Narrative:

Night departure from vny runway 16R - both HSI's aligned with runway heading and checked. On glendale 7 departure turned left to 110 degree, then given a heading of 080 degree to intercept V186. On that 'heading' we went screaming across the airway and were called by ATC with urgent warning about high terrain. On checking against the copilot's HSI found the pilot's HSI was off by about 45 degrees (indicating about 90 degrees with an actual heading of about 45 degrees). Sorting out the problem required some time, but thank god I was in visual conditions and could sight the terrain. Compass was in automatic slave position, and required the manual position plus reslaving, then worked (in manual) the remainder of the flight. The giant precession error must have occurred during the big turn, but there was no annunciation of any problem. 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 problem 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!

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Original NASA ASRS Text

Title: 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!

Data retrieved from NASA's ASRS site as of July 2007 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.