Both VOR indicators showed aircraft to be on course when in fact aircraft was 8 miles north of cours...

Attributes
ACN	102609
Time
Date	198901
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : dru
State Reference	MT
Altitude	msl bound lower : 14000 msl bound upper : 14000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Operator	other
Make Model Name	Small Aircraft, Low Wing, 2 Eng, Retractable Gear
Flight Phase	cruise other cruise other
Route In Use	enroute airway : v343
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 100 flight time total : 5000
ASRS Report	102601
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

The flight was an air ambulance run from mso to hln to shr and return. All en route (between VOR's) legs were short except V343 between bzn and dru. At the midpoint of that leg the navigation's were indicating on centerline and did not change indications after frequency change to dru. At an estimated (dru has no DME) 25-30 mi from dru center advised the aircraft was 8 mi north of centerline. Indications were on centerline for both VOR CDI's and the autoplt heading bug hadn't had to be moved but 2 or 3 degrees the whole time. A request to fly a particular heading was received and complied with. At approximately 20 mi out both CDI's swung hard left and an intercept of 45 degrees would have been needed to cross dru. The problem was written up in the maintenance logs. Radio tech's state that because the system uses shared components, both CDI's would respond alike to an erroneous input when tuned to the same frequency. In this case a lack of receiver sensitivity may also have had an effect. Had the ATC specialist commented earlier an 8 mi course correction would have been unnecessary and would have alerted the crew that the navigation's were not to be entirely trusted.

Original NASA ASRS Text

Title: BOTH VOR INDICATORS SHOWED ACFT TO BE ON COURSE WHEN IN FACT ACFT WAS 8 MILES NORTH OF COURSE.

Narrative: THE FLT WAS AN AIR AMBULANCE RUN FROM MSO TO HLN TO SHR AND RETURN. ALL ENRTE (BTWN VOR'S) LEGS WERE SHORT EXCEPT V343 BTWN BZN AND DRU. AT THE MIDPOINT OF THAT LEG THE NAV'S WERE INDICATING ON CENTERLINE AND DID NOT CHANGE INDICATIONS AFTER FREQ CHANGE TO DRU. AT AN ESTIMATED (DRU HAS NO DME) 25-30 MI FROM DRU CENTER ADVISED THE ACFT WAS 8 MI N OF CENTERLINE. INDICATIONS WERE ON CENTERLINE FOR BOTH VOR CDI'S AND THE AUTOPLT HDG BUG HADN'T HAD TO BE MOVED BUT 2 OR 3 DEGS THE WHOLE TIME. A REQUEST TO FLY A PARTICULAR HDG WAS RECEIVED AND COMPLIED WITH. AT APPROX 20 MI OUT BOTH CDI'S SWUNG HARD LEFT AND AN INTERCEPT OF 45 DEGS WOULD HAVE BEEN NEEDED TO CROSS DRU. THE PROB WAS WRITTEN UP IN THE MAINT LOGS. RADIO TECH'S STATE THAT BECAUSE THE SYS USES SHARED COMPONENTS, BOTH CDI'S WOULD RESPOND ALIKE TO AN ERRONEOUS INPUT WHEN TUNED TO THE SAME FREQ. IN THIS CASE A LACK OF RECEIVER SENSITIVITY MAY ALSO HAVE HAD AN EFFECT. HAD THE ATC SPECIALIST COMMENTED EARLIER AN 8 MI COURSE CORRECTION WOULD HAVE BEEN UNNECESSARY AND WOULD HAVE ALERTED THE CREW THAT THE NAV'S WERE NOT TO BE ENTIRELY TRUSTED.

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.

Both VOR indicators showed aircraft to be on course when in fact aircraft was 8 miles north of course.