Pilatus PC-12 Smoke Incident in Romanian Airspace (OK-NGX)

Incident Overview and Crew Response On April 24, 2024, at 10:10 local time, a Pilatus PC-12 registered OK-NGX was conducting a private flight from Bratislava (LZIB) to Bucharest Băneasa (LRBS). While cruising at flight level 210 within the Budapest Flight Information Region, shortly before entering Romanian airspace, the crew detected a smell and white smoke in the rear cabin area. The smoke, described as resembling an electrical fire, entered the cabin through a ventilation opening at the lower rear of the passenger compartment. The pilot and co-pilot, both holding valid licenses and medical certificates, decided to use oxygen masks and requested a diversion to the nearest airport, Oradea (LROD). During the descent, the smoke density decreased. The crew did not declare a MAYDAY emergency but requested fire and rescue assistance upon landing. At 10:30 local time, the aircraft landed safely at Oradea and taxied to a parking position. Firefighters inspected the aircraft but found no active source of fire or smoke. ## Aircraft and Maintenance Background The 2022-manufactured Pilatus PC-12, serial number 2230, is a low-wing, single-engine turboprop aircraft powered by a Pratt & Whitney PT6A-67P engine. At the time of the incident, the aircraft had accumulated 762 flight hours and 643 cycles. The aircraft was privately owned and held a valid certificate of airworthiness. The last maintenance inspection was a 12-month check completed on January 31, 2024, when the aircraft had 617 hours and 526 landings. This inspection included operational tests of the emergency shut-off system, cabin pressurization, and manual pressurization systems. No defects were identified, and no major components were replaced. The last certificate of airworthiness was issued on November 29, 2023. ## Technical Investigation and Findings Following the incident, the operator's technical team traveled to Oradea to inspect the aircraft. They checked all electrical cables under the cargo and cabin floors, finding no damage. The cabin heater and its protection system were tested according to the Airplane Maintenance Manual (AMM). During testing, the team observed random disconnection of the cabin heater and fan due to a faulty actuation relay. Additionally, low air flow was detected at the outlet of the distribution ducts. Inspection of the cabin heating ducts revealed a red protective pipe cap causing a blockage. The auxiliary cabin heating system was subsequently isolated in accordance with the Minimum Equipment List (MEL) to allow the aircraft to be relocated. After the aircraft was moved to the maintenance organization, a detailed verification of the heating system identified overheating marks on the top of the cabin heater and the duct in the heater area. The cabin heater and its associated duct required replacement. ## Analysis of the Event The investigation determined that the cabin heating system was partially obstructed by the protective cap found at the end of the heating duct. The origin of the cap, as well as when and how it reached the difficult-to-access location, could not be determined. According to the system's operating scheme, the fan distributes heated air through this opening into the cabin. During operation, the hot air flow generated by the fan moved the protective cap, positioning it to partially block the ventilation duct. This obstruction led to a decrease in air flow to the cabin and an increase in static pressure within the duct, which in turn raised the temperature of the air flow to the activation threshold of the heater's thermal protection system. During testing, the thermal protection activation relay did not function consistently. It is possible that the thermal protection system failed to operate correctly due to the faulty actuation relay. When the temperature exceeded the activation value, the heating system continued to operate because the circuit breaker was not automatically disconnected. The protective cap and heating system elements overheated, generating smoke and a burnt plastic odor in the air distributed to the cabin.