Boeing 767-338ER, VH-OGL

The Boeing 767 aircraft was at cruise altitude when the flight crew noticed a burning odour from the captain's light panel on the glareshield. The odour intensified and the flight crew donned supplemental oxygen masks. The crew then isolated the odour to the map and chart light rheostat switches, which were hot to the touch as was the surrounding panel. The switches were selected off and the odour dissipated. The crew descended the aircraft to flight level 250 in anticipation of a diversion and monitored the situation. As the odours did not recur, the crew elected to continue the flight as planned.

The operator reported that their maintenance personnel checked all instrument lighting control switches with no faults found. They then attempted to replicate the problem by turning on all switch rheostats to maximum intensity for a long period and observed no malfunction. A subsequent systems check revealed all operations normal. The aircraft technical log had noted a similar discrepancy two days prior to the occurrence. A decision was made to replace the map panel light rheostat as a precaution. No further problems were noted on subsequent flights.