Narrative:

We had been cleared to depart the john wayne airport (sna) using the musel 5 departure, modified so as to intercept the sxc 084 degree radial to sxc, rather than the published 061 degree radial. On an assigned intercept heading of 200, with both pilots' HSI courses set to 264, and autoplt on with VOR mode armed, with both CDI needles still pegged full left and about 24 mi from the station the mode annunciator changed to 'cap' and the aircraft banked right to about a 240 heading, then the mode annunciator defaulted to 'VOR course', an ambiguity mode usually triggered by impending station passage or very low signal strength. We both fixated for a moment on the conflicting scan cues, then the first officer, who was flying engaged 'heading sel' and began to command a turn back to the left to reestablish an appropriate intercept heading. As he was reaching for the knob, the controller asked us our heading, then immediately modified our clearance to proceed direct to sxc. When I phoned an apology to the TRACON later, it became apparent that the controller assumed we had misset the course bars to the published, and usual, inbound course. Compliments to the controller. We couldn't have been rolled out on the inappropriate heading more than 5 seconds before his inquiry. And traffic and communications in the sector were quite heavy. Add a new one to my 'automatic airplanes glitch list', along with failure to level off even in 'altitude cap' without intervention, interception of a parallel (and bogus) localizer course with all annunciations normal. So far this pilot hasn't 'failed' (or relaxed!) but I feel the odds piling up.

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Original NASA ASRS Text

Title: FLT CREW OF ADVANCED COCKPIT AIR CARRIER ACFT GIVEN MODIFIED MUSEL 5 STANDARD INSTRUMENT DEP. THE VOR INDICATES CAPTURE AND AUTOPLT COMMANDS HEADING CHANGE WELL BEFORE INTERCEPT POINT.

Narrative: WE HAD BEEN CLRED TO DEPART THE JOHN WAYNE ARPT (SNA) USING THE MUSEL 5 DEP, MODIFIED SO AS TO INTERCEPT THE SXC 084 DEG RADIAL TO SXC, RATHER THAN THE PUBLISHED 061 DEG RADIAL. ON AN ASSIGNED INTERCEPT HDG OF 200, WITH BOTH PLTS' HSI COURSES SET TO 264, AND AUTOPLT ON WITH VOR MODE ARMED, WITH BOTH CDI NEEDLES STILL PEGGED FULL LEFT AND ABOUT 24 MI FROM THE STATION THE MODE ANNUNCIATOR CHANGED TO 'CAP' AND THE ACFT BANKED RIGHT TO ABOUT A 240 HDG, THEN THE MODE ANNUNCIATOR DEFAULTED TO 'VOR COURSE', AN AMBIGUITY MODE USUALLY TRIGGERED BY IMPENDING STATION PASSAGE OR VERY LOW SIGNAL STRENGTH. WE BOTH FIXATED FOR A MOMENT ON THE CONFLICTING SCAN CUES, THEN THE F/O, WHO WAS FLYING ENGAGED 'HDG SEL' AND BEGAN TO COMMAND A TURN BACK TO THE LEFT TO REESTABLISH AN APPROPRIATE INTERCEPT HDG. AS HE WAS REACHING FOR THE KNOB, THE CTLR ASKED US OUR HDG, THEN IMMEDIATELY MODIFIED OUR CLRNC TO PROCEED DIRECT TO SXC. WHEN I PHONED AN APOLOGY TO THE TRACON LATER, IT BECAME APPARENT THAT THE CTLR ASSUMED WE HAD MISSET THE COURSE BARS TO THE PUBLISHED, AND USUAL, INBND COURSE. COMPLIMENTS TO THE CTLR. WE COULDN'T HAVE BEEN ROLLED OUT ON THE INAPPROPRIATE HDG MORE THAN 5 SECONDS BEFORE HIS INQUIRY. AND TFC AND COMS IN THE SECTOR WERE QUITE HEAVY. ADD A NEW ONE TO MY 'AUTOMATIC AIRPLANES GLITCH LIST', ALONG WITH FAILURE TO LEVEL OFF EVEN IN 'ALT CAP' WITHOUT INTERVENTION, INTERCEPTION OF A PARALLEL (AND BOGUS) LOC COURSE WITH ALL ANNUNCIATIONS NORMAL. SO FAR THIS PLT HASN'T 'FAILED' (OR RELAXED!) BUT I FEEL THE ODDS PILING UP.

Data retrieved from NASA's ASRS site as of August 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.