Fairchild Industries Inc SA227-AC, VH-VEH

During cruise flight, the crew of the Metro III aircraft noticed
a burning smell and smoke in the cockpit. The crew discovered that
the problem was associated with avionics bus number 2, and isolated
the bus. Following this action, the smoke and burning smell
dissipated and the flight was successfully completed.

The investigation revealed that a short circuit had occurred in
the right avionics bus circuit breaker panel and that the aircraft
was being operated with a minimum equipment list item open for a
faulty press-to-talk (PTT) for the co-pilot's audio system. Damage
was sustained to insulation of the wiring and the bus link in the
co-pilot's audio loom, including the PTT wires. The damage had
resulted in a short circuit between the right avionics bus circuit
breaker bus supply link, and the co-pilot's audio wiring loom. The
wiring loom was physically secured to the circuit breaker
bus-bar.

The damage to the insulation of the link wire was in the form of
cuts, possibly from manoeuvring the right avionics bus circuit
breaker access panel past sharp edges on the panel housing during
routine maintenance. The insulation damage ranged in depth and in
some areas exposed or damaged the copper conductor. There was also
evidence of heat moulding from physical contact between the bus
link and the co-pilot's audio wiring loom, due to excess current
draw from the faulty PTT.

The routing of the PTT wiring loom ran along the circuit breaker
bus-bar fro the right avionics circuit breaker bus. Chafing against
this bus may have caused the original PTT defect, and combined with
the damage to the bus link wire, produced a short circuit from the
bus link wire through to the earth wires in the co-pilot's audio
wiring loom. The short circuit bypassed individual circuit breakers
allowing a current draw of up to 100 amperes through the right
essential bus 100 ampere circuit breaker switch.