What happened
On November 16, 2015, an AW139 helicopter, registration PR-OHD, operated by Omni Air Taxi S.A., was completing a transport flight from Macaę Aerodrome to Jacarepaguá Aerodrome in Rio de Janeiro. The flight carried two pilots and nine passengers.
After landing and while the aircraft was parked, the crew noticed a burning smell and observed smoke filling the cabin. The pilots immediately proceeded to shut down the engines and, with the assistance of ground maintenance personnel, disconnected the aircraft batteries. The smoke was successfully extinguished, and the aircraft sustained only minor damage to the upper panel wiring. All eleven occupants were unharmed.
The investigation
CENIPA investigators determined that the smoke originated from the upper electrical circuit breaker (CB) panel. The investigation revealed that two specific electrical wires—one serving the EGPWS and the other the TCAS—were rubbing against a fixing screw within the panel. This friction caused a hole in the wire insulation, leading to a short circuit and subsequent overheating of the screw and the surrounding fiber material.
Further research by the operator's engineering department discovered that the wiring arrangement inside the upper panel was random and did not follow the approved design specifications. This condition was not limited to a single aircraft; inspections of the operator's entire fleet revealed that the same improper wiring arrangement was present in all their AW139 models.
Findings
- The primary cause was a manufacturing quality escape at the Agusta Westland production facility, where production drawings were not updated to the latest revisions, leading to incorrect cable accommodation.
- The improper arrangement of the wiring induced undue contact between the cables and a panel screw.
- The lack of protective coating on the screw facilitated the friction that led to the short circuit.
- There was no existing task in the aircraft maintenance program to inspect the condition of the wiring, nuts, or screws inside the upper CB panel.