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31 Mar 2015 - The Accident Investigation Board Norway have issued their final report on the serious aircraft incident during approach to Kittilä airport in Finland (EFKT) on 26. December 2012 with a Boeing 737-800, LN-DYM, operated by Norwegian Air Shuttle ASA

The report is available at the following link here.

On 8 Aug 2016 the FAA ised an SAIB on the issue here recommending a change in de-icing procedures around the position of the stab.

 

Synopsis:

During approach to Kittilä (EFKT) in Finland on 26 December 2012, LN-DYM, a Boeing 737-800 NG on Norwegian Air Shuttle's (NAS') air service NAX5630 from Helsinki airport (EFHK), came close to stalling. The outcome of a stall would most likely have been catastrophic, primarily because the elevator system at that time did not function normally. The elevator system worked only at a ratio of 1:250.

 

 

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Synopsis:

During approach to Kittilä (EFKT) in Finland on 26 December 2012, LN-DYM, a Boeing 737-800 NG on Norwegian Air Shuttle's (NAS') air service NAX5630 from Helsinki airport (EFHK), came close to stalling. The outcome of a stall would most likely have been catastrophic, primarily because the elevator system at that time did not function normally. The elevator system worked only at a ratio of 1:250.

De-icing was carried out prior to departure in order to remove about 25 cm of snow that had settled on the aircraft. The departure and flight en route to the destination were normal. During the approach to Kittilä, the aircraft was established on the localizer at 4 421 ft (AMSL) with flaps 5 configuration, and the autopilot as well as autothrottle were engaged. As the aircraft was in the process of intercepting the glide slope, the elevator trim started to pitch the nose up. This trim continued for 12 seconds. At the same time, the aircraft started to unintentionally ascend while the autothrottle commanded full engine thrust. Both pilots eventually pushed the elevator control column with full force, but the aircraft’s nose continued to pitch up to an angle of +38.5° before slowly decreasing. The aircraft's speed dropped to 118 kt (Calibrated Airspeed, CAS) and the Angle of Attack (AOA) reached a maximum of approximately 25°. The aircraft was thus close to stalling. The aircraft's autopilot was disengaged just after the aircraft's nose angle was at its highest. Control over the aircraft was slowly regained. A new approach was carried out without additional problems.

AIBN's investigation has uncovered that de-icing fluid had ingressed the tail section and frozen on three or four of the input cranks for the aircraft's two elevator Power Control Units (PCUs) and thus prevented them from functioning as intended. The investigation has documented that, even after the introduction of new de-icing procedures from Boeing, considerable amounts of fluid and humidity are entering the tail section (Tail Cone Compartment) during de-icing. AIBN questions whether this satisfies the certification requirements for the aircraft type. AIBN believes there is a need for measures that prevent ice formation on the input cranks and thus reduce the risk of blocking normal elevator function on the Boeing 737 series.

The investigation has also uncovered a potential for improvement in relation to registration of inquiries received by the company's center for maintenance management, and the fact that LN-DYM continued to operate after the incident.

Three safety recommendations are proposed in connection with the submission of this report. (See below)

Photo showing fluid penetration and spray potential toward the Input cranks on the PCUs. (Photo taken in the aircraft's direction of travel in connection with simulated de-icing of the aircraft's right side). The pipe at the bottom of the photo is the exhaust pipe from the aircraft's Auxiliary Power Unit (APU). Photo: AIBN

 

Safety recommendation SL 2015/01T

During its approach to Kittilä on 26 December 2012, LN-DYM came close to stalling as a result of a blocked elevator. AIBN's investigation has documented that, even after the introduction of new de-icing procedures from Boeing, considerable volumes of fluid and pertaining humidity are penetrating the Tail Cone Compartment during de-icing of the Boeing 737 aircraft type. The investigation shows fluid penetration toward the four Input cranks on the aircraft's two Power Control Units. If this fluid freezes in the narrow gap between the Input cranks, this may result in blockage of the Power Control Units. This prevents operation of the elevator on Boeing 737 with potentially catastrophic outcome.

AIBN recommends that the aircraft manufacturer Boeing conduct a new safety assessment of the Boeing 737 aircraft type as regards blockage of the aircraft type's elevator system, and establish measures in order to satisfy the requirements in FAR Part 25 § 25.671 and EASA CS-25 §25.671. (Similar safety recommendations are also issued to the FAA and EASA).

Safety recommendation SL 2015/02T

During its approach to Kittilä on 26 December 2012, LN-DYM came close to stalling as a result of a blocked elevator. AIBN's investigation has documented that, even after the introduction of new de-icing procedures from Boeing, considerable volumes of fluid and pertaining humidity are penetrating the Tail Cone Compartment during de-icing of the Boeing 737 aircraft type. The investigation shows fluid penetration toward the four Input cranks on the aircraft's two Power Control Units. If this fluid freezes in the narrow gap between the Input cranks, this may result in blockage of the Power Control Units. This prevents operation of the elevator on Boeing 737 with potentially catastrophic outcome.

AIBN recommends the FAA to ensure that the aircraft manufacturer Boeing conduct a new safety assessment of the Boeing 737 aircraft type as regards blockage of the aircraft type's elevator system, and that the analysis result and established measures satisfy the requirements in FAR Part 25 § 25.671 . (Similar safety recommendations are also issued to Boeing and EASA).

Safety recommendation SL 2015/03T

During its approach to Kittilä on 26 December 2012, LN-DYM came close to stalling as a result of a blocked elevator. AIBN's investigation has documented that, even after the introduction of new de-icing procedures from Boeing, considerable volumes of fluid and pertaining humidity are penetrating the Tail Cone Compartment during de-icing of the Boeing 737 aircraft type. The investigation shows fluid penetration toward the four Input cranks on the aircraft's two Power Control Units. If this fluid freezes in the narrow gap between the Input cranks, this may result in blockage of the Power Control Units. This prevents operation of the elevator on Boeing 737 with potentially catastrophic outcome.

AIBN recommends EASA to ensure that the aircraft manufacturer Boeing conduct a new safety assessment of the Boeing 737 aircraft type as regards blockage of the aircraft type's elevator system, and that the analysis result and established measures satisfy the requirements in EASA CS-25 §25.671. (Similar safety recommendations are also issued to Boeing and the FAA).

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