Norwegian B738 at Kittila on Dec 26th 2012, unintentional steep climb on ILS approach
Last Update: March 28, 2015 / 16:50:33 GMT/Zulu time
- During its approach to Kittilä, LN-DYM was close to stalling. The outcome of a stall could have been catastrophic.
- The analysis of data from the flight data recorder shows that three or four of the Input cranks on the aircraft's elevator Power Control Units were blocked, most likely due to ice. Calculations made by Boeing indicate that the elevator was extremely slow in responding. With only 0.2°/second, compared with the normal 50°/second. This indicates 1:250 compared with the normal response. At the end of the scenario, the blockage ceased.
- Fluid and humidity will penetrate the Tail Cone Compartment in connection with de-icing of Boeing 737. Increased humidity and spray settling on cold Input cranks may result in ice formation.
- AIBN's investigation has documented that, even after the introduction of new deicing procedures from Boeing, large volumes of fluid and pertaining humidity can enter the Tail Cone Compartment during de-icing.
- AIBN questions whether certification requirements in FAR Part 25 § 25.671 and EASA CS-25 §25.671 for the Boeing 737 Classic and Next Generation series are satisfied.
- AIBN believes that training must be carried out at a quality and volume which ensure the establishment of system understanding and the automatic reflex of return to manual flying.
- LN-DYM should have been grounded at Kittilä after the incident occurred. This is because the background of the serious control issues had not been clarified, which meant that the aircraft's air-worthiness had not been verified.
The AIBN stated that even though the procedures were changed for the stab trim position during de-icing significant ingress of de-icing fluid, which together with cold input cranks could cause another blockade of the stabilizer trim. AIBN wrote following Boeing's modification of de-icing procedures: "The simulations showed that, by changing the stabiliser trim position from the full forward position to the centre part of the green band, this reduced the fluid penetration, but a considerable volume of fluid still entered the Tail Cone Compartment and splashed in the direction of the Input cranks on the Power Control Units. AIBN's investigation results were communicated to Boeing." and continued: "In August 2013, Boeing informed AIBN that change of procedures regarding stabilizer position and de-ice direction was not intended to fully mitigate the ingress of fluid towards the elevator system in the Tail Cone Compartment. Based on the engineering geometric analysis, Boeing still expected fluid to get inside the cavity. Boeings long term mitigation strategy was to make some sort of change of the design of the system which will prevent fluid impingement onto the control system components. In that connection, Boeing informed they planned to modify all Boeing 737s in the interest of achieving better protection against spray into the Tail Cone Compartment and towards components belonging to the elevator system. Later, Boeing notified AIBN that the considered changes would introduce an unacceptable risk of Foreign Object Debris (FOD) that could impair safety negatively."
The AIBN reported that they rated the occurrence a serious incident arguing that the aircraft was close to stalling at low altitude in instrument meteorologic conditions and with the blocked stabilizer trim it was difficult to regain control of the aircraft. The AIBN annotated however, that this assessment is in stark contrast to the perceiption of the occurrence by the flight crew, who did not consider the upset as serious and were ready to take the aircraft onto its next leg, which completed without further incident. The AIBN continued however that after Boeing reviewed the flight data Boeing immediately recommended the replacement of both elevator power control units underlining the seriousness of the occurrence.
Following the recommendation both PCUs were replaced under supervision by the AIBN, the replaced PCUs were taken into custody by the AIBN. Examination revealed no malfunction and no damage, only traces of dried up de-icing fluid were found.
The AIBN analysed: "AIBN's investigations of the flight data show that, while en route at cruising altitude to Kittilä on 26 December 2012, the elevator on LN-DYM had gradually become blocked. However, there were no error indications on the flight deck, and the pilots perceived the flight as normal until the aircraft was in the process of being established on the glide slope for approach. As a result of the mentioned blockage and because autopilot commanded stabilizer trim to change angle on horizontal stabilizer, this resulted the aircraft's nose to unintentionally pitch up rapidly and to a dangerous steep angle. The pilots had to use their full force in an attempt to lower the aircraft's nose angle. The pilots did not perform measures that would have improved the situation (disengaging the aircraft's autopilot, autothrottle and/or reducing engine thrust) were not initiated before at a late sequence of the scenario. 184.108.40.206 The AIBN have not decided which procedure mentioned in Chapter 220.127.116.11 that should have been used. The general guideline in Boeing/Norwegian's operational procedures was, and is, to return to manual flying by disengaging the autopilot and autothrottle, which was not done in this case. Only when the aircraft's nose position passed +35°, on its way down, was the stabiliser trim activated by the pilots and the autopilot thereby automatically disengage. As mentioned in Chapter 18.104.22.168, the commander has explained that he thought the aircraft's autopilot had disengaged automatically when the aircraft's nose was in the process of rising. The aircraft eventually passed the definition of an Upset situation (e.g. more than +25° nose position), and the procedures indicate return to manual flying. The commander believing that the autopilot had already been disengaged may explain why the pilots still did not disengage the autopilot. Several of the company's pilots AIBN has talked with have understood that the autopilot disengages automatically when there is used great force on the control column."
The AIBN released following safety recommendation to Boeing, EASA, FAA:
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 ... 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.
This article is published under license from Avherald.com. © of text by Avherald.com.
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