Wideroe DH8C enroute on Jan 20th 2020, severe icing, engines affected

Last Update: February 9, 2023 / 20:19:02 GMT/Zulu time

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Incident Facts

Date of incident
Jan 20, 2020

Classification
Incident

Airline
Wideroe Airlines

Flight number
WF-577

Departure
Kristiansund, Norway

Destination
Bergen, Norway

Aircraft Registration
LN-WFO

Aircraft Type
De Havilland Dash 8 (300)

ICAO Type Designator
DH8C

A Wideroe de Havilland Dash 8-300, registration LN-WFO performing flight WF-577 from Kristiansund to Bergen (Norway), was enroute at FL220 about 250nm north of Bergen, when the aircraft entered severe icing. The crew turned west over the coastline while descending to FL160, where no icing had been reported. Eventually the ice broke off but affected both engines (PW123), the aircraft needed to descend further to FL120. The crew finally managed a safe landing in Bergen about 45 minutes after leaving FL220 and about 25 minutes after the ice broke loose.

Norway's Statens Havarikommisjon for Transport (AIBN) rated the occurrence a serious incident and opened an investigation.

The occurrence aircraft remained on the ground for 5 days before returning to service.

On Feb 9th 2023 the AIBN released their final report concluding the probable causes of the serious incident were:

While climbing from Kristiansund Airport Kvernberget, WF577 inadvertently flew into an area of severe icing conditions, and the crew took corrective action by changing course and altitude.

However, they were unable to sufficiently limit the duration of the aircraft’s exposure to severe icing conditions. As a result, ice formed on the aircraft and inside the engine’s air inlets.

On approach to Bergen Airport Flesland, the aircraft lost engine power on the left engine, then on the right engine, and then on the left engine again. The aircraft’s automatic ignition system restarted both engines, but the start-up sequence took time, and the aircraft was completely without engine power for a brief period. The engines flamed out due to ice detaching from the engine’s air inlets. The ice either entered the combustion chamber as slush and water and caused a flameout, or disrupted the airflow into the engine sufficiently to stall it.

The crew acted professionally in a highly demanding situation and landed the aircraft safely at Flesland.

The AIBN summarized the sequence of events:

On 20 January 2020, the crew of LN-WFO were scheduled to fly four flights between Bergen Airport Flesland (ENBR) and Kristiansund Airport Kvernberget (ENKB). The crew arrived at Flesland 45 minutes before the first scheduled departure time which was planned at 1410 hours.

They reviewed the operational flight plans for the sectors they were scheduled to fly, as well as weather information and NOTAM reports. There were strong winds in Kristiansund, and particular attention was devoted to this. They also noted the presence of a warm front in the area they would be flying in, and a forecast for mountain waves. The first flight to Kristiansund went as normal, and after landing, the crew retrieved updated flight plans and weather information in preparation for their return flight to Bergen. A SIGMET report regarding heavy turbulence and mountain winds had been issued for the northern part of Western Norway. Moderate icing had also been reported in the areas where they would be flying. They experienced no icing during the next two flights.

Before the final departure for Bergen, the commander carried out an external visual inspection of the aircraft without finding signs of snow or ice. The temperature at the time was 9°C and it was raining, and the crew did not consider de-icing necessary. For the same reason, the air inlet was not checked for ice during this inspection.

The aircraft with the callsign WIF77G took off for Bergen at 1939 hours. When the crew contacted the air traffic control (Norway Control) for the northern part of Western Norway after departure, they were cleared directly to the first approach point (NIDGI) inbound Flesland. This clearance took them on a more direct course to Flesland (see Figure 1). They entered clouds at about 4,000 ft, by which time they had activated the aircraft’s de-icing system. The aircraft’s weather radar was not actively used during the climb.

Ice began to form on the aircraft around flight level FL140, and the crew observed a thin layer of ice on the wings. The crew expected to pass through the icing layer and climb above the clouds as they had on the three preceding flights. However, the icing intensified as they climbed.

When they levelled out at FL220, the aircraft was still in clouds with considerable icing. They did not notice changes to the aircraft’s stability or performance other than the speed being somewhat lower than normal, which they considered to be the result of drag due to ice. This was confirmed by the propellers shedding ice that hit the fuselage and by vibrations in the aircraft. According to Widerøe’s internal report, the experienced cabin attendant found the situation dramatic, with abnormally strong shaking, vibrations and loud bangs when ice shed from the propeller and hit the side of the fuselage.

The crew were actively working the aircraft’s de-icing systems and tried different propeller RPMs in an attempt to remove the ice. The rotational speed was increased from 900 RPM to 1,050 RPM.

They decided to descend to a lower altitude to find a layer of higher air temperature and requested an altitude adjustment at the same time as they submitted an aircraft report about moderate icing to the air traffic service. The commander confirmed in an interview with the NSIA that he considered the intensity of the icing to be moderate throughout. They were granted an altitude adjustment down to FL160 and began a gradual descent. Shortly after, they requested a course change to 250° to get closer to the coast. The course was later adjusted to 270°. They considered that this would take them to areas of warmer air and lower terrain. The aircraft was controlled by autopilot during the descent and as it continued its approach to Flesland. The crew did not consider disengaging the autopilot.

The icing continued even after the aircraft approached FL160. Using the inspection lights, they observed a great deal of ice on the leading edge of the wings, on the propeller spinners and on the underside of the flap fairing. Abnormal amounts of ice accumulated on the left and right windshield wiper arm. There was also ice on the lower part of the flight deck windows. The pilots have stated that the windows were nearly completely iced over at one point. They cannot remember seeing ice on or in the air inlet. They also stated that they considered the icing a nuisance, but not dangerous. The RPM was increased to 1,200 RPM for a while during the descent. The propeller blades shed ice that hit the fuselage with loud bangs, and they felt vibrations. They have later stated that the phenomenon was more annoying than worrying. They levelled out at FL160 approximately five minutes after starting the descent.

The crew did not wish to stay on a westerly course for too long, as that would eat into their fuel reserves. The course was adjusted 260° to shorten the distance to Flesland. They requested permission to proceed towards BR638 to continue south towards Flesland, and the request was granted. This is an approach point to Flesland that is located further west than NIDGI, for which they were originally cleared. They did not set a direct course immediately, but adjusted it incrementally on the way south. Some of the ice came off, but new ice also accumulated. The crew were aware that the level of the zero-degree isotherm6 was about 6,000 ft and requested clearance to descend to FL100 in an attempt to stop the icing. The descent was gradual, and all the time they were varying the propeller’s RPM. However, they did not succeed in shedding all the ice.

While descending to FL100, they found that the ice increasingly separated from the aircraft, and the intensity of icing decreased. A direct course was then set for BR638. They heard on the radio that there was another Widerøe flight in the area enroute from Bergen to Florø at FL140. They asked the air traffic service to check with the crew if they had experienced icing and received confirmation that this was not the case. They therefore decided to stop the descent at FL120. The icing conditions decreased in step with the decreasing altitude, and as they flew further west. They submitted another AIREP to the air traffic service about moderate icing in the area they had flown through. When the propeller vibrations subsided, the propeller speed was again reduced to 900 RPM.

The aircraft continued at FL120 on a southerly course towards Flesland. The crew still observed thick ice on the spigot, flap fairings and spinner (see Figure 2, Figure 3 and Figure 4). In interviews with the NSIA, they have stated that they had great confidence in the aircraft’s ability to withstand icing, as they had experienced similar situations several times before.

WF577 was cleared for approach to ILS W runway 17 and began its approach and descent towards Flesland. At this time, Flesland reported low cloud cover with precipitation, south-westerly winds and a temperature of 8°C. Ice dislodged from the aircraft at irregular intervals throughout the flight. As they approached 7,000 ft, the amount of ice that came off increased noticeably. Among other things, all the ice disappeared from the spigot. When the aircraft passed 6,700 ft, 20 seconds after the ice dislodged from the spigot, they lost power on the left engine. The first officer noticed a loss of torque and a drop in the turbine temperature and RPM, and he immediately informed the commander.

The crew quickly concluded that the engine had flamed out7 and awaited the situation. They did not start the process of shutting down the engine and feather the propellers, as they expected the engine to restart automatically. This was soon confirmed, and they concluded that the occurrence could be due to ice in the engine. Data from the flight data recorder (FDR) show that the engine regained power and showed normal values after 25 seconds.

After another 51 seconds, the right engine lost power with the same indications as the left engine had shown. The aircraft passed 5,200 ft, and the crew declared an emergency to air traffic control.

The crew believed the cause to be the same as with the left engine and waited to see whether the engine would restart automatically. The right engine took a little longer to regain power, as it took two attempts to restart it. It stopped immediately after the first automatic restart. When the right engine’s power seemed to increase again, the power from the left engine dropped again. Thirteen seconds elapsed from the left engine lost power until the right engine had regained normal power.

The aircraft was thus completely without engine power for a few seconds. This happened while the aircraft was starting its approach to runway 17 at Flesland. The left engine regained power after another 18 seconds, as the aircraft was passing 4,500 ft.

The loss of power on the left engine lasted for 25 seconds, the loss of power on the right engine lasted for 35 seconds, and the second loss of power on the left engine lasted for 31 seconds. The rest of the approach and landing was normal, and the crew cancelled the distress call. The aircraft landed at 2052 hours.

There was still a lot of ice on the aircraft when it was parked after landing, despite rainy weather and a temperature of 8°C. The commander contacted Widerøe’s flight operations duty manager and it was decided to remove the crew from active service. The aircraft was towed into a hangar to be checked by technical personnel, and the CVR’s circuit breaker was pulled just over an hour after the aircraft was parked at the gate.
Aircraft Registration Data
Registration mark
LN-WFO
Country of Registration
Norway
Date of Registration
P Apjlhneei Subscribe to unlock
Manufacturer
DE HAVILLAND AIRCRAFT OF CANADA LIMITED
Aircraft Model / Type
DHC-8-311
ICAO Aircraft Type
DH8C
Serial Number
I h Subscribe to unlock
Maximum Take off Mass (MTOM) [kg]
Qfdcp Subscribe to unlock
Engine Count
Subscribe to unlock
Main Owner
Q bl cmck bbqknpceljffimd db emmbdknjhiqjipjcpcgkf Subscribe to unlock
Incident Facts

Date of incident
Jan 20, 2020

Classification
Incident

Airline
Wideroe Airlines

Flight number
WF-577

Departure
Kristiansund, Norway

Destination
Bergen, Norway

Aircraft Registration
LN-WFO

Aircraft Type
De Havilland Dash 8 (300)

ICAO Type Designator
DH8C

This article is published under license from Avherald.com. © of text by Avherald.com.
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