The Accident Investigation Board of Norway (AIBN) is conducting a “thematic investigation” of incidents in which pilots flying Boeing 737s for several Scandinavian airlines had to use larger-than-normal control yoke forces to unstick their aircraft’s elevators.
The action follows two stuck-elevator investigations. One involved a Norwegian Air 737-800 that came close to stalling on approach into Finland’s Kittila Airport in December 2012. The other, which involved a Scandinavian Airlines 737-600 on short final to Oslo in March 2015, was tied to deicing fluid that had entered the tail section during preflight activities and froze at altitude.
“The AIBN is aware that in recent years, there has been a greater number of incidents involving the Boeing where it has been necessary to use larger forces on the elevator controls than normal,” the Norwegian board states in a report issued in June.
In its final report on the Oslo incident, AIBN found that the captain had to apply approximately 130 lb. of force to “break [the elevator] loose” when he attempted to pull up the nose when flaring the aircraft for landing. Typically, pilots use 27-36 lb. of force on the yoke to flare the aircraft, the board noted.
Based on flight-recorder data, the AIBN said the elevator became free when the aircraft was 6 ft. above the runway, and the aircraft landed normally with no damage or injuries.
The mishap occurred on the same day the AIBN published its final report on the 2012 stuck-elevator incident.
The board issued several recommendations in the report to address the problem. It says the issue could affect all 737 models. It includes a recommendation for Boeing to perform a new risk-assessment on 737 controls and urges the FAA and European Aviation Safety Agency (EASA) to ensure that Boeing performs the assessment and addresses any certification concerns.
Boeing did not agree to a new risk- assessment but did make two procedural changes. One change involves how elevator trim is set before deicing operations start. The other instructs deicing teams to apply deicing fluid from the front to the back of the elevator and to avoid pointing the spray into the opening in the tail.
As of April, the AIBN had not received responses from the FAA or EASA.
As part of an investigation, the AIBN performed simulations of the new Boeing procedures. It found that fluid penetration into the tail was reduced, but some still entered.
The stuck elevator survey will be mainly based on flight data recorder information from approximately 800,000 flights made by Boeing 737NGs flying for Norwegian Air Shuttle, Norwegian Air Norway and Scandinavian Airlines System, the board says.