posted on Nov, 26 2013 @ 02:13 PM
On 26 December 2012, a Norwegian 737 on approach to Kittila almost stalled on final approach. The aircraft was under autothrottle/autopilot control
when it began to nose up. The autothrottle commanded more power, which led to higher pitch. The crew pushed forward as they reached 20 degrees nose
up, and were able to recover although the aircraft fought.
It was determined by investigators that deicing fluid can enter the tail cone of the aircraft, and contaminate the flight control cables. The fluid
freezes on the PCU arms, restricting their ability to travel.
Boeing plans to release a modification to the deicing procedure, as well as to the aircraft itself to prevent fluid entry into the tail cone area, but
hasn't specified what exactly.
Boeing is to modify 737s to improve protection against potential freezing of elevator systems, after investigation into a near-stall by a
Norwegian aircraft highlighted the risk of de-icing fluid contaminating power control units for the horizontal stabiliser.
The investigation into the 737-800 incident, on approach to Kittila in Finland, has already resulted in Boeing changing de-icing procedures on the
type. Under the new procedures the trim is set to take-off position, rather than fully-forward, and de-icing fluid is applied at an angle, not
directly from the side.
While the cause of the incident is still being explored, Norwegian investigation authority SHT demonstrated that de-icing fluid was capable of
entering the tail cone in “quite considerable” amounts.
“Under certain circumstances it is possible that the input arms [to the power control units] may be exposed to fluid which in turn freezes solid and
blocks [them],” says SHT, adding that Boeing was “not aware” of this potential problem before the investigation.