Narrative:

At cruise; the heading comparator appeared and we noticed the headings disagreed by 7 degrees. Later; the difference was as much as 10 degrees. I decided to see whether having the navigation lights off was affecting the equipment. I turned the lights on; and the system returned the headings to within 3 degrees inside of 5 mins. I turned the lights back off and the disparity returned. Through 3 complete cycles; the headings were in tolerance with the lights on; but out of limits with them off. I was complying with the new company procedure to turn navigation lights off during daytime flts when I experienced the possible anomaly impacting the ahrs system that could potentially degrade navigation. I left the navigation lights on for the remainder of the flight. I made an aircraft maintenance log entry. I discussed the event with maintenance personnel; who stated that the navigation lights could not cause this type of anomaly. I also briefed the captain scheduled to fly the aircraft next. Callback conversation with reporter revealed the following information: the reporter stated the navigation lights were positioned to 'off' following the company cost saving policy. In cruise; noted first comparator in view with a 7 degree disagreement between system and shortly after increasing to 10 degrees. The navigation lights were switched on and the system returned to normal within 5 mins. The navigation lights were again switched off and the error returned. The report was written in the maintenance log and was explained to maintenance when on the ground. The technician said 'it can't happen' but the airplane was removed from service. The next time the reporter flew the same airplane; the navigation light anomaly was corrected but the fix is unknown; as the maintenance log page had been removed from the logbook.

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Original NASA ASRS Text

Title: AN EMB 145 IN CRUISE AT FL370 NOTED HDG COMPARATOR APPEARED WITH 7 DEGS DISAGREEMENT BTWN SYS AND LATER INCREASING TO 10 DEGS. SWITCHED ON NAV LIGHTS AND HDG INDICATIONS RETURNED TO NORMAL.

Narrative: AT CRUISE; THE HDG COMPARATOR APPEARED AND WE NOTICED THE HDGS DISAGREED BY 7 DEGS. LATER; THE DIFFERENCE WAS AS MUCH AS 10 DEGS. I DECIDED TO SEE WHETHER HAVING THE NAV LIGHTS OFF WAS AFFECTING THE EQUIP. I TURNED THE LIGHTS ON; AND THE SYS RETURNED THE HDGS TO WITHIN 3 DEGS INSIDE OF 5 MINS. I TURNED THE LIGHTS BACK OFF AND THE DISPARITY RETURNED. THROUGH 3 COMPLETE CYCLES; THE HDGS WERE IN TOLERANCE WITH THE LIGHTS ON; BUT OUT OF LIMITS WITH THEM OFF. I WAS COMPLYING WITH THE NEW COMPANY PROC TO TURN NAV LIGHTS OFF DURING DAYTIME FLTS WHEN I EXPERIENCED THE POSSIBLE ANOMALY IMPACTING THE AHRS SYS THAT COULD POTENTIALLY DEGRADE NAV. I LEFT THE NAV LIGHTS ON FOR THE REMAINDER OF THE FLT. I MADE AN ACFT MAINT LOG ENTRY. I DISCUSSED THE EVENT WITH MAINT PERSONNEL; WHO STATED THAT THE NAV LIGHTS COULD NOT CAUSE THIS TYPE OF ANOMALY. I ALSO BRIEFED THE CAPT SCHEDULED TO FLY THE ACFT NEXT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE NAV LIGHTS WERE POSITIONED TO 'OFF' FOLLOWING THE COMPANY COST SAVING POLICY. IN CRUISE; NOTED FIRST COMPARATOR IN VIEW WITH A 7 DEG DISAGREEMENT BTWN SYS AND SHORTLY AFTER INCREASING TO 10 DEGS. THE NAV LIGHTS WERE SWITCHED ON AND THE SYS RETURNED TO NORMAL WITHIN 5 MINS. THE NAV LIGHTS WERE AGAIN SWITCHED OFF AND THE ERROR RETURNED. THE RPT WAS WRITTEN IN THE MAINT LOG AND WAS EXPLAINED TO MAINT WHEN ON THE GND. THE TECHNICIAN SAID 'IT CAN'T HAPPEN' BUT THE AIRPLANE WAS REMOVED FROM SVC. THE NEXT TIME THE RPTR FLEW THE SAME AIRPLANE; THE NAV LIGHT ANOMALY WAS CORRECTED BUT THE FIX IS UNKNOWN; AS THE MAINT LOG PAGE HAD BEEN REMOVED FROM THE LOGBOOK.

Data retrieved from NASA's ASRS site as of January 2009 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.