Narrative:

On departure, we were given a vector to intercept a radial. The first officer was flying, and he set the autoplt to 'VOR arm' to automatically intercept the radial. The first officer saw the course needle come in on his HSI, and the autoplt began a turn towards the VOR. Several seconds later, the course needle returned to full-scale deflection, and the autoplt leveled its wings and went to 'VOR crs,' while looking for the VOR signal. The first officer decided it had been a premature intercept, and began to select a heading back to our original vector. I had been cleaning up a checklist and asked him what was going on, when ZNY told us to stop our climb and asked us what our heading was. We resolved our questions, and ATC pointed out conflicting traffic to our left, the direction we had turned on our premature intercept. Some of these vors (pxt, rbv, spa) are notoriously erratic, and I believe the ground transmitter was the cause of this problem. Neither the aircraft VOR radio, nor the autoplt gave any subsequent problems, and were checked ok by maintenance.

Related reports:

Google
 

Original NASA ASRS Text

Title: MLG CREW ON VECTORS, WAS CLRED TO INTERCEPT A GIVEN RADIAL. PF SET THE AUTOPLT TO DO THE INTERCEPT AUTOMATICALLY. AUTOPLT STARTED THE TURN EARLY. BOTH THE CTLR AND PLT DISCOVERED THE EARLY TURN. CONFLICT WITH OTHER TFC OCCURRED.

Narrative: ON DEP, WE WERE GIVEN A VECTOR TO INTERCEPT A RADIAL. THE FO WAS FLYING, AND HE SET THE AUTOPLT TO 'VOR ARM' TO AUTOMATICALLY INTERCEPT THE RADIAL. THE FO SAW THE COURSE NEEDLE COME IN ON HIS HSI, AND THE AUTOPLT BEGAN A TURN TOWARDS THE VOR. SEVERAL SECONDS LATER, THE COURSE NEEDLE RETURNED TO FULL-SCALE DEFLECTION, AND THE AUTOPLT LEVELED ITS WINGS AND WENT TO 'VOR CRS,' WHILE LOOKING FOR THE VOR SIGNAL. THE FO DECIDED IT HAD BEEN A PREMATURE INTERCEPT, AND BEGAN TO SELECT A HEADING BACK TO OUR ORIGINAL VECTOR. I HAD BEEN CLEANING UP A CHKLIST AND ASKED HIM WHAT WAS GOING ON, WHEN ZNY TOLD US TO STOP OUR CLB AND ASKED US WHAT OUR HEADING WAS. WE RESOLVED OUR QUESTIONS, AND ATC POINTED OUT CONFLICTING TFC TO OUR L, THE DIRECTION WE HAD TURNED ON OUR PREMATURE INTERCEPT. SOME OF THESE VORS (PXT, RBV, SPA) ARE NOTORIOUSLY ERRATIC, AND I BELIEVE THE GND XMITTER WAS THE CAUSE OF THIS PROB. NEITHER THE ACFT VOR RADIO, NOR THE AUTOPLT GAVE ANY SUBSEQUENT PROBS, AND WERE CHKED OK BY MAINT.

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.