Large transport descended below assigned altitude due to controller hearback error. Less than standard separation resulted with an small transport at lower altitude.

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Attributes
ACN	106618
Time
Date	198903
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : tpa
State Reference	FL
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute : direct enroute airway : zma
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	106618
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	faa : investigated
Miss Distance	horizontal : 12000 vertical : 600
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Large transport was in holding and had been reclred to tpa and descended to 11000'. Pilot read back 10000', but controller missed readback. When controller observed that the large transport was descending through 11000' it took 3 calls to the aircraft to stop the descent due to other aircraft keying the frequency to check in. At the time of the initial clearance there was no other noise on the tape recording that would indicate that the large transport had received a garbled clearance. Neither was the pilot's readback garbled. Both parties involved heard what they wanted to hear. We all need to listen better.

Original NASA ASRS Text

Title: LGT DESCENDED BELOW ASSIGNED ALT DUE TO CTLR HEARBACK ERROR. LESS THAN STANDARD SEPARATION RESULTED WITH AN SMT AT LOWER ALT.

Narrative: LGT WAS IN HOLDING AND HAD BEEN RECLRED TO TPA AND DSNDED TO 11000'. PLT READ BACK 10000', BUT CTLR MISSED READBACK. WHEN CTLR OBSERVED THAT THE LGT WAS DSNDING THROUGH 11000' IT TOOK 3 CALLS TO THE ACFT TO STOP THE DSCNT DUE TO OTHER ACFT KEYING THE FREQ TO CHK IN. AT THE TIME OF THE INITIAL CLRNC THERE WAS NO OTHER NOISE ON THE TAPE RECORDING THAT WOULD INDICATE THAT THE LGT HAD RECEIVED A GARBLED CLRNC. NEITHER WAS THE PLT'S READBACK GARBLED. BOTH PARTIES INVOLVED HEARD WHAT THEY WANTED TO HEAR. WE ALL NEED TO LISTEN BETTER.

Data retrieved from NASA's ASRS site as of August 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.