The aircraft had been radar vectored for a pilot intercept of the outbound track. The controller had monitored the aircraft's track, which indicated compliance with the planned track. Later the aircraft was observed diverging from track and potentially in conflict with an overflying aircraft. The controller instructed the pilot of the diverging aircraft to adopt a new heading to avoid a loss of separation. A subsequent report from the pilot indicated that he had been using the course deviation indicator bar to maintain track. The bar had later moved to one side, and he had turned to centre the bar. He later found that the bar was giving erroneous indications compared with the RMI needle.
