A NORRA Nordic Regional Airlines Avions de Transport Regional ATR-72-212A on behalf of Finnair, registration OH-ATG performing flight AY-1014 from Tallinn (Estonia) to Helsinki (Finland) with 15 passengers and 4 crew, was on approach to Helsinki's runway 04R at 12:03L (09:03Z) when the crew reported smoke in the cockpit. The aircraft landed and vacated the runway onto a taxiway about 2200 meters past the runway threshold and stopped clear of the runway. The aircraft was evacuated, there were no injuries.
The airline reported the smoke was caused by a failure in the power supply of the propeller anti-icing unit.
Emergency services reported the aircraft had a slight smoke generation in the cockpit after the aircraft landed at Vantaa Airport. The passengers were safely taken to the terminal.
In the evening of Jun 30th 2020 Finland's Onnettomuustutkintakeskus (AIBF) announced, they have opened a preliminary investigation into the occurrence rated a serious incident. The passengers of a NORRA aircraft had to be evacuated due to smoke.
On Jul 22nd 2020 the AIBF reported the preliminary investigation has been completed, the findings do not warrant a continuation of the investigation. The cause of the smoke was a faulty relay in the propeller anti-icing system. The AIBF released their "preliminary" final report concluding the probable causes of the incident were:
1. The passengers were rapidly, and in a controlled manner, evacuated through one door
Conclusion: The evacuation was made easier by the small number of passengers and the fact that most carry-on luggage was left behind.
2. The alert was made as full emergency, even though in this instance the correct response should have been aircraft accident. The alerts are made using automatic pushbuttons at the ATC.
Conclusion: Sufficient rescue equipment and personnel in accordance with
contingency planning are critical for effective rescue and firefighting operations.
3. Smoke generation inside the cockpit is always problematic. The relay is positioned inside a metallic avionics bay, which reduces the risk of a fire spreading outside it. During the flight the bay vents into open air.
Conclusion: The manufacturer can reduce the consequences of risk-prone part failures by positioning them appropriately in the aircraft.
4. The damaged relay is not a life limited part, nor does it have a designated service life.
Conclusion: The relay will be used more frequently in Nordic conditions compared to the south. Therefore, the relay will probably wear out sooner.
5. The smoke detected by the pilots and aircraft systems came from the faulty relay. Burning dust inside the avionics bay increased the smoke generation.
Conclusion: Avionics bays are only opened during more extensive periodic
maintenance, or when systems fail. Dust accumulation is not monitored. Excessive dust and grime speed up equipment overheating because of degraded cooling and strengthen a developing fire.
6. The propeller heating circuits are protected against short circuit by 50A circuit breakers.
Conclusions: The fault and the consequent relay damage did not generate
sufficient resistance to trip the circuit breaker. However, they did generate enough heat to start a fire inside the avionics bay. When the contact surfaces melt together the aircraft system continues to provide current to the propeller in an uncontrollable manner. If the fault continues for an extended period, it will likely damage the anti-icing system.
The AIBF analysed:
Propeller anti-icing was used on the occurrence flight. Anti-icing was turned on three minutes after take-off at 11:47. The system was turned off nine minutes before landing, at 11:54. While the aircraft was taxiing after landing, the MASTER WARNING activated at 12:04.
In the inspections conducted by the aircrafts maintenance organisation the propeller 1-3-5 anti-icing relay, position 18DH, proved to be the cause of the smoke. Its main moving contacts and the end of the relay had melted. The relay had experienced a fault 10 minutes after the anti-icing system was turned off. The fault caused the relays contacts to overheat, which in turn damaged and partly melted the propeller control relay inside the avionics bay. There was abundant dust around the relay and wiring inside the avionics bay. Traces of burnt dust were also detected in the compartment above the burnt avionics bay.
The avionics bay is normally vacuumed and cleaned of dust and grime during more extensive periodic maintenance. Judging by the large amount of dust the cleaning had possibly been omitted, or poorly done, during the previous maintenance.
The preliminary investigation did not establish the cause of the failed relay. Its control wiring was intact and attached to the unit. The electrical wiring to the propellers had melted off of the relay. The contacts with which the wires are connected to the relay were still attached to the ends of the wires. The insulations in the propeller control relay had completely burnt on the side of the propeller electricity supply. The relay was partly disassembled but no clear indication of the cause of the fault was found inside it. The contact surfaces and the swivelling insulating plate had melted completely.
Excluding the propeller control relay, the propeller anti-icing system was deemed to be fully functioning, and after the relay was replaced the system has performed normally.
The 50A circuit breaker in the anti-icing system worked in this situation as designed.