What was found
RETURN TO FIELD DUE TO CAPTAIN'S HORIZONTAL SITUATION INDICATOR PROBLEM. CREW REPORTED THAT CAPT'S HSI STUCK IN WX TEST PATTERN AND ILS MODE. CAPT'S EHSI DISPLAY (P/N 622-7999-003, S/N 114851) WAS REPLACED. THE SHOP FOUND NO FAULTS WITH THE DISPLAY AND RETURNED IT TO SERVICE. THE LT EFIS CONTROL PANEL (622-5048-381, S/N 121958) ALSO WAS REMOVED AS A PRECAUTION. THE CONTROL PANEL PASSED ALL SHOP TESTS, INCLUDING ENDURANCE AND THERMAL TESTING TO SEE IF THE UNIT WOULD LOCK UP IF POWERED FOR AN EXTENDED PERIOD. NO FAULTS WERE FOUND ON THE EFIS CONTROL PANEL AND IT WAS RETURNED TO SERVICE. THERE IS NO EXPLANATION FOR THE REPORTED EVENT. TO DATE THERE HAS BEEN NO RECURRENCE OF THE EVENT ON THE AIRPLANE.