West Sweden ATP enroute on Sep 25th 2014, control problems
Last Update: October 30, 2015 / 15:24:09 GMT/Zulu time
Date of incident
Sep 25, 2014
West Air Sweden
BRITISH AEROSPACE ATP
ICAO Type Designator
Norway's Statens Havarikommisjon for Transport rated the occurrence a serious incident and opened an investigation reporting the investigation currently suspects the failure of the anti ice systems at the tail plane.
On Oct 30th 2015 Norway's Statens Havarikommisjon (SHT, AIBN) released their final report concluding the probable causes of the serious incident were:
The vibrations and loss of control experienced by the crew was most likely a result of a stall or incipient stall. Ice that had accumulated on the aircraft increased the drag and reduced the lift, while mountain waves probably also contributed to lowering the speed. The crew was unfamiliar with the aircraft type's minimum speeds in icing conditions, and implemented countermeasures too late. The incident can to some degree be linked to deficiencies in the operator’s training program and weaknesses in the aircraft's authority approved documentation.
The SHT reported that the first officer (29, CPL, 560 hours total, 200 hours on type) was pilot flying, the captain (41, ATPL, 4,500 hours total, 2,400 hours on type) was pilot monitoring.
The aircraft levelled off at FL150 on autopilot, climb power was maintained for about 3 minutes before cruise power was set. The aircraft reached 195 KIAS. The crew aware of forecasts of moderate icing in connection with an approaching weather front had activated propeller and engine air intake heating throughout the flight. After being in cruise flight for a few minutes the first officer commented that the airspeed seemed to drop gradually, by that time they had lost about 4 knots indicated airspeed, the crew observed some ice forming on the windshield frame and wiper. The commander thought some drop of speed was natural in icing conditions. When the speed dropped below 180 KIAS she commented that they should not let the airspeed drop below 140 KIAS. At that time about 3-4cm of ice had accumulated on the windshield frame and wiper, the airframe de-icing was activated. The inspection lights made it easy to see that the boots were working and ice was breaking off the leading edges of the wings.
The speed however continued to reduce and suddenly the speed trent went "all the way down". The commander decided that they had to descent and instantly received clearance to descend to FL130. Before the descent commenced the aircraft began to violently vibrate and shake, the aircraft pitched up slowly first accelerating the pitch up, the aircraft banked to the left uncontrolled.
The first officer, in first instinct to avoid a stall, pushed the yoke with both hands, the commander noticed the autopilot was still engaged and instructed the first officer to disconnect the autopilot, he disconnected the autopilot, however, the controls did not respond as expected. In order to avoid a spin the first officer focussed on lowering the nose to accelerate the aircraft while being careful with the bank which continued to increase. After a while the nose did come down, the speed stopped reducing and then increased relatively rapidly, the vibrations and shaking ceased and control was regained. The aircraft had turned by about 50 degrees, the control was lost for about 30 seconds.
The commander had determined they had sound margin to the ground and there was no other traffic in the area, therefore she saw no need to take over control.
Once the aircraft had levelled off at FL130 the aircraft's track was corrected, the autopilot was reengaged. The crew received de-icing warning lights, counted the kicks and determined, that only 4 of the 6 kicks occurred. The crew concluded that the last 2 kicks of the sequence, that should de-ice the tail plane, were not occurring and concluded, that a malfunction of the de-icing system prevented the tail plane from being de-iced.
The flight continued to destination without further incident.
The AIBN annotated that none of the flight crew found it "natural" to manipulate the engine controls to offset the loss of speed before losing control or during the loss of control.
Following the flight the airframe's de-ice timer was replaced, 6 kicks of the de-ice boots were observed.
The AIBN stated: "The aircraft is equipped with a stick shaker, which means that the control yokes starts shaking if the angle of attack exceeds a set value. This artificial warning is activated before an aerodynamic stall occurs, provided that the wing is “clean” i.e. free of ice. If the wing is contaminated by snow or ice, the aircraft will stall at a lower angle of attack than the angle that activates the stick shaker. There is no indicator in the cockpit to show the aircraft's angle of attack."
The AIBN summarizd weather forecasts that moderate icing was expected between FL060 and FL150 as result of an occluded weather front moving southwards and a hot front moving northwest. Mountain wave activity could not be seen on satellite images due to being covered by cloud, however, there were indications of mountain wave activity on the lee side of the mountains in Scotland. Based on model data the Norwegian Institute of Meteorology assessed: "The aircraft may have experienced varying degrees of icing during the flight. It is likely that the aircraft experienced severe icing within a minor horizontal area and within a limited vertical band. Icing in connection with lee clouds can be local and short-term and is therefore not necessarily detected by the model at the relevant location and at the relevant time. ... Based on the available information, it is likely that there were mountain waves over the area in question during that evening. Lee clouds will form in connection with mountain waves, and icing may be encountered in these lee clouds in rising and humid air. The icing can often be concentrated to small areas horizontally and thin bands vertically. It is not unlikely that the aircraft experienced severe icing at approx. 15,000ft."
The AIBN reported that according to the flight data recorder the airspeed increased for 2-3 minutes after the aircraft had levelled off, then the airspeed dropped by 3 knots over the next minute, then another 3, 6, 5, 11, 5, 7 and 18 knots per minute. A total of 58 knots were lost, the lowest indicated airspeed was 136 KIAS within 8 minutes. The highest pitch reached was +7.1 degrees, the nose dropped to -6.2 degrees, the aircraft lost about 1000 feet of altitude while out of control, the maximum left bank angle was 32.3 degrees, the highest rate of descent was between 2500 and 3000 fpm.
The AIBN stated: "It emerges that the lowest speed during the incident with SE-MAF was 30 kt higher than the “clean” stalling speed and 16 kt higher than the “clean” stick shaker onset speed, but 22 kt lower than the recommended minimum speed in icing conditions."
The AIBN analysed:
The analyses performed by BAE Systems show that the lift coefficient was significantly reduced in relation to what one would expect from a clean wing (cf. 1.16). Airframe buffeting indicated an incipient stall, even though the speed was higher than the stick shaker onset speed. The fact that buffeting can occur this early when the aircraft is in icing conditions is a known fact and was taken into account when the aircraft was certified. The certification requirements in this area are complex and have later been made stricter.
Unexpected stalling is a serious safety issue, and stalling without adequate forewarning can be considered an unsafe condition, sowing doubts about the airworthiness of an aircraft. It was not obvious to the crew of SE-MAF that a stall was imminent. The first officer experienced the vibrations and the aircraft's response as very different from what he experienced in the simulator (cf. 1.1.14). The AIBN nevertheless believes that this incident does not provide a basis for putting forward far-reaching airworthiness recommendations. There exist recommended minimum speeds in icing conditions that give higher safety margins, provided that they are known and adhered to. The significant safety problems uncovered in this investigation can be solved with simple measures. The AIBN will therefore not look closer into matters relating to construction and certification.
The AIBN further analysed: "In AIBN’s opinion, rime ice probably formed on large parts of the unprotected areas of the aircraft, and that this increased the drag. It is known that small droplets of freezing drizzle that from the pilots viewpoint may not even be discernible, can form rime ice with very severe aerodynamic effects. Thin rough ice generally cannot be removed by deicing boots."
The AIBN continued analysis: "The Accident Investigation Board believes that it is likely that icing in combination with mountain waves caused the airspeed to decrease. Additional engine power was not applied to compensate for the speed loss. Countermeasures like reduction of angle of attack and initiation of descend were implemented too late, and control was lost."
The AIBN wrote in the continuing analysis: "However, the AIBN considers it worrying that the information concerning the aircraft's considerable speed loss was available to the senior operational personnel from the day after the incident, without anyone reacting. The commander's report shows that she thought the safety margin to be sufficient if they stayed above 140 kt (cf. 1.1.4), whereas the recommended minimum speed for the relevant mass in reality was as high as 158 kt (cf. 1.6.7 and 1.16.1). Knowledge about special procedures for approach in icing conditions seemed to be in place for all operational personnel the AIBN spoke to in connection with the investigation. However, there was an impression that knowledge, or at least awareness, was lacking as regards icing in other phases."
As an immediate safety action, after the flight data recorder had been evaluated, BAE Systems issued a Flight Operations Safety Information Letter, "where the main message was that ATP crews did not seem to be fully aware of the recommended minimum speed in icing conditions (cf. 184.108.40.206)."
Two safety recommendations were issued as result of the investigation both focussing on ice accretion causing aircraft to stall prematurely.
Date of incident
Sep 25, 2014
West Air Sweden
BRITISH AEROSPACE ATP
ICAO Type Designator
This article is published under license from Avherald.com. © of text by Avherald.com.
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