Rudder pedals that became stuck in neutral position during the landing of a Boeing 737 Max 8 operated by United Airlines may have been related to a cold-soaked rudder roll-out guidance servo. 

According to a preliminary investigation released on 7 March by the National Transportation Safety Board, the 737’s captain reported that the aircraft’s rudder pedals did not respond to normal foot pressure upon landing at Newark Liberty International airport during a flight 6 February flight from the Bahamas. 

Rather, the pedals remained stuck, forcing the pilot to use the nose-wheel steering tiller to “keep the airplane near the runway” centreline before taking a high-speed turn-off and slowing to a safe speed for taxiing.  

During the turn-off, the United captain asked the aircraft’s first officer to check his rudder pedals, which were stuck in the same position. Shortly afterwards, the both pilots’ rudder pedals began operating normally. 

United Airlines' first Boeing 737 Max 8

Source: United Airlines

The NTSB is investigating the effects of cold-soaking a guidance servo within the 737 Max 8’s rudder-control system 

The aircraft – a four-year-old 737 Max 8 assembled at Boeing’s facility in Renton, Washington – taxied to its gate and passengers de-boarded without further incident. 

A review of the aircraft’s flight data recorder supported the captain’s account. 

”Data showed that during the landing and subsequent roll-out, the rudder surface position remained near its neutral position even though the force inputs to the rudder pedals were observed to be increasing,” the NTSB says. 

”About 30 seconds after touchdown, a significant pedal force input was observed along with corresponding rudder surface movement,” it adds. ”Afterwards, the rudder pedals and rudder surface began moving as commanded and continued to function normally for the remainder of the taxi.” 

On 9 February, United duplicated the stuck-rudder phenomenon during a test flight in New Jersey. Then, it notified the NTSB of the flight-control issue and the investigation was launched. 

Inspection and troubleshooting of the rudder-control system ”found no obvious malfunctions with the system or any of its components whose failure would have resulted in the restricted movement observed during flight”, the NTSB says. 

Testing of the roll-out guidance servo removed from the 737 conducted on 28 February at Collins Aerospace’s facility in Iowa examined the potential effects of cold-soaking the components. 

”Testing at room temperature found that the torque to rotate the servo’s output crank arm was within design specifications,” the agency says. The unit was then cold-soaked for 1h and re-tested, and investigators found ”that the torque to move the servo’s output crank arm was significantly beyond the specified design limits.” 

The NTSB’s investigation continues as “further examination of the guidance servo will be conducted”.