What happened
On 7 March 2023, an ATR 72-202, registration G-NPTF, was operating a cargo flight from East Midlands Airport to Belfast International Airport. During the pre-flight control check, the flight crew noted that the rudder felt unusually stiff. Despite this observation, the crew decided to proceed with the flight, noting that the stiffness was manageable and the expected crosswinds at the destination were minimal.
During the landing flare at Belfast, the co-pilot attempted to correct a slight leftward drift but discovered the rudder pedals were almost impossible to move. The commander immediately took control, utilizing the nosewheel steering to maintain directional control on the runway. After the aircraft arrived at its parking stand, the crew found that the rudder pedals had become completely immobile.
The investigation
An investigation by the AAIB revealed that the rudder was extremely difficult to move manually from both the cockpit and from outside the aircraft. Upon detailed examination of the aircraft's rear bay, investigators found significant moisture accumulation. The drain hole in the bay was ineffective, allowing water to pool and create a damp environment.
Technical analysis identified that the corrosion of the steel rudder rear quadrant support bearings was the primary cause of the control stiffness. The sealed design of these bearings prevented visual detection of their degradation. Additionally, investigators found that the aircraft's lubrication had been inconsistent; two different types of grease had been used alternately, and the lack of a proper purging process may have allowed the grease to become hygroscopic, attracting moisture.
Findings
- The primary cause of the extreme rudder stiffness was the degradation of the steel rudder rear quadrant support bearings due to corrosion.
- Moisture had accumulated in the aircraft's rear bay because the existing drain hole was not functioning effectively.
- A Service Bulletin recommending the replacement of these bearings with corrosion-resistant stainless steel versions had not been implemented on G-NPTF.
- Inconsistent lubrication practices, specifically the mixing of different grease products, likely contributed to the moisture-retention issues in the pivot points.
- The crew had identified the stiffness during the pre-flight check but elected to continue the flight based on the low expected crosswind.