Aurigny AT72 near Guernsey on Dec 21st 2016, temporary loss of control due to ice accretion

Last Update: December 14, 2017 / 16:12:37 GMT/Zulu time

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Incident Facts

Date of incident
Dec 21, 2016

Classification
Incident

Flight number
GR-678

Aircraft Registration
G-COBO

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

An Aurigny Air Services Avions de Transport Regional ATR-72-212A, registration G-COBO performing flight GR-678 from Guernsey,CI to Manchester,EN (UK) with 61 passengers and 4 crew, was climbing through FL130 out of Guernsey when the crew temporarily lost control of the aircraft due to ice accretion. The crew was able to recover the aircraft, descended to 10,000 feet and returned to Guernsey for a safe landing on runway 27 about 40 minutes after departure.

The British AAIB rated the occurrence a serious incident and opened an investigation.

On Dec 14th 2017 the AAIB released their bulletin concluding the probable causes of the serious incident were:

The aircraft suffered an in-flight upset at FL130 after accruing airframe icing during the climb, resulting in the adverse aerodynamic effect of ice build-up on the wings. The crew were presented with a degraded perf caution but did not action the relevant checklist because they focused on climbing out of the icing conditions. The IAS was not maintained at or above red bug +10 kt and control of the aircraft was lost when a turn was initiated in the lnav mode of the flight director.

The AAIB reported prior to departure the crew noted a frontal weather system would be encountered during their flight over the English Channel with associated cloud, precipitation and moderate icing.

The line training captain (46, ATPL, 6,040 hours total, 1,401 hours on type) was pilot flying and monitoring the first officer undergoing line training, who assumed the role of the pilot monitoring.

Soon after takeoff the crew selected "High Bank" permitting bank angles up to 27 degrees by the automation, engaged the autopilot and selected the target of climb at FL170. While climbing through about 5300 feet the aircraft's anti-icing systems were activated, at FL090 the aircraft's de-icing systems were activated as the airframe began the accumulate ice. The aircraft continued the climb at the minimum icing speed (165 KIAS), the crew reviewed the QRH for the severe icing checklist memory items in case they would become necessary later.

Climbing through FL110 the crew received a "DEGRADED PERF" and "INCREASE SPEED" caution indication, the external icing lights were turned on prompting the commander to comment "we have got a bit [of icing] haven't we?". The captain made a reference to the checklists for the caution indications, however, the checklist was not put into action, yet, the captain increased the speed to 175 KIAS which reduced the climb rate from about 420 fpm to 25 fpm. The caution lights extinguished. The commander commented the aircraft was "not climbing very well", acknolwedged the checklist required the aircraft to maintain 175 KIAS but as they were flying level it was safe to reduce the speed to 165 KIAS again, which resulted in an increased pitch attitude. The captain commented "just see if we can get above [the clouds]", the aircraft climbed now at 200 fpm. The airspeed was increased again to 175 KIAS requiring the aircraft to descend at 540 fpm down to about FL120, where the aircraft levelled off again, reduced the speed to 165 KIAS and began to climb again.

As it became apparent that the aircraft had insufficient performance to reach FL170, the crew requested to level off at FL130 to accelerate the aircraft, which was approved by ATC instructing the aircraft to turn 10 degrees direct towards NORRY waypoint. The crew reprogrammed their Multi Function Display and selected LNAV. While levelling off and turning towards NORRY the aircraft experienced an inflight upset with the aircraft rolling left through 32 degrees causing the autopilot to disconnect, then rolled right to 38 degrees, rolled left to 78 degrees with the nose dropping to 16 degrees below the horizont.

The captain instructed the first officer to perform the recovery items when commanded to do so. The commander instructed the flaps to be extended to 15 degrees. After losing about 1000 feet the captain recovered the aircraft to stable flight. During the upset the pitch had varied between 16 degrees nose down and 19 degrees nose up and the airspeed had varied between 190 KIAS and 123 KIAS. During the upset the first officer transmitted a Mayday Call, after stabilizing the aircraft again the crew decided to return to Guernsey where the aircraft landed safely.

Maintenance performed functional checks of anti-ice, de-ice, ice-detection as well as aircraft performance monitoring systems of the aircraft with no anomalies detected. A FDR read out revealed that during recovery the maximum flap speed had been exceeded by 5 knots. The aircraft was returned to service.

The AAIB reported weather services had forecast moderate icing over the English Channel from FL100 to FL190. Post event analysis revealed the aircraft was operating in thick cloud layers with tops at FL190 which would have caused moderate icing. There was no evidence to cumulonimbus cloud or severe icing. The AAIB commented: "Prolonged flight in moderate icing conditions could lead to an increasing amount of ice accretion that could result in severe ice accretion."

The AAIB analysed:

The aircraft was climbed into a known area of frontal weather that was forecast to have moderate icing conditions. As the aircraft’s altitude increased its performance decreased to a point where it had reached its operational ceiling due to the accretion of airframe icing. Although the DEGRADED PERF caution illuminated, the crew did not action the DEGRADED PERF check list or the SEVERE ICING procedure.

The forecast and aftercast icing conditions, and the visible extent of the icing encountered, were not entirely consistent. However, the poor climb performance was an indication to the crew of the severity of the ice accretion. Had the crew actioned the QRH procedure for the DEGRADED PERF they would have been directed to carry out the SEVERE ICING checklist. The crew had reviewed the memory items in the checklist, but not the notes on detection which listed ‘Unexpected decrease in speed or rate of climb’ as being one of the indicators of severe icing.

The DEGRADED PERF checklist and SEVERE ICING memory procedure both required (among other things) that the speed be maintained at or above red bug +10 kt and that the autopilot be disconnected. The crew did not observe these actions, varying the speed instead between 165 kt (red bug) and 175 kt (red bug +10 kt). Consequently departure from controlled flight was more likely because the aircraft was flown slower than required. Also as the autopilot remained engaged, the crew would not have been aware of any handling indications of an imminent departure. With the controls in a dynamic condition, an extreme upset was more likely if the crew were not holding the controls firmly at that time.

The crew were focussed on climbing out of the icing conditions into VMC above the clouds, and in trying to achieve this, they had made speed selections which they knew were below that required by the DEGRADED PERF and SEVERE ICING checklists. An earlier level off or a descent would have been required, but this would have involved the aircraft remaining in icing conditions, contrary to what the crew intended to achieve. High terrain was not a factor, and a descent, with the associated increase in IAS, would have avoided the occurrence.

Eventually, control of the aircraft was lost with extreme roll and pitch as a result of a combination of icing, an inappropriate speed, and a turn that was initiated by a change from hdg mode to lnav which permitted a hi bank turn. The crew’s actions enabled a recovery to normal flight and an uneventful landing at Guernsey.

There was evidence that pilots lacked confidence in the APM system, with technical log reports of spurious warnings including on the incident aircraft from the day before. This may have predisposed the commander to partially disregard the cautions and therefore not carry out the appropriate checklist appropriately. The APM system was subsequently checked and found serviceable.

The occurrence highlights that poor decisions are possible in stressful or otherwise high workload situations. In such circumstances, it may be necessary to abandon the immediate goal and pursue an alternative, safer course of action, even if that course of action is perceived as taking the aircraft further away from the desired state. In his case, the desired state was a climb out of icing conditions and thus a return to more normal performance, but the well-intentioned pursuit of the goal led directly to the upset.

Several safety actions were taken by the operator and the aircraft manufacturer to remind flight crew of the required response to the aircraft performing monitoring messages and reduce spurious fault messages. In addition flight crew were reminded:

APM / Reduced Performance

Crews are also reminded that Severe Icing may be encountered without the presence of the normally associated visual cues, and reduced rate of climb or cruise airspeed are sometimes the only indication of significant ice accretion. Whenever crews encounter or suspect severe icing, the full checklist at QRH page 1.09 [Severe Icing] must be completed.
Incident Facts

Date of incident
Dec 21, 2016

Classification
Incident

Flight number
GR-678

Aircraft Registration
G-COBO

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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