HDG DEV CAUSED BY CELLULAR PHONE USE IN FLT.

Narrative

FLT DEPARTED ROANOKE REGIONAL ARPT FOR WASHINGTON DULLES. PRIOR TO DEP BOTH THE CAPT AND FO DIRECTIONAL GYROS WERE CHKED AND FOUND ACCURATE AGAINST THE DEP RWY. UPON DEP; ZDC CLRED US DIRECT MONTEBELLO VOR; V-143; AS FILED. AT APPROX 10 MI PRIOR TO MONTEBELLO; THE HSI AND RMI; WHICH WAS TUNED AND IDENTED TO MONTEBELLO; BECAME ERRATIC. WE PROCEEDED ON THE CURRENT HDG OF APPROX 060 DEGS AND UPON STATION PASSAGE; TURNED TO 036 DEGS; ALONG WITH THE APPROPRIATE HSI SELECTION TO NAV NEBOUND ON V-143. THE VOR REMAINED SOMEWHAT ERRATIC AND AT APPROX 10 DME FROM MONTEBELLO; ZDC INFORMED US THAT WE WERE WELL R OF COURSE. WE INFORMED CTR THAT WE APPEARED TO BE ON COURSE AND THEY ISSUED A HDG FOR DOCCS INTXN. UPON COMPARING DIRECTIONAL GYROS; WE NOTICED A 15 DEG DISCREPANCY. A CABIN INSPECTION WAS CONDUCTED AND A PAX WAS FOUND USING A CELLULAR TELEPHONE. WHEN THE TELEPHONE WAS TURNED OFF THE VOR SIGNALS STABILIZED AND THE DIRECTIONAL GYROS LINED UP WITHIN 2 DEGS OF EACH OTHER. ALTHOUGH THE PAX WERE BRIEFED ON NOT USING PHONES; RADIOS; AND ELECTRONIC DEVICES; WITHOUT A FLT ATTENDANT; MONITORING PAX FOR THE USE OF THESE ITEMS IS VIRTUALLY IMPOSSIBLE. SUBSEQUENT FLTS IN THE SAME ACFT FOUND NO OTHER NAVIGATIONAL DISCREPANCIES.