What happened
On April 24, 2024, at approximately 10:10 LT, a Pilatus PC 12, registration OK-NGX, was conducting a private flight from Bratislava (LZIB) to Bucharest Băneasa (LRBS). While cruising at FL 210 within the Budapest FIR, the crew detected white smoke and an odor similar to an electrical fire entering the cabin through a lower rear ventilation outlet.
In response to the smoke, the crew donned oxygen masks and requested a diversion to the nearest suitable airport, Oradea (LROD). During the descent, the density of the smoke decreased. Although the crew did not declare a formal emergency, they requested fire services to meet the aircraft upon arrival. The aircraft landed safely at Oradea at 10:30 LT and taxied to its parking position for inspection by airport firefighters, who found no active fire or remaining smoke.
The investigation
The investigation focused on the cabin environmental control system (ECS) and the auxiliary electric heating units located under the cabin floor. Technical teams inspected the electrical wiring in the cargo and cabin compartments, finding no damage. While testing the cabin heater, fan, and thermal protection systems according to the Airplane Maintenance Manual (AMM), investigators discovered that the heater and fan were experiencing intermittent disconnections due to a malfunctioning relay.
Furthermore, inspectors noted low airflow at the ventilation outlet. Upon inspecting the heating ducts, they located a red protective cap lodged inside the ductwork. Subsequent detailed inspections at a maintenance facility revealed overheating marks on both the upper portion of the cabin heater and the associated ducting.
Findings
- A red protective cap was found obstructing the ventilation duct, which partially blocked the airflow.
- The obstruction caused a reduction in airflow to the cabin, leading to an increase in static pressure within the duct and a subsequent rise in air temperature.
- The malfunctioning relay prevented the thermal protection system from consistently disconnecting the electrical supply when temperatures reached critical levels.
- As a result of the blockage and the failure of the thermal protection to interrupt the circuit, the heating elements and the protective cap overheated, generating the smoke and plastic odor detected by the crew.