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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.
Is this related at all to the 747 and 787 GenX icing issues with the current no fly within 50 kilometers of electrical storms?
Under the new procedures the trim is set to take-off position, rather than fully-forward