Light transport descended below assigned altitude and conflicted with another aircraft.

Attributes
ACN	122955
Time
Date	198909
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mke
State Reference	WI
Altitude	msl bound lower : 7400 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mke tower : geg
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
ASRS Report	122955
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 9000 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cga X was inbound to mke from the east. Cga X was assigned 8000' and a heading of 250 degrees due to air carrier Y, level at 7000' also heading 250 degrees. Traffic was issued to both aircraft as both were on parallel headings approximately 1 1/2 mi apart. While monitoring the progress on cga X, I observed the altitude readout on cga X indicated 7900'. I then restated 'maintain 8000' and he rogered the altitude. The altitude readout continued to descend. I then issued a turn to 210 degrees to cga X to vector the aircraft away from each other. Cga X's altitude was observed to go to 7400' before a positive rate of climb was indicated. Cga X leveled at 8000' with no further incident.

Original NASA ASRS Text

Title: LTT DESCENDED BELOW ASSIGNED ALT AND CONFLICTED WITH ANOTHER ACFT.

Narrative: CGA X WAS INBND TO MKE FROM THE E. CGA X WAS ASSIGNED 8000' AND A HDG OF 250 DEGS DUE TO ACR Y, LEVEL AT 7000' ALSO HDG 250 DEGS. TFC WAS ISSUED TO BOTH ACFT AS BOTH WERE ON PARALLEL HDGS APPROX 1 1/2 MI APART. WHILE MONITORING THE PROGRESS ON CGA X, I OBSERVED THE ALT READOUT ON CGA X INDICATED 7900'. I THEN RESTATED 'MAINTAIN 8000' AND HE ROGERED THE ALT. THE ALT READOUT CONTINUED TO DSND. I THEN ISSUED A TURN TO 210 DEGS TO CGA X TO VECTOR THE ACFT AWAY FROM EACH OTHER. CGA X'S ALT WAS OBSERVED TO GO TO 7400' BEFORE A POSITIVE RATE OF CLB WAS INDICATED. CGA X LEVELED AT 8000' WITH NO FURTHER INCIDENT.

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.