Norwegian near-stall to spur 737 modification

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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.

Examination of Norwegian’s fleet revealed that there had been similar fluid ingress into other 737-800, as well as 737-300, aircraft.

Boeing has also simulated a comparable restriction of the power control unit arms, by de-icing fluid, in a cold-chamber rig.

The aircraft (LN-DYM) involved in the Kittila incident had been delivered new to the carrier in 2011.

It had been de-iced before the flight from Helsinki, on 26 December last year, in temperatures of minus 17C.

At a height of 3,250ft during approach to Kittila runway 34 – with its autothrust and autopilot engaged – the aircraft’s began to pitch up in line with a nose-up trim actuation. To counter the resulting loss of airspeed the autothrust commanded full power, but this led the pitch to increase and the airspeed to bleed away rapidly.

Analysis shows that both the primary and secondary input arms on the right-hand power control unit were blocked when the autopilot “unintentionally” elevated the nose of the aircraft, says SHT.

When the 737 reached 20° nose-up the pilots began pushing with “full force” on their control columns, it states. Flight-data recorder information shows they applied a combined 920N (207lb) of force in a bid to stop the jet climbing.

The pitch increased to 38.5° and airspeed fell to 118kt – activating the stick-shaker and generating a stall warning. Although this was below the 121kt stall threshold, lower wing loading meant a stall was averted.

SHT says the pilots’ efforts managed to bring the nose down. But it notes that, during the initial ascent, there was “no attempt” to disengage the autopilot, autothrust or manually adjust the stabiliser trim to nose-down.

“One or more of these measures would have improved the situation,” it says, and also suggests that the control column force should have disengaged the autopilot automatically.

The aircraft subsequently landed safely after a 30min hold for the crew to conduct a systems check.

Examination of the power control units found traces of dried de-icing fluid, although the units passed all function tests and met specifications. There was no indication of internal component abnormalities.

SHT says it is looking into error codes, regarding automatic Mach trim, from one of the two flight control computers of the aircraft – although it points out that the other computer had been engaged during the approach, and passed all function tests.

Boeing has informed the investigators that it plans to modify all 737 to achieve “better protection” against the risk of the elevator system “freezing solid”, says SHT. The authority says it plans to release a related safety recommendation, but stresses that it has yet to reach conclusions about the incident.