Mia controller described near midair collision event when aircraft descended below assigned altitude...

Attributes
ACN	749890
Time
Date	200708
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mia.tracon
State Reference	FL
Altitude	msl single value : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mia.tracon
Operator	common carrier : air carrier
Make Model Name	EMB ERJ 170/175 ER&LR
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : mia.tracon
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar controller : approach
Qualification	controller : radar
Experience	controller military : 3.0 controller non radar : 3.5 controller radar : 23.8 controller time certified in position1 : 5.4
ASRS Report	749890
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : nmac non adherence : clearance
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	controller : issued alert controller : issued new clearance flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Aircraft X was inbound to mia. I issued aircraft X a descent clearance to 8000 ft. The pilot of aircraft X read back the altitude correctly as 8000 ft. I subsequently issued aircraft X a clearance of; 'turn right heading 280; reduce speed to 170.' the pilot responded and said that he was; 'getting an RA.' at this point; I observed the mode-C readout showing that aircraft X was actually level at 7000 ft and converging with aircraft Y; a B757 departure who was climbing out of 6600 ft for 7000 ft northbound. I immediately issued the crossing traffic and advised the crew of aircraft X that they were assigned 8000 ft. The crew responded that they had been assigned 7000 ft and that they had the B757 in sight. At this point; the 2 aircraft were converging; only 1 mi apart and both level at 7000 ft. The ground speed of aircraft X was 300 KTS. The ground speed of the B757 was approximately 250 KTS. I instructed the crew of aircraft X to maintain visual separation from the B757. The 2 targets merged as aircraft X climbed through 7200 ft and the B757 descended through 6800 ft.

Original NASA ASRS Text

Title: MIA CTLR DESCRIBED NMAC EVENT WHEN ACFT DESCENDED BELOW ASSIGNED ALT.

Narrative: ACFT X WAS INBOUND TO MIA. I ISSUED ACFT X A DSCNT CLRNC TO 8000 FT. THE PLT OF ACFT X READ BACK THE ALT CORRECTLY AS 8000 FT. I SUBSEQUENTLY ISSUED ACFT X A CLRNC OF; 'TURN R HDG 280; REDUCE SPD TO 170.' THE PLT RESPONDED AND SAID THAT HE WAS; 'GETTING AN RA.' AT THIS POINT; I OBSERVED THE MODE-C READOUT SHOWING THAT ACFT X WAS ACTUALLY LEVEL AT 7000 FT AND CONVERGING WITH ACFT Y; A B757 DEP WHO WAS CLBING OUT OF 6600 FT FOR 7000 FT NBOUND. I IMMEDIATELY ISSUED THE XING TFC AND ADVISED THE CREW OF ACFT X THAT THEY WERE ASSIGNED 8000 FT. THE CREW RESPONDED THAT THEY HAD BEEN ASSIGNED 7000 FT AND THAT THEY HAD THE B757 IN SIGHT. AT THIS POINT; THE 2 ACFT WERE CONVERGING; ONLY 1 MI APART AND BOTH LEVEL AT 7000 FT. THE GND SPD OF ACFT X WAS 300 KTS. THE GND SPD OF THE B757 WAS APPROX 250 KTS. I INSTRUCTED THE CREW OF ACFT X TO MAINTAIN VISUAL SEPARATION FROM THE B757. THE 2 TARGETS MERGED AS ACFT X CLBED THROUGH 7200 FT AND THE B757 DSNDED THROUGH 6800 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.