Aircraft descended to an occupied altitude resulting in less than standard separation.

Attributes
ACN	137431
Time
Date	199002
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : geg
State Reference	WA
Altitude	msl bound lower : 8000 msl bound upper : 9000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : geg
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air taxi
Make Model Name	Small Aircraft, High Wing, 1 Eng, Retractable Gear
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller radar : 1
ASRS Report	137431
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : required legal separation non adherence : published procedure
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 9900 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Pilot Deviation Operational Error

Narrative:

At approximately XA47L, I instructed aircraft a to turn left heading 030 degrees, descend and maintain 5000'. After 9 seconds west/O a reply, I asked aircraft a if he copied. He acknowledged a left turn to 030 degrees and descending to 5000'. 16 seconds later I instructed aircraft a to maintain 9000'. Again the aircraft did not reply. However, due to other aircraft that had called and were awaiting my reply (I had 7 aircraft on my frequency at the time) I did not observe aircraft a descending below 9000' until he was within a close proximity of aircraft B. I instructed aircraft a to turn right immediately heading 070 degrees and issued traffic as an small aircraft, 10 O'clock, 3 mi. I then turned the small aircraft right heading 270 degrees and issued traffic at 9 O'clock, 3 mi, eastbound. Upon arriving at geg, one of the pilots from aircraft a called to ask for information. After explaining the situation to him, he advised that both crew members were monitoring ATC and that neither of them heard the restriction to maintain 9000'. The restriction to maintain 9000' is very clear on the ATC recording.

Original NASA ASRS Text

Title: ACFT DESCENDED TO AN OCCUPIED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: AT APPROX XA47L, I INSTRUCTED ACFT A TO TURN LEFT HDG 030 DEGS, DSND AND MAINTAIN 5000'. AFTER 9 SECS W/O A REPLY, I ASKED ACFT A IF HE COPIED. HE ACKNOWLEDGED A LEFT TURN TO 030 DEGS AND DSNDING TO 5000'. 16 SECS LATER I INSTRUCTED ACFT A TO MAINTAIN 9000'. AGAIN THE ACFT DID NOT REPLY. HOWEVER, DUE TO OTHER ACFT THAT HAD CALLED AND WERE AWAITING MY REPLY (I HAD 7 ACFT ON MY FREQ AT THE TIME) I DID NOT OBSERVE ACFT A DSNDING BELOW 9000' UNTIL HE WAS WITHIN A CLOSE PROX OF ACFT B. I INSTRUCTED ACFT A TO TURN RIGHT IMMEDIATELY HDG 070 DEGS AND ISSUED TFC AS AN SMA, 10 O'CLOCK, 3 MI. I THEN TURNED THE SMA RIGHT HDG 270 DEGS AND ISSUED TFC AT 9 O'CLOCK, 3 MI, EBND. UPON ARRIVING AT GEG, ONE OF THE PLTS FROM ACFT A CALLED TO ASK FOR INFO. AFTER EXPLAINING THE SITUATION TO HIM, HE ADVISED THAT BOTH CREW MEMBERS WERE MONITORING ATC AND THAT NEITHER OF THEM HEARD THE RESTRICTION TO MAINTAIN 9000'. THE RESTRICTION TO MAINTAIN 9000' IS VERY CLEAR ON THE ATC RECORDING.

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.