Air carrier X descended below assigned altitude. No loss of separation occurred.

Attributes
ACN	108975
Time
Date	198904
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : bna
State Reference	TN
Altitude	msl bound lower : 8000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme
Operator	common carrier : air taxi
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	108975
Person 2
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

Air carrier X, a turbo propeller was given a clearance to 9000'. There was 8000' traffic, an small transport. Air carrier X wanted pilot's discretion to 9000' and I concurred. I told air carrier X to expect to cross 30 north of bna at 6000'. He read back what I thought was correct, however the pilot was foreign, so the readback was very hard to understand. I told air carrier X to descend through 10000' to 9000'. Air carrier X acknowledged. When I noticed air carrier X descending through 9000', I told him to maintain 9000' and immediately turned him to 270 degrees to miss the small transport at 8000'. I had already turned both aircraft prior to this, so by then I could descend air carrier X to 6000'. It took 2 or 3 xmissions to get air carrier X to acknowledge, however both aircraft were separated. I think it's both the pilot and the controller's responsibility to catch all erroneous readbacks, not just the controller's. Also, pilots should be aware of restrictions in the TCA and be ready to comply. Speaking good english also would help this problem.

Original NASA ASRS Text

Title: ACR X DESCENDED BELOW ASSIGNED ALT. NO LOSS OF SEPARATION OCCURRED.

Narrative: ACR X, A TURBO PROP WAS GIVEN A CLRNC TO 9000'. THERE WAS 8000' TFC, AN SMT. ACR X WANTED PLT'S DISCRETION TO 9000' AND I CONCURRED. I TOLD ACR X TO EXPECT TO CROSS 30 N OF BNA AT 6000'. HE READ BACK WHAT I THOUGHT WAS CORRECT, HOWEVER THE PLT WAS FOREIGN, SO THE READBACK WAS VERY HARD TO UNDERSTAND. I TOLD ACR X TO DSND THROUGH 10000' TO 9000'. ACR X ACKNOWLEDGED. WHEN I NOTICED ACR X DSNDING THROUGH 9000', I TOLD HIM TO MAINTAIN 9000' AND IMMEDIATELY TURNED HIM TO 270 DEGS TO MISS THE SMT AT 8000'. I HAD ALREADY TURNED BOTH ACFT PRIOR TO THIS, SO BY THEN I COULD DSND ACR X TO 6000'. IT TOOK 2 OR 3 XMISSIONS TO GET ACR X TO ACKNOWLEDGE, HOWEVER BOTH ACFT WERE SEPARATED. I THINK IT'S BOTH THE PLT AND THE CTLR'S RESPONSIBILITY TO CATCH ALL ERRONEOUS READBACKS, NOT JUST THE CTLR'S. ALSO, PLTS SHOULD BE AWARE OF RESTRICTIONS IN THE TCA AND BE READY TO COMPLY. SPEAKING GOOD ENGLISH ALSO WOULD HELP THIS PROB.

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.