Performance monitoring of the auxiliary power unit (APU) was
being carried out during ground maintenance of the Boeing 737-300.
Maintenance staff reported that the electrical system voltage had
surged twice and that the aircraft interior lights had dimmed
before voltage and frequency indications returned to normal. No
circuit breakers had tripped. When maintenance personnel in the
aircraft cockpit noticed a burning smell, they shut down the APU to
investigate the fault.
The forward No. 2 galley ovens were found to be inoperative.
During further troubleshooting, ground power was applied twice,
which resulted in smoke emanating from a ceiling access panel above
the forward No. 2 galley assembly.
The fault was traced to the electrical connector supplying
115VAC three-phase power to the forward No. 2 galley ovens. The
connector and associated wiring behind an overhead panel were
burnt. The seven-pin connector had a five-wire bundle comprising
phases A, B, C, neutral D and static ground. The three-phase wires
had desoldered from the pins on the connector. Arcing damage was
found on the endbell and cable clamp halves of the connector.
The insulation blanket and galley panels next to the connector
were blackened and heat-damaged. Charring of the insulation
material had occurred over an area of about 75 mm x 100 mm. The
film covering the blanket had burnt for a further 150 mm to 250
mm.
Figure 1: VH-CZB damaged insulation blanket
The galley manufacturer and operator established that arcing and
heating of the connector and wiring was due to inadequate soldering
and insertion of the phase wires into the connector-pin buckets.
Only the static ground-wire connection was found to meet the
manufacturer's soldering process standard.
The connector at the other end of the damaged cable was also
examined. The solder terminations of phases A, B, C and neutral D
were also found to be substandard, with solder smeared on the
outside of the contact pin wall and only a small amount of solder
bonding the cable strands to the contact pin.
The phase and neutral wire-identification numbers stamped on the
insulation were of a different size and orientation from those
supplied by the galley manufacturer (as shown on the static ground
wire insulation). The wire-identification number stamps matched
those held by the operator.
The arcing damage found on the endbell and cable-clamp halves of
the connector had no matching wire-bundle insulation damage,
showing that the damage had occurred before this incident. The
resulting damage to the cable probably explains why the phase A, B,
C and neutral D wires had been replaced and re-identified.
Forward No. 2 galley was the original fitment on this aircraft,
delivered in September 1986. The operator reported that
disconnection of the subject connector would occur only during
scheduled galley removal at each D-check. The sole D-check on this
aircraft occurred in July 1994. The operator reviewed maintenance
and pilot discrepancies raised against electrical power and galley
equipment systems since aircraft delivery, but did not find any
failures requiring loom replacement. Similarly, a review of D-check
work documents showed no galley electrical system
discrepancies.
The insulation blanket and film-like covering from above the
forward No. 2 galley were heat damaged as a result of the
electrical connector fault. The operator reported that the
blanket-covering material (Orco Film AN-26) did not appear to be
flame retardant to the level of FAR 25.853. The operator believed
that the accumulation of organic material on the surface of the
film over 13 years, especially corrosion inhibiting compound mist,
would have changed the film's flammability performance. As a result
of the accident involving Swissair flight 111, the U.S. Federal
Aviation Administration is researching the flammability of thermal
acoustical insulation materials.
