UK investigators probing a rudder jam involving a West Atlantic ATR 72-200 freighter have found that degraded steel bearings had not been replaced by corrosion-resistant ones.
The replacement had been recommended in a service bulleting published 30 years before the incident at Belfast International on 7 March 2023.
As the ATR flared to land on runway 25, the first officer, who was flying, attempted to correct a slight drift but found the rudder pedals “almost immovable”, says the Air Accidents Investigation Branch.
The captain took control of the landing and, upon touchdown, quickly derotated in order to use nose-wheel steering. Neither crew member was injured during the occurrence.
“Once the unloading was complete, the aircraft was moved to a remote stand,” says the inquiry. “At this point the flight crew found the rudder pedals would not move at all.”
While the crew experienced rudder stiffness during a full-and-free control check before the flight from East Midlands airport, the pilots were aware of recent maintenance work which was considered to have resolved the problem – an issue which had generated an “extensive history” of rudder-stiffness reports.
Believing the maintenance intervention had been effective, the crew opted to proceed with the flight.
But examination of the aircraft, following the incident on arrival at Belfast, found two support bearings on the rudder rear quadrant shaft were corroded, probably as a result of trapped moisture in the aft bay.
“Unable to rotate freely, the bearings would have resisted the movement of the rudder rear quadrant shaft, leading to the stiffness,” says the inquiry, which adds that “other anomalies” in the rudder system could also have contributed to the situation, although to a lesser extent.
Investigators state that a service bulletin was published in 1993 recommending replacement of the bearings with corrosion-resistant equivalents, but this work was not carried out on the aircraft involved – a 1990 airframe registered G-NPTF.
The degradation probably took place over a long period of time but the first crew report of rudder stiffness on the aircraft emerged in early February 2023, about a month before the incident.
Failure by maintenance personnel to detect the specific problem which led to the rudder jam has prompted ATR to establish a new fault-isolation task, intended to assist operators in the event that a pre-flight rudder check reveals problems such as stiffness in movement.
Troubleshooting instruction sequences will be based on the most probable root causes, including measures to check the condition of rear quadrant shaft support bearings. ATR plans to implement this change in January 2025.
