Aircraft descended to wrong altitude and came very close to another aircraft.

Attributes
ACN	148242
Time
Date	199006
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lex
State Reference	KY
Altitude	msl bound lower : 11000 msl bound upper : 11400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid tracon : lex
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller non radar : 2 controller radar : 6
ASRS Report	148242
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : nmac non adherence : clearance non adherence : far non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The aircraft above called on frequency descending to 11000'. I amended the clearance to maintain 12000'. The pilot replied one two zero, one two thousand small transport. I called traffic at 1 O'clock, 7 mi, southbound at 11000' the aircraft was westbound. I switched the aircraft to lex approach. He called on their frequency descending to 11000'. The lex radar controller and I talked. They climbed the aircraft (now at 11400') back to 12000'. In the meantime I immediately descended the other aircraft to 10000' and turned him to a 080 degree heading. The aircraft saw the small transport. I advised the pilot to just miss him. By this time the targets were one. The pilot of the small transport said he forgot to put the revised altitude into the aircraft alert window. I thought they were going to hit. I was scared to death. As a pilot I see both sides of the microphone. First, if a controller calls traffic above or below you intercepting why in the heck would you think that you should descend to the same altitude. Second, don't guess at a clearance, ask again!

Original NASA ASRS Text

Title: ACFT DESCENDED TO WRONG ALT AND CAME VERY CLOSE TO ANOTHER ACFT.

Narrative: THE ACFT ABOVE CALLED ON FREQ DSNDING TO 11000'. I AMENDED THE CLRNC TO MAINTAIN 12000'. THE PLT REPLIED ONE TWO ZERO, ONE TWO THOUSAND SMT. I CALLED TFC AT 1 O'CLOCK, 7 MI, SBND AT 11000' THE ACFT WAS WBND. I SWITCHED THE ACFT TO LEX APCH. HE CALLED ON THEIR FREQ DSNDING TO 11000'. THE LEX RADAR CTLR AND I TALKED. THEY CLBED THE ACFT (NOW AT 11400') BACK TO 12000'. IN THE MEANTIME I IMMEDIATELY DSNDED THE OTHER ACFT TO 10000' AND TURNED HIM TO A 080 DEG HDG. THE ACFT SAW THE SMT. I ADVISED THE PLT TO JUST MISS HIM. BY THIS TIME THE TARGETS WERE ONE. THE PLT OF THE SMT SAID HE FORGOT TO PUT THE REVISED ALT INTO THE ACFT ALERT WINDOW. I THOUGHT THEY WERE GOING TO HIT. I WAS SCARED TO DEATH. AS A PLT I SEE BOTH SIDES OF THE MIKE. FIRST, IF A CTLR CALLS TFC ABOVE OR BELOW YOU INTERCEPTING WHY IN THE HECK WOULD YOU THINK THAT YOU SHOULD DSND TO THE SAME ALT. SECOND, DON'T GUESS AT A CLRNC, ASK AGAIN!

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.