On 28 July 2004, a Piper PA-31T Cheyenne, VH-TNP, with one pilot and five passengers, on a private, instrument flight rules flight from Bankstown to Benalla, collided with terrain 34 km south-east of Benalla. All occupants were fatally injured and the aircraft was destroyed by impact forces and fire. Instrument meteorological conditions existed at the time and the pilot had reported commencing a Global Positioning System (GPS) non-precision approach (NPA) to Benalla.
The experienced pilot was familiar with the aircraft and its navigation and autoflight systems. The flight did not follow the usual route to Benalla, but diverted south along the coast before tracking to the northernmost initial approach waypoint BLAED of the Benalla Runway 26L GPS NPA. While tracking to BLAED the aircraft diverged left of track, without the pilot being aware of the error. The air traffic control Route Adherence Monitoring (RAM) system triggered alerts, but controllers believed the aircraft was tracking to a different waypoint and did not question the pilot about the aircraft's position. The destruction of the aircraft navigation and flight control systems did not permit verification of their operational status. The investigation found that instructions to controllers relating to RAM alerts could be ambiguous. Actions were taken by Airservices Australia to enhance alerts and clarify controllers' responses to them.
The occurrence drew pilots' attention to the need to pay careful attention to the use of automated flight and navigation systems and also demonstrated the need for effective communication between controllers and pilots to clarify any apparent tracking anomalies. The Australian Transport Safety Bureau's (ATSB) final report was released on 7 February 2006.
In July 2008, during the subsequent coronial inquest, additional information about the possibility of dead reckoning navigation by the GPS receiver was provided. The ATSB investigation was reopened to examine that possibility and an amended report issued. That investigation found that dead reckoning navigation could not be positively established as there were inconsistencies between dead reckoning principles and the recorded radar data. Neither could it reconcile how a pilot would continue navigation by GPS with the alerts and warnings provided by the GPS receiver and the instrument indications. As a result of the reopened investigation, the ATSB issued a safety advisory notice alerting users of GPS navigation receivers to take appropriate action to ensure familiarity with dead-reckoning operation and any associated receiver-generated warning messages.