On 16 January 2003, while conducting pre-flight checks during passenger boarding, the flight crew of a Boeing 737-700 aircraft, registered VH-VBS, detected a pungent burning smell. The pilot in command contacted the company by radio and requested that an aircraft engineer attend the aircraft.
The cabin supervisor advised the pilot in command that she could smell fumes near the forward door. The pilot in command then instructed her to disembark the passengers.
The first officer reported feeling faint and the pilot in command felt dizzy and weak at the knees as he stood to leave the flight deck. The pilot in command also reported experiencing shaking hands, watering eyes and tingling fingers. He opened the windows in the flight deck for ventilation and contacted air traffic control to request paramedic assistance. The cabin supervisor administered oxygen to the pilot in command and the first officer, but when the pilot in command attempted to stand, his legs collapsed from under him.
Airport Rescue and Fire Fighting services attended the aircraft to assist the pilot in command and requested an ambulance. The pilot in command was transported to hospital for observation and was advised by medical personnel that he showed signs similar to mild carbon dioxide poisoning.
A subsequent engineering examination of the aircraft found a burned diode on the master dim and test module circuit board on the P6 panel, located behind the first officer's seat. The plastic cased diode was the only component damaged on the circuit board. Failure analysis conducted by the aircraft manufacturer determined that the diode failure mode was due to excessive heating while under electrical load.
An examination of the circuit board conducted by the ATSB determined that the diode used on the master dim and test module circuit board was a plastic cased component which differed from the diode outlined by silk screening on the circuit board of the master dim and test module. The original diode specified for use was a hermetically sealed, metal cased, glass diode with heat dissipation properties superior to the plastic cased diode, however both diodes were approved by the aircraft manufacturer for use in this application.
The aircraft manufacturer conducted further testing on a diode with the same part number from the event master dim and test module to identify major components of combustion and reported the following:
`The plastic cased diode was manufactured of Bisphenol-A epoxy resin, which decomposed to form Phenol (C6H5OH) and variations of Phenol, eg. Methyl phenol (C6H4CH3OH). The mass loss at 427 degrees C was approximately 12 percent, as measured by thermal gravimetric analysis (TGA).'
The flight crew were physically affected as a result of exposure to the fumes produced from the combustion of the failed diode on the master dim and test module.