTR WATCHING A TRNEE; TURNED AWAY FOR A SHORT TIME; AND DURING THAT TIME; THE TRNEE GAVE THE DSCNT CLRNC TO RPTR USING RPTR'S CALL SIGN. TRNEE MEANT TO DSND ANOTHER ACFT; BUT MISSPOKE THE CALL SIGN. (RPTR STATED THAT HE HAD THOUGHT THEIR ALT WAS 11000 FT WHEN THEY STARTED DOWN TO 6000 FT. THE RPT SHOWED AN ALT OF 10000 FT.) SUPPLEMENTAL INFO FROM ACN 90376: I INITIALLY SPOTTED THE ACFT AT MY 10 O'CLOCK POS; APPROX DISTANCE 3 MI; CONVERGING COURSE AND STEADY BEARING. SINCE WE WERE AT OUR ASSIGNED ALT OF 10000 FT; I ASSUMED; AND IT APPEARED; THE OTHER ACFT WAS AT 11000 FT. I COMMENTED TO MY F/O; AS I POINTED THE TARGET OUT; 'I WONDER WHY DEP DOESN'T CALL HIM OUT.' THE RELATIVE BEARING DID NOT CHANGE AND AS HE GOT CLOSER; MY INSTINCTS; BORNE OF A 20 YR NAVY BACKGROUND OF DAY AND NIGHT RENDEZVOUS IN HIGH PERFORMANCE ACFT; TOLD ME SOMETHING WAS NOT RIGHT AND THAT HE WAS IN FACT AT MY ALT! I ORDERED THE COPLT WHO WAS AT THE CTLS: 'GO DOWN NOW!' HE DID SO AND AVOIDED WHAT COULD HAVE BEEN A MIDAIR COLLISION AS THE OTHER ACFT PASSED DIRECTLY OVER US IN WHAT APPEARED TO BE A SLIGHT DSCNT. I ESTIMATE THE VERTICAL SEP; AT CPA; TO BE 200-300 FT. BECAUSE OF OUR EXTREME SIT OUR MANEUVER WAS ABRUPT ENOUGH TO POSSIBLY STIR PAX ATTN. OUR RECOVERY BOTTOMED OUT AT 9200 FT. I ESTIMATE WE MAY HAVE BEEN AT 9400 FT DURING CROSSOVER AND THE OTHER ACFT AT 9700 FT. SUPPLEMENTAL INFO FROM ACN 90440: CAPT CALLED OUT TFC; A COMPANY LGT; AT 10 O'CLOCK; SLIGHTLY HIGH. AFTER 5 SECONDS OR SO IT BECAME APPARENT WE WERE ON A CONSTANT BEARING AND CLOSING. THE CAPT CALLED FOR A DSCNT AND WE STARTED DOWN. IT WASN'T ENOUGH; SO I ROLLED L AND SHOVED THE YOKE OVER. MY REFLECTIONS: HAD WE BEEN IMC; WE WOULD NOT BE. HAD IT BEEN NIGHT; OR SUNSET; WE WOULD NOT BE. HAD THE CAPT BEEN FLYING; AND CONSEQUENTLY A BIT MORE COMMITTED INSIDE THE COCKPIT; MY SCAN MIGHT NOT HAVE PICKED UP THE LGT AT ABOUT 5 MI. I'VE EXPERIENCED CLOSURE RATES LIKE THIS ONE DURING AIR COMBAT MANEUVERING IN THE NAVY; BUT FOUND CHANGING THE VELOCITY VECTOR OF A MLG SURPRISINGLY MORE DIFFICULT--SOMETHING I'VE THOUGHT ABOUT; BUT NOT ENOUGH! LASTLY; WE HAD EVERYTHING IN OUR FAVOR TO EFFECT A SEE AND AVOID REACTION: LOW POS; GOOD LIGHT; A SCANNING PLT ON THE SIDE OF THE THREAT; NO AUTOPLT TO DISCONNECT; AND WE STILL ALMOST HIT THIS GUY.