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.