The Aerospatiale AS332L helicopter was tracking inbound to Darwin from the Timor Sea, in instrument meteorological conditions (IMC). The pilot-in-command was using the global positioning system (GPS) to track to the initial approach fix for a runway 11 VOR/DME approach.
The pilot reported that when air traffic control cleared the helicopter for the approach, he switched his navigation source control switch from A NAV (the selection for navigation using GPS) to NAV 2 and VOR 2 for the approach. Immediately, the navigation EMERG MODE light on the pilot's bearing pointer's control panel illuminated. In accordance with the emergency checklist, the pilot selected his radio navigation sources to ADF 1, VOR 2 and NAV 2.
After the helicopter had passed the initial approach fix and was established on the final approach leg, the pilot observed that the course bar on his horizontal situation indicator (HSI) was central but his NAV 2 bearing pointer indicated that the helicopter was between 5 and 10 degrees right of track. The pilot, who was occupying the right pilot seat, asked the co-pilot to check the instrument indications on the left side HSI. The course bar on the co-pilot's HSI was indicating half scale left of track and the co-pilot's bearing pointer was also showing that the helicopter was to the left of track.
At that point the helicopter descended into visual meteorological conditions (VMC) and the pilot discontinued the instrument approach. The pilot changed his navigation source switches to the same selections as the co-pilot's switches and the pilot's course bar and bearing needle both showed the helicopter to be left of track, which it was. No OFF flags appeared on either pilot's navigation instruments prior to making the change. The pilot landed the helicopter normally and by the time the helicopter arrived at its base, the fault was no longer evident.
Engineers were unable to reproduce the fault on the ground. Engineers later reported that illumination of an EMERG MODE light indicates one of three types of fault;
(a) a fault in the navigation signal being received by the aircraft;
(b) a fault in the navigation switching power supply; or
(c) an internal fault in the navigation switching system.
Although engineers could not be certain, they suspected an internal fault in the switching system, probably a sticking relay. This was indicated by the appearance of the fault when the pilot switched navigation sources from A.NAV to NAV.2 for the approach.