What happened
On June 30, 2020, an ATR 72-212A, registration OH-ATG, operating a scheduled flight from Tallinn to Helsinki, experienced an in-flight electrical malfunction. Shortly after landing at Helsinki-Vantaa Airport at 12:03 local time, a series of pops were heard behind the first officer's seat, followed by an electrical smoke warning and a master warning. Heavy smoke began to fill the cockpit while the aircraft was taxiing.
The captain immediately stopped the aircraft on the taxiway and notified the cabin crew of the need for readiness. After shutting down the engines and confirming that the smoke had ceased, the captain initiated an evacuation via the main door. There were 15 passengers and four crew members on board. The evacuation was completed safely and without injuries.
The investigation
OTKES examined the aircraft's flight data recorder, air traffic control recordings, and emergency service communications. The investigation focused on the electrical systems, specifically the propeller de-icing system. Investigators also reviewed the maintenance history and the response of the airport's rescue services.
Technicians inspected the equipment bay and found that a specific relay, responsible for the propeller blade heating elements, had malfunctioned. The investigation also looked into the accumulation of dust within the equipment bay and the procedures used by air traffic controllers to trigger emergency responses.
Findings
- The primary cause of the smoke was a malfunctioning de-icing relay (type 101CC01A2) located in the equipment bay. The relay's contacts had overheated and melted, causing the electrical components and surrounding wiring to burn.
- The accumulation of excessive dust in the equipment bay, which had likely been missed during previous maintenance cleaning, contributed to the smoke formation by providing additional fuel for the overheating component.
- The failure of the relay's contacts caused them to weld together, meaning the power supply to the propeller heating elements could no longer be controlled or disconnected.
- While the evacuation was successful, the air traffic controller triggered an "aircraft accident hazard" alert rather than a full "aircraft accident" alert, which resulted in a slightly reduced emergency response level.
- The relay in question was not a life-limited part, meaning it did not have a mandatory replacement interval, though it was subject to manufacturer testing guidelines.