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.