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 busbar.
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 busbar from 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.