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

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Description

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.

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.

Spray test 5 - video

Safety recommendation

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).

Location
Kittilâ airport Finland (EFKT)
Occurrence date
26.12.2012
ICAO Location indicator
EFKT
Aircraft
Boeing 737-600/700/800
Operator
Norwegian Air Shuttle
Registration
LN-DYM
Meteorological conditions
IMC
Type of occurrence
Serious incident
Type of operation
Scheduled air transport
Category of operation
Heavy, aeroplane (> 10 000kg)
Aircraft category
Landplane, Multi-engine, Turbofan/Turbojet
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