ACFT EQUIP PROB - CAPT'S HSI FAILED TO AUTOSLAVE AND THE HDG WAS OFF BY 45 DEGS. AN ALERT CTLR; GOOD WX AND RAPID XCHKING SAVED THE ACFT.

Narrative

NIGHT DEP FROM VNY RWY 16R - BOTH HSI'S ALIGNED WITH RWY HDG AND CHKED. ON GLENDALE 7 DEP TURNED L TO 110 DEG; THEN GIVEN A HDG OF 080 DEG TO INTERCEPT V186. ON THAT 'HDG' WE WENT SCREAMING ACROSS THE AIRWAY AND WERE CALLED BY ATC WITH URGENT WARNING ABOUT HIGH TERRAIN. ON CHKING AGAINST THE COPLT'S HSI FOUND THE PLT'S HSI WAS OFF BY ABOUT 45 DEGS (INDICATING ABOUT 90 DEGS WITH AN ACTUAL HDG OF ABOUT 45 DEGS). SORTING OUT THE PROB REQUIRED SOME TIME; BUT THANK GOD I WAS IN VISUAL CONDITIONS AND COULD SIGHT THE TERRAIN. COMPASS WAS IN AUTO SLAVE POS; AND REQUIRED THE MANUAL POS PLUS RESLAVING; THEN WORKED (IN MANUAL) THE REMAINDER OF THE FLT. THE GIANT PRECESSION ERROR MUST HAVE OCCURRED DURING THE BIG TURN; BUT THERE WAS NO ANNUNCIATION OF ANY PROB. TURNS WERE NO MORE THAN STANDARD RATE; AND UNIT WORKED NORMALLY EARLIER. PERHAPS IN THE INITIAL CONFUSED PERIOD OF OVERSHOOTING AIRWAY AND SORTING OUT THE PROB I SHOULD HAVE REQUESTED NO GYROSCOPE VECTORS. HSI GYROSCOPE DID NOT TUMBLE OR FREEZE; WHICH MADE IT APPEAR TO FUNCTION NORMALLY. THIS COULD HAVE BEEN VERY SERIOUS IN A NON-RADAR ENVIRONMENT WITH IFR CONDITIONS!