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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.

 

Local safety action

As a result of this occurrence, the aircraft operator has taken action to re-route the PTT wiring away from the circuit breaker supply bus and removed sharp edges from the panel housing to prevent wire chafing.

 
General details
Date: 06 January 2002 Investigation status: Completed 
Time: 2315 hours ESuT  
Location   (show map):56 km NE Melbourne, Aero. Investigation type: Occurrence Investigation 
State: Victoria Occurrence type: Smoke 
Release date: 16 October 2002 Occurrence class: Operational 
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
 
Aircraft details
Aircraft manufacturer: Fairchild Industries Inc 
Aircraft model: SA227 
Aircraft registration: VH-VEH 
Serial number: AC-663B 
Type of operation: Charter 
Damage to aircraft: Nil 
Departure point:Canberra, ACT
Departure time:2215 hours ESuT
Destination:Melbourne, VIC
Crew details
RoleClass of licenceHours on typeHours total
Pilot-in-CommandATPL
 
 
 
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Last update 13 May 2014