What happened
On March 25, 2015, a SAS Boeing 737-600, registration LN-RPA, was performing a scheduled flight from Tromsø to Oslo Gardermoen. Prior to departure from Tromsø, the aircraft underwent a two-step de-icing process involving both Type 1 and Type 2 fluids due to heavy snowfall and windy conditions.
During the approach to runway 01R at Oslo, the autopilot and auto-throttle were disconnected at approximately 700 feet. The pilot flying immediately noticed abnormal resistance in the pitch controls. As the aircraft reached 20 feet above the runway, the pilot attempted to pull back on the control column to flare for landing, but the elevator felt stuck. The pilot had to apply an immense force of 58 daN—roughly four times the typical landing force of 12-16 daN—to "break it loose" and complete the landing safely.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined flight recorder data and conducted inspections of the aircraft. Following the landing, the crew reported the issue to maintenance, and the aircraft was removed from service. Inspections revealed a significant amount of de-icing fluid on the balance panels and noted that very little water was removed during a flushing of the elevator pitot-static system.
The investigation also reviewed a previous serious incident involving a Boeing 737-800 in Finland, where similar heavy control forces were observed. In that case, it was determined that de-icing fluid had entered the tail cone compartment, leading to ice formation on the Power Control Unit (PCU) control arms.
Findings
- The pilot was required to use unusually high physical force to move the elevator control column during the landing flare.
- The force applied (58 daN) was approximately four times the normal operational range for this aircraft type.
- Heavy snowfall and subsequent de-icing procedures likely contributed to the presence of fluid in the tail section.
- While the pilot successfully landed the aircraft, the resistance in the pitch controls presented a significant safety concern regarding the reliability of the elevator system under these conditions.