Jettime AT72 near Bergen on Nov 14th 2016, drops left then right wing in icing conditions

Last Update: September 10, 2020 / 17:16:06 GMT/Zulu time

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Incident Facts

Date of incident
Nov 14, 2016

Classification
Incident

Airline
Jettime

Flight number
SK-4144

Destination
Aalesund, Norway

Aircraft Registration
OY-JZC

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A Jettime Avions de Transport Regional ATR-72-212A on behalf of Scandinavian Airlines, registration OY-JZC performing flight SK-4144 from Bergen to Aalesund (Norway), was cleared to climb to FL170 out of Bergen. When the aircraft climbed through FL150 in icing conditions, the crew received a "Degraded Perf" caution. Subsequently the stick shaker activated, the aircraft stopped the climb at FL160 at 08:05L (07:05Z) and began to descend again, the left hand wing dropped, then the right hand wing dropped. The crew regained control manually, turned northwest towards the sea and descended the aircraft to FL100. At FL100 the crew turned northeast towards Aalesund and landed safely.

The aircraft continued the next sector to Trondheim on schedule and continued schedule throughout the day.

Norway's AIBN rated the occurrence a serious incident and opened an investigation.

On Sep 10th 2020 Norway's AIBN released their final report concluding the probable causes of the serious incident were:

En route from Bergen to Alesund control of SAS4144 was temporarily lost in severe icing conditions. The AIBN is of the opinion that the control loss was the result of insufficient planning and inappropriate decisions en route, such as the attempt to climb above the icing conditions despite degraded aircraft performance as well as use of the autopilot in altitude hold mode.

Recovery of control of the aircraft may have been impeded by the commander's initial response, pulling the control wheel back as the stick shaker activated. The commander is likely to have become startled when the aircraft stick shaker activated and the autopilot automatically deactivated, while the aircraft banked sharply, pitching its nose down. He may consequently have pulled on control wheel as the result of a temporary startle effect.

Another factor that contributed to the non-optimal recovery was that two memory items on the checklist for stalling, deploying flaps and increasing engine power, were omitted.

However, the crew quickly regained their composure and the rest of the flight and landing at Alesund Airport Vigra proceeded without any further problems.

The AIBN is of the opinion that icing should be a priority item in risk analyses for airlines that plan to operate in Norway during the icing season, and that it is important to take the characteristics of the aircraft type into account. Such analysis should conjointly consider the flown routes, the flown flight levels, expected icing conditions, and mitigation actions to adverse conditions including icing, with regard to the aircraft type and specific performance.

The AIBN reported the first officer (51, CPL, 7000 hours total, 5850 hours on type) was pilot flying, the captain (60, ATPL, 9,423 hours total, 3,523 hours on type) was pilot monitoring.

When the aircraft climbed through FL120 ice began to build up, the climb rate reduced to about half, the aircraft performance monitoring system (APM) indicated "DEGRADED PERF". Climbing through FL140 the first officer observed two streams of water or ice running down her windshield and interpreted this as freezing rain. Both captain and first officer observed ice forming on their side windows. The crew selected Maximum Continuous Thrust (MCT) and reduced the target speed to 165 KIAS. Climbing through FL160 the APM indicated "INCREASE SPEED", the aircraft was maintaining 164 KIAS at that time. The crew selected the target speed back to 170 KIAS, the aircraft levelled off and accelerated to 172 KIAS. The crew agreed to terminate the climb.

The aircraft was cleared to descend to FL150, 500 fpm sink rate were achieved, the thrust mode was reduced to CRZ (Cruise), the airspeed stabilized at 170 KIAS. When the aircraft neared FL150, Altitude Capture activated and the aircraft levelled off at FL150, the airspeed began to decay and the Pitch and Angle of Attack increased. The captain noticed the aircraft did not accelerate.

As more and more ice formed on the airframe the crew requested to turn to a westerly direction, were cleared and selected a heading of 330 degrees into the autopilot heading, the autopilot rolled the aircraft to 29.4 degrees bank angle. 12 seconds into the turn the autopilot automatically deactivated, the airspeed was 163 KIAS and the AoA had increased to 11.8 degrees nose up. The bank angle to the left increased abruptedly and the stick shaker activated.

The AIBN described the sequence of events further:

The first officer reacted by trying to push the control wheel forward. In addition, she responded by right rudder pedal input and turning the control wheel to the right. However, the bank angle still increased uncontrollably to 68.2° left.

At the same time, the commander took hold of the control wheel, initially pulling back, in the opposite direction of the first officer’s move. The first officer has explained to the AIBN that she tried to push the aircraft's nose down when the stick shaker activated. She noticed that it was unusually difficult to push the control wheel forward and wondered if ice had formed on the horizontal tail.

The crewmembers have explained that they both operated the controls during this phase. The commander did not take over the controls, but "joined her" on the control wheel to help her regain control of the aircraft. According to their statements in retrospect, neither of them was aware of the fact that the other gave an opposite elevator input. Both crewmembers have stated that they thought cooperation and communication between them were good.

As the aircraft banked sharply to the left, the nose pitched down to 3.3° below the horizon. Immediately after, the aircraft banked uncontrollably to the right and the nose pitched further down to 8.1° below the horizon. However, the angle of attack had increased to 14.5°. The stick shaker and stick pusher then activated for two seconds. The bank angle to the right reached 66.2° before it reversed back to the left. This time, the bank angle to the left stopped at 36°. The first officer continuously responded to the uncontrolled banking with opposite rudder and aileron inputs.

Three seconds after their first activation, the stick shaker and stick pusher activated again. The local angle of attack was then 15.9°. Two seconds later, the nose pitched to 11.9° below the horizon and the aircraft quickly lost altitude while the airspeed increased to 190 knots. At the most, the sink rate was 6,448 ft/min before the nose started to pitch up again.

When asked directly during separate interviews with the AIBN, both crew members stated that they did not notice that the stick pusher activated (see Section 1.11.2.4).

At its lowest, the aircraft was at an altitude of 13,425 ft, before it started to ascend again. According to the ICAO Aeronautical Chart Southern Norway, the minimum safe altitude for terrain and obstruction clearance was 5,800 ft in the area where control of SAS4144 was lost. This meant clearance to terrain and obstructions was 7,600 ft or more. The aircraft's course (330°) took it to areas with lower terrain height.

During stall recovery, the nose pitched up to 12.1° above the horizon. Altitude increased again, while the speed decreased. The crew reacted by selecting maximum continuous thrust (PWR MGT selector to MCT) and pushing the power levers forward from 70° to 77°. Despite this, the APM indicated almost immediately: INCREASE SPEED. 13 seconds had then passed since SAS4144 started its climb. The APM message was displayed for 28 seconds. SAS4144 then climbed about a thousand feet to the highest registered altitude of 14,531 ft. The speed had then dropped to 150 kt.

The crew requested clearance to descend and initiated a continuous descent towards FL100. Norway Control gave SAS4144 clearance to operate within altitude block FL100 – FL140. SAS4144 continued on course 330, along a more westerly route than originally planned. This meant they flew closer to the coastline and lower terrain.

At 08:13:36 hours, SAS4144 descended below FL127. The airspeed at the time was 212 kt, and the ice that had formed on the airframe started to disappear. The crew reconnected the autopilot 3 minutes and 23 seconds after the autopilot automatically deactivated and temporary loss of control occurred. When the aircraft reached FL110, all the ice had disappeared and the DEGRADED PERF. alert from APM, which had displayed continually since 08:00:08, discontinued.

After the situation was resolved, the commander informed the aircraft passengers over the PA system.

The remainder of the flight was without further incident.

The AIBN analysed:

The crew planned to follow a common flight path on their flight from Bergen to Ålesund. They were aware that moderate icing had been forecast and were consequently prepared to divert westward and towards the coast if the icing should become too severe.

After the incident, the crew stated that they should have chosen a more westerly track from the outset. The AIBN agrees that a different route could have been advantageous in terms of potentially lower precipitation and icing intensity.

Furthermore, the AIBN believes that the choice of flight level might be a learning point. The temperature profile for Bergen (ENBR) indicates that moderate icing could be expected from FL80 up to FL160, whereas the icing area was higher up further north towards Bringelandsåsen (ENBL), where moderate icing could be expected up to FL200. Even though the aircraft is certified for flight in icing conditions, the planned flying altitude, FL190, may not have been the most prudent choice in terms of avoiding prolonged exposure to icing conditions.

...

The departure from Bergen and the flight until SAS4144 reached FL100 passed without incident. The crew quickly (within five seconds) activated the pneumatic de-icing system when the ice detector warned of ice accretion on the aircraft. It is not clear whether they had any visually observations of ice accretion before the ice detector warning light illuminated. The FCOM procedure requires activation of the de-icing equipment at the first indication of ice accretion, not waiting for the detection by the ice detector.

Over the next four minutes, the crew got several indications that the icing was developing into a problem. Ice continued to form, and the climb speed dropped gradually. Then, after one and two minutes respectively, the crew got two clear indications that the aircraft had encountered severe icing and that the icing intensity surpassed the capacity of the aircraft ice protection systems. These indications were performance degradation (decreased climb rate and speed) eventually causing the APM alert Degraded perf. to display at FL137, and the streaks of water/ice along the window at FL140 (see "Detection" in the severe icing checklist, item 1.6.11.3). According to all procedures and checklists, the aircraft should immediately have left the area at this time.

When the crew increased PWR MGT to MCT and reduced the airspeed to 165 kt, the climb speed increased temporarily. However, it dropped relatively quickly although the aircraft to a certain degree continued to climb. It was not until the aircraft reached FL160 and the APM alert Increase speed was displayed, that the crew increased the speed back to 170 kt and requested clearance to descend to FL150. In the meantime, it became necessary to use the three checklists Severe Icing, Degraded Performance, Increase Speed, one after the other. One item in all the checklists is "IAS above ICING BUG + 10 kt", i.e. the speed should not drop below 166 kt. The AIBN believes that in general the airspeed was slightly low throughout the climb, and that this may have resulted in increased ice accretion on the aircraft.

Descending to FL150, the crew reduced PWR MGT to CRZ. Engine power was not increased again after the aircraft levelled off at FL150. That may have contributed to the speed dropping more rapidly. According to the severe icing checklist, PWR MGT must be in MCT (1.6.11.3).

In the AIBN's opinion, it was particularly unfortunate to use the autopilot in altitude hold mode when SAS4144 levelled off at FL150 and the aircraft was still experiencing icing. Regardless of aircraft model, it is easy in such conditions to find oneself in a situation where the autopilot gradually and imperceptibly increases the angle of attack in order to compensate for the loss of lift caused by reduced lifting capacity and increased drag as ice builds up on the aircraft35. This, in turn, will expose the aircraft to further icing, which can lead to a vicious circle ending in the aircraft stalling. When asked what they would have done differently, both pilots said they waited too long before disengaging the autopilot. ATR's manuals, procedures and checklists also did warn against the use of autopilot if an aircraft encounters severe icing.

Shortly after the aircraft had levelled off at FL150, the SAS4144 crew requested clearance to change the course due to icing and were granted such clearance by air traffic control. This clearance was given together with a question about whether the pilots wanted to change their approach to Ålesund. This led to a radio exchange between SAS4144 and Norway Control. The change of course towards the west was not initiated until after this exchange was terminated. 44 seconds elapsed from the time when SAS4144 requested to change course and before the actual change of course was initiated by the crew.

This resulted in unnecessary loss of time in an already tight time frame. No more than just over a minute passed from the time the aircraft levelled off at FL150 and until control was lost. In retrospect, it appears that it would have been better had the crew first initiated the change of course and then requested clearance. This would have been more in line with the old, but still current, task prioritization mantra for pilots: "Aviate – Navigate – Communicate".

SAS4144 had, in two calls, three minutes apart, indicated that they were experiencing problems due to icing. The AIBN has the impression that Norway Control understood the situation and was offering help, when they not only approved the change of course, but also offered an alternative approach to Ålesund. In stressful situations, an aircraft crew may not have capacity to consider more than what is ongoing in the present moment. Thus, the AIBN believes that one lesson air traffic control can draw from this is that they should have just given SAS4144 the clearance they requested, and then raised the issue of a changed approach after the change of course had been completed.

In addition to too much time passing from when SAS4144 encountered severe icing until the crew initiated the turn towards the west to escape the icing conditions, the situation was aggravated by the fact that the turn was performed using the autopilot heading function in high bank mode. The system caused the aircraft to bank at an angle of approx. 30 degrees, while the angle of attack continued to increase.

About five minutes passed from the first indications of icing until the aircraft encountered icing conditions that were too severe for the aircraft ice protection systems to handle. Ten minutes later the aircraft was out of control. In retrospect, it appears that the crew did not make optimal use of these ten minutes, as they tried to climb above the clouds and the ice. Should one try to understand the crew's priorities, it is important to remember that they expected, at any time, to clear the clouds and escape the icing conditions, after having climbed above FL140. The AIBN interprets the fact that they, after having abandoned this approach, temporarily levelled off at FL150 and failed to change the course immediately, as an indication that the crew still did not fully understand how serious the situation had in fact become. The same applies to the fact that the autopilot was not disengaged earlier.

...

In interviews, the crewmembers have independently of each other stated that they felt cooperation functioned well, both before, during and after loss of control. There is nothing to indicate that the crewmembers were not getting along, that there was disagreement between them, or that one was working against the other or was not contributing.

However, the AIBN believes that what took place immediately after the stick shaker activated must be characterized as a temporary breakdown of crew resource management – at a non-verbal level. The most obvious indication was that the first officer and the commander applied opposite forces on the control wheel without being aware of it, but also that two memory items on the checklist were disregarded. It appears that cooperation in fact broke down without the crew being aware of it.

The breakdown of crew resource management on SAS4144 did not last long. The crew regained control of the aircraft and continued its flight to Ålesund where the aircraft was subsequently landed normally. The AIBN is of the opinion that this shows resilience38 on behalf of the crew, also as a team. They managed to recollect themselves and gain control of the situation, thus averting an accident.

...

However, the AIBN believes that focus also should be directed on the issue of aircraft type and specific performance in icing.

There is general agreement that the former certification regulations were not comprehensive enough, and that they did not sufficiently take into account recent knowledge about icing that emerged in the wake of the Roselawn accident, in particular the effect of supercooled large droplets (SLD). This means that the majority of existing aircraft types that are approved for operation in icing conditions could have significant differences in their ability to resist icing.

Compared with N401AM (ATR 72-212) that crashed in the Roselawn accident, OY-JZC (ATR 72-212A) was no doubt better equipped against icing. Although the aircraft operated in severe icing conditions for a relatively long period, the aircraft was not exposed to aileron hinge moment reversal, and it was possible to recover and continue the flight after control had been lost. The AIBN believes that a number of improvements of the aircraft model and better aircrew training have no doubt had a positive effect. It should also be taken into account that Jet Time AS had operated this aircraft model on Norwegian air routes for more than two years, apparently without any problems with icing.

...

AIBN is of the opinion that icing should be a priority item in the risk analyses for airlines that plan to operate in Norway during the icing season, and that it is important to take the characteristics of the aircraft type into account. Such analysis should conjointly consider the flown routes, the flown flight levels, expected icing conditions, and mitigation actions to adverse conditions including icing, with regard to the aircraft type and specific performance.
Incident Facts

Date of incident
Nov 14, 2016

Classification
Incident

Airline
Jettime

Flight number
SK-4144

Destination
Aalesund, Norway

Aircraft Registration
OY-JZC

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

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