Narrative:

I was the PF. We checked in with departure control. They immediately assigned us an altitude of 4000 ft MSL. Upon reaching 4000 ft MSL, ATC assigned us 6000 ft MSL. The captain and I both heard and understood a climb to 6000 ft MSL. The captain clearly read back 'climb to 6000 ft MSL' and dialed 6000 ft MSL into the altitude selector. Reaching 5000 ft MSL, ATC advised us that a DC9 would be crossing over our altitude 1000 ft above us. What that meant to me is that since we were cleared to, and climbing to, 6000 ft MSL. He must be at 7000 ft MSL. At about that time the TCASII system gave us a TA warning and ATC asked us if we had the traffic in sight at 3-4 O'clock above us, a DC9. I looked out the window and saw the traffic and the captain acknowledged to ATC, 'traffic in sight.' climbing through about 5100 ft MSL, the TCASII gave us an RA warning, warning us 'don't climb.' the captain then told me, 'as long as you have the traffic in sight, we can disregard the RA and continue our climb.' reluctantly, I did, and continued to climb to about 5500 ft MSL. When suddenly, with the TCASII telling us to 'don't climb and monitor vertical speed' and the DC9 closing in on us. I sensed something wasn't right and I leveled off at 5500 ft MSL. The DC9 then asked ATC 'what was our altitude?' ATC then told us to 'maintain 5000 ft MSL.' my captain replied to ATC, 'we are descending back down to 5000 ft MSL.' the DC9 passed over us and no further conflict arose. When on the ground, the captain and I called stl departure control and spoke with the supervisor on duty. He told us, after listening to the ATC tapes, we were only cleared to 5000 ft MSL, however, that we clearly read back 6000 ft MSL. The problem is that ATC did fail to catch and correct an improper altitude readback. Problem #2, ATC should have advised us of exactly the altitude the DC9 was at, ie, traffic at 6000 ft MSL, instead of just say altitude 1000 ft above you. The lesson learned here: whenever in doubt and something doesn't feel right, trust your TCASII -- it doesn't make human errors!

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Original NASA ASRS Text

Title: LTSS OCCURRED BTWN A BA41 AND DC9 WHEN CTLR MISSED INCORRECT ALT READBACK BY THE FLC OF BA41. OPERROR.

Narrative: I WAS THE PF. WE CHKED IN WITH DEP CTL. THEY IMMEDIATELY ASSIGNED US AN ALT OF 4000 FT MSL. UPON REACHING 4000 FT MSL, ATC ASSIGNED US 6000 FT MSL. THE CAPT AND I BOTH HEARD AND UNDERSTOOD A CLB TO 6000 FT MSL. THE CAPT CLRLY READ BACK 'CLB TO 6000 FT MSL' AND DIALED 6000 FT MSL INTO THE ALT SELECTOR. REACHING 5000 FT MSL, ATC ADVISED US THAT A DC9 WOULD BE XING OVER OUR ALT 1000 FT ABOVE US. WHAT THAT MEANT TO ME IS THAT SINCE WE WERE CLRED TO, AND CLBING TO, 6000 FT MSL. HE MUST BE AT 7000 FT MSL. AT ABOUT THAT TIME THE TCASII SYS GAVE US A TA WARNING AND ATC ASKED US IF WE HAD THE TFC IN SIGHT AT 3-4 O'CLOCK ABOVE US, A DC9. I LOOKED OUT THE WINDOW AND SAW THE TFC AND THE CAPT ACKNOWLEDGED TO ATC, 'TFC IN SIGHT.' CLBING THROUGH ABOUT 5100 FT MSL, THE TCASII GAVE US AN RA WARNING, WARNING US 'DON'T CLB.' THE CAPT THEN TOLD ME, 'AS LONG AS YOU HAVE THE TFC IN SIGHT, WE CAN DISREGARD THE RA AND CONTINUE OUR CLB.' RELUCTANTLY, I DID, AND CONTINUED TO CLB TO ABOUT 5500 FT MSL. WHEN SUDDENLY, WITH THE TCASII TELLING US TO 'DON'T CLB AND MONITOR VERT SPD' AND THE DC9 CLOSING IN ON US. I SENSED SOMETHING WASN'T RIGHT AND I LEVELED OFF AT 5500 FT MSL. THE DC9 THEN ASKED ATC 'WHAT WAS OUR ALT?' ATC THEN TOLD US TO 'MAINTAIN 5000 FT MSL.' MY CAPT REPLIED TO ATC, 'WE ARE DSNDING BACK DOWN TO 5000 FT MSL.' THE DC9 PASSED OVER US AND NO FURTHER CONFLICT AROSE. WHEN ON THE GND, THE CAPT AND I CALLED STL DEP CTL AND SPOKE WITH THE SUPVR ON DUTY. HE TOLD US, AFTER LISTENING TO THE ATC TAPES, WE WERE ONLY CLRED TO 5000 FT MSL, HOWEVER, THAT WE CLRLY READ BACK 6000 FT MSL. THE PROB IS THAT ATC DID FAIL TO CATCH AND CORRECT AN IMPROPER ALT READBACK. PROB #2, ATC SHOULD HAVE ADVISED US OF EXACTLY THE ALT THE DC9 WAS AT, IE, TFC AT 6000 FT MSL, INSTEAD OF JUST SAY ALT 1000 FT ABOVE YOU. THE LESSON LEARNED HERE: WHENEVER IN DOUBT AND SOMETHING DOESN'T FEEL RIGHT, TRUST YOUR TCASII -- IT DOESN'T MAKE HUMAN ERRORS!

Data retrieved from NASA's ASRS site as of July 2007 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.