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.