GA pilot deviates 2-3 mi off course while relying on autoplt to track course.

Attributes
ACN	439180
Time
Date	199905
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : hve.vortac
State Reference	UT
Altitude	msl single value : 17000
Environment
Flight Conditions	VMC
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zlc.artcc
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : multi engine pilot : atp
Experience	flight time last 90 days : 75 flight time total : 10000 flight time type : 100
ASRS Report	439180
Person 2
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Resolutory Action	controller : provided flight assist flight crew : returned to intended course
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

HSI was coupled to autoplt. Heading and altitude hold HSI started deviation without cause. The discovery of this action was simultaneous with controller and pilot. Pilot noted magnetic compass heading change, visual reference and map reference. Deviation was approximately 2-3 mi right of course. Correction was made immediately by reference to magnetic compass. The controller inquired about intentions and was satisfied with the corrective action. The IFR flight plan was canceled shortly thereafter.

Original NASA ASRS Text

Title: GA PLT DEVIATES 2-3 MI OFF COURSE WHILE RELYING ON AUTOPLT TO TRACK COURSE.

Narrative: HSI WAS COUPLED TO AUTOPLT. HDG AND ALT HOLD HSI STARTED DEV WITHOUT CAUSE. THE DISCOVERY OF THIS ACTION WAS SIMULTANEOUS WITH CTLR AND PLT. PLT NOTED MAGNETIC COMPASS HDG CHANGE, VISUAL REF AND MAP REF. DEV WAS APPROX 2-3 MI R OF COURSE. CORRECTION WAS MADE IMMEDIATELY BY REF TO MAGNETIC COMPASS. THE CTLR INQUIRED ABOUT INTENTIONS AND WAS SATISFIED WITH THE CORRECTIVE ACTION. THE IFR FLT PLAN WAS CANCELED SHORTLY THEREAFTER.

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.