Atx small transport descends 500 ft below assigned altitude causing a conflict.

Attributes
ACN	242719
Time
Date	199305
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : osh
State Reference	WI
Altitude	msl bound lower : 10500 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air taxi
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 145 flight time total : 2200 flight time type : 120
ASRS Report	242719
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	faa : investigated
Miss Distance	horizontal : 24000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

After a 7 hour flight I was given a descent from 14000 ft to 11000 ft. I started a descent to 11000 ft and continued to 10000 ft. The controller caught me at 10500 ft and told me to climb back to 11000 ft. At this time I was not sure which altitude I was given because of lack of rest. (I had rested but I was tired because of the long flight.) there was another aircraft involved at 10000 ft which I had in sight but I do not believe we were closer than 5 mi. I think that there is no question that my fatigue had something to do with this.

Original NASA ASRS Text

Title: ATX SMT DSNDS 500 FT BELOW ASSIGNED ALT CAUSING A CONFLICT.

Narrative: AFTER A 7 HR FLT I WAS GIVEN A DSCNT FROM 14000 FT TO 11000 FT. I STARTED A DSCNT TO 11000 FT AND CONTINUED TO 10000 FT. THE CTLR CAUGHT ME AT 10500 FT AND TOLD ME TO CLB BACK TO 11000 FT. AT THIS TIME I WAS NOT SURE WHICH ALT I WAS GIVEN BECAUSE OF LACK OF REST. (I HAD RESTED BUT I WAS TIRED BECAUSE OF THE LONG FLT.) THERE WAS ANOTHER ACFT INVOLVED AT 10000 FT WHICH I HAD IN SIGHT BUT I DO NOT BELIEVE WE WERE CLOSER THAN 5 MI. I THINK THAT THERE IS NO QUESTION THAT MY FATIGUE HAD SOMETHING TO DO WITH THIS.

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.