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Notification reports received from air traffic control and the pilot of a Cessna 404 revealed that the pilot was instructed by air traffic control to maintain 5,000 ft due to crossing traffic at 6,000 ft. He reported that he trimmed the aircraft to maintain 5,000 ft, but it climbed to 5,140 ft. The pilot noticed the aircraft climbing and immediately initiated a descent. Air traffic control reported that as the crossing aircraft passed, the Cessna 404 was observed on radar at 5,400 ft. The crew of the crossing aircraft did not sight the Cessna 404 or receive a TCAS advisory or alert.

The pilots report also advised that, following the occurrence, the Cessna 404s altitude encoder was checked and was found to have an error. The encoder was subsequently repaired.

The ATSB commenced a category 4 investigation to determine if safety was compromised. An assessment of available information indicates that although there was a breakdown of separation standards there was limited safety benefit in continuing the investigation.

Status: Downgraded the occurrence to category 5 and investigation discontinued.

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