Narrative:

Air carrier X was established on the localizer for runway 17C at dfw at 4000 ft. Air carrier Y was on a vector to runway 18R at 3000 ft. Air carrier X was cleared for ILS approach to runway 17C to cross penny intersection at 4000 ft. The aircraft crew read back approach clearance; confused about the speed assignment; and stated something about 3000 ft. The aircraft descended out of 4000 ft and separation was lost between air carrier X and air carrier Y. The things that caused this were: the flight crew missed the 4000 ft crossing restr and I did not hear the statement about 3000 ft. In order to help prevent this situation from recurring: flight crews must know that when you are within 15 mi from the airport of intended landing (especially a busy one like dfw); that they must pay attention to what they are instructed to do. If one of the crew members is busy doing something that they are unable to listen to ATC; the one that is receiving ATC instructions must not assume they heard something they did not. In the clearance that was issued to this aircraft; the speed assignment and crossing restr were clearly stated. If there is any question of what was said; ask for a clarification. Do not choose altitudes or speeds on your own. Also; as an ATC specialist; I must also listen more closely to what flight crews read back. A controller issues many instructions hourly and the majority of them are read back correctly. When air carrier X read back its clearance; I caught the statement about the speed being incorrect; but not the part about altitude. The incorrect speed was caught and I knew I had to correct this. Possibly this triggered a reflex thought action that momentarily made me not hear the incorrect altitude. Standard phraseology must be used by ATC and instructions must be read back by flight crews in a logical sequence. Read back the clearance as it was stated. If there are any questions about the clearance; ask for clarification. Supplemental information from acn 676213: vectors for ILS runway 17C at 4000 ft cleared to 3000 ft at 190 KTS. Thought we were cleared to 3000 ft and believed that we were by penny intersection and cleared to 3000 ft.

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Original NASA ASRS Text

Title: DFW APCH CTLR RPTS LOSS OF SEPARATION WHEN B757 DSNDS FROM CLRED ALT PRIOR TO PENNY INTXN ON APCH TO RWY 17C.

Narrative: ACR X WAS ESTABLISHED ON THE LOC FOR RWY 17C AT DFW AT 4000 FT. ACR Y WAS ON A VECTOR TO RWY 18R AT 3000 FT. ACR X WAS CLRED FOR ILS APCH TO RWY 17C TO CROSS PENNY INTXN AT 4000 FT. THE ACFT CREW READ BACK APCH CLRNC; CONFUSED ABOUT THE SPD ASSIGNMENT; AND STATED SOMETHING ABOUT 3000 FT. THE ACFT DSNDED OUT OF 4000 FT AND SEPARATION WAS LOST BTWN ACR X AND ACR Y. THE THINGS THAT CAUSED THIS WERE: THE FLT CREW MISSED THE 4000 FT XING RESTR AND I DID NOT HEAR THE STATEMENT ABOUT 3000 FT. IN ORDER TO HELP PREVENT THIS SITUATION FROM RECURRING: FLT CREWS MUST KNOW THAT WHEN YOU ARE WITHIN 15 MI FROM THE ARPT OF INTENDED LNDG (ESPECIALLY A BUSY ONE LIKE DFW); THAT THEY MUST PAY ATTN TO WHAT THEY ARE INSTRUCTED TO DO. IF ONE OF THE CREW MEMBERS IS BUSY DOING SOMETHING THAT THEY ARE UNABLE TO LISTEN TO ATC; THE ONE THAT IS RECEIVING ATC INSTRUCTIONS MUST NOT ASSUME THEY HEARD SOMETHING THEY DID NOT. IN THE CLRNC THAT WAS ISSUED TO THIS ACFT; THE SPD ASSIGNMENT AND XING RESTR WERE CLRLY STATED. IF THERE IS ANY QUESTION OF WHAT WAS SAID; ASK FOR A CLARIFICATION. DO NOT CHOOSE ALTS OR SPDS ON YOUR OWN. ALSO; AS AN ATC SPECIALIST; I MUST ALSO LISTEN MORE CLOSELY TO WHAT FLT CREWS READ BACK. A CTLR ISSUES MANY INSTRUCTIONS HRLY AND THE MAJORITY OF THEM ARE READ BACK CORRECTLY. WHEN ACR X READ BACK ITS CLRNC; I CAUGHT THE STATEMENT ABOUT THE SPD BEING INCORRECT; BUT NOT THE PART ABOUT ALT. THE INCORRECT SPD WAS CAUGHT AND I KNEW I HAD TO CORRECT THIS. POSSIBLY THIS TRIGGERED A REFLEX THOUGHT ACTION THAT MOMENTARILY MADE ME NOT HEAR THE INCORRECT ALT. STANDARD PHRASEOLOGY MUST BE USED BY ATC AND INSTRUCTIONS MUST BE READ BACK BY FLT CREWS IN A LOGICAL SEQUENCE. READ BACK THE CLRNC AS IT WAS STATED. IF THERE ARE ANY QUESTIONS ABOUT THE CLRNC; ASK FOR CLARIFICATION. SUPPLEMENTAL INFO FROM ACN 676213: VECTORS FOR ILS RWY 17C AT 4000 FT CLRED TO 3000 FT AT 190 KTS. THOUGHT WE WERE CLRED TO 3000 FT AND BELIEVED THAT WE WERE BY PENNY INTXN AND CLRED TO 3000 FT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.