In-flight upset of ATR 72 due to airframe icing and improper speed management

Casualties unknown • 5 nm north of reporting point ORTAC, GB

An ATR 72-212 A experienced a significant in-flight upset while climbing through icing conditions, leading to a loss of control and an emergency return to Guernsey.

What happened

On 21 December 2016, an ATR 72-2 and12 A, registration G-COBO, was conducting a scheduled passenger flight from Guernsey to Manchester. During the climb, the aircraft entered a weather front and began accumulating airframe ice. As the aircraft climbed, the crew received 'degraded performance' and 'increase speed' caution messages from the Aircraft Performance Monitoring system.

In an effort to climb above the cloud layer, the crew did not fully execute the required checklists. Instead, they varied the airspeed between the minimum icing speed and the recommended safety margin. While attempting to level off at FL130 and initiating a turn via the lnav mode, the aircraft experienced a severe upset. The aircraft rolled as much as 73 degrees and pitched nose-down by 16 degrees, triggering the stick shaker and stick pusher. The crew managed to recover the aircraft to controlled flight and subsequently declared a MAYDAY, returning to Guernsey for an uneventful landing. There were no injuries among the 61 passengers and 4 crew members.

The investigation

The AAIB investigation examined the aircraft's flight data, cockpit voice recordings, and the crew's response to the icing conditions. The investigation looked into the functionality of the Aircraft Performance Monitoring (APM) system, noting that recent technical logs had recorded reports of spurious warnings on this specific aircraft. The investigators also reviewed the manufacturer's Quick Reference Handbook (QRH) procedures and the crew's training regarding icing encounters.

Findings

  • The aircraft encountered significant airframe icing while climbing through a frontal weather system.
  • The crew focused on climbing out of the icing conditions rather than following the prescribed 'DEGRADED PERF' and 'SEVERE ICING' checklists.
  • The crew failed to maintain the required minimum speed of red bug +10 kt, which increased the risk of a loss of control.
  • The transition to lnav mode for the turn deactivated high bank protection, contributing to the upset.
  • A lack of confidence in the APM system, due to previous reports of spurious warnings, may have led the commander to partially disregard the caution messages.

Probable cause

The loss of control was caused by a combination of airframe ice accumulation, the failure to adhere to required speed and checklist procedures, and the initiation of a turn using a flight mode that permitted an excessive bank angle.

Frequently asked questions

What happened in the 2016-12-21 ATR 72-212 A, 500 Version accident near 5 nm north of reporting point ORTAC, GB?

An ATR 72-212 A experienced a significant in-flight upset while climbing through icing conditions, leading to a loss of control and an emergency return to Guernsey.

What aircraft was involved and where did it happen?

The accident on 2016-12-21 involved a ATR 72-212 A, 500 Version, registration G-COBO , at 5 nm north of reporting point ORTAC, GB.

What was the probable cause of the accident?

The loss of control was caused by a combination of airframe ice accumulation, the failure to adhere to required speed and checklist procedures, and the initiation of a turn using a flight mode that permitted an excessive bank angle.

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