Narrative:

I did not hear the london director's (approach control) initial transmission tell us to descend to 5000 ft, but the captain responded to their transmission to 'descend, maintain 5000 ft.' the radio transmission was apparently blocked. First officer was flying manually, and he started to leave FL080. In the meantime, ATC came back to say that the 2 aircraft responded to the last transmission, and said thing like 'that's for air carrier Y, descend and maintain 5000 ft.' there was a moment of confusion to who said what and what exactly ATC wanted each aircraft to do. At that time, we were already approaching FL078. Captain responded to ATC again that 'air carrier X, descend maintain 5000 ft.' several radio xmissions were blocked. ATC eventually came back and said, 'negative air carrier X, that is for air carrier Y, not even close. Air carrier X, maintain FL080.' first officer immediately started to climb back to FL080 from FL078. There was an aircraft (not air carrier Y) crossing right to left in front of us 1000 ft above. Since the climb rate was somewhere around 500-800 FPM, and first officer slightly passed FL080 and reaching FL081. TCASII RA triggered. First officer immediately leveled around FL081, then descended to FL080. We had conflicting aircraft in sight all the time, way above us and there was absolutely no threat of collision. The RA lasted only a few seconds. The chain of events happened very fast. It started with several blocked xmissions with the ATC and led to the TCASII RA. After that, we made an uneventful normal landing to runway 9L in lhr.

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Original NASA ASRS Text

Title: B777-200 CREW ACCEPTED CLRNC FOR ANOTHER ACFT FOR DSCNT. AFTER THE APCH CTLR TOLD THE CREW TO MAINTAIN THEIR ALT, THE CREW HAD A TCASII RA WHEN THEY OVERSHOT THEIR ALT BY A RPTED 100 FT.

Narrative: I DID NOT HEAR THE LONDON DIRECTOR'S (APCH CTL) INITIAL XMISSION TELL US TO DSND TO 5000 FT, BUT THE CAPT RESPONDED TO THEIR XMISSION TO 'DSND, MAINTAIN 5000 FT.' THE RADIO XMISSION WAS APPARENTLY BLOCKED. FO WAS FLYING MANUALLY, AND HE STARTED TO LEAVE FL080. IN THE MEANTIME, ATC CAME BACK TO SAY THAT THE 2 ACFT RESPONDED TO THE LAST XMISSION, AND SAID THING LIKE 'THAT'S FOR ACR Y, DSND AND MAINTAIN 5000 FT.' THERE WAS A MOMENT OF CONFUSION TO WHO SAID WHAT AND WHAT EXACTLY ATC WANTED EACH ACFT TO DO. AT THAT TIME, WE WERE ALREADY APCHING FL078. CAPT RESPONDED TO ATC AGAIN THAT 'ACR X, DSND MAINTAIN 5000 FT.' SEVERAL RADIO XMISSIONS WERE BLOCKED. ATC EVENTUALLY CAME BACK AND SAID, 'NEGATIVE ACR X, THAT IS FOR ACR Y, NOT EVEN CLOSE. ACR X, MAINTAIN FL080.' FO IMMEDIATELY STARTED TO CLB BACK TO FL080 FROM FL078. THERE WAS AN ACFT (NOT ACR Y) XING R TO L IN FRONT OF US 1000 FT ABOVE. SINCE THE CLB RATE WAS SOMEWHERE AROUND 500-800 FPM, AND FO SLIGHTLY PASSED FL080 AND REACHING FL081. TCASII RA TRIGGERED. FO IMMEDIATELY LEVELED AROUND FL081, THEN DSNDED TO FL080. WE HAD CONFLICTING ACFT IN SIGHT ALL THE TIME, WAY ABOVE US AND THERE WAS ABSOLUTELY NO THREAT OF COLLISION. THE RA LASTED ONLY A FEW SECONDS. THE CHAIN OF EVENTS HAPPENED VERY FAST. IT STARTED WITH SEVERAL BLOCKED XMISSIONS WITH THE ATC AND LED TO THE TCASII RA. AFTER THAT, WE MADE AN UNEVENTFUL NORMAL LNDG TO RWY 9L IN LHR.

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.