During a smoke and fumes event on an Airbus A319-111 (G-EZNM), the lenses of both pilots’ oxygen masks misted when donned, obscuring their vison. The commander removed his mask so he could see the flight instruments. The aircraft diverted to Birmingham and landed without further event.
The investigation found that the masks misted up due to a combination of the environment in which they were stowed and the condition of the lenses. The source of the smoke was probably an accumulation of dust in the transformer rectifier unit (TRU).
The aircraft manufacturer has added a cleaning procedure for the TRU in the Aircraft Maintenance Manual. The operator has added a practical demonstration on the use of the mask and its selectors during its recurrent training cycle.