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.