Date: 7th March 2023
Location: Enroute to Belfast International Airport, Northern Ireland
Aircraft: ATR-72-200, registration G-NPTF
Operator: West Atlantic
Flight Number: NPT-07B
Route: East Midlands, England, to Belfast International, Northern Ireland
Passengers and Crew: 2 crew (cargo flight)
Injuries: None
Summary of Events
On 7th March 2023, a West Atlantic ATR-72-200 cargo aircraft encountered restricted rudder travel during flight. The issue arose as the aircraft was enroute to Belfast International Airport. Despite the restriction, the crew safely landed the aircraft on Runway 25. Post-landing, the rudder restriction persisted, preventing its movement during taxi operations.
The UK Air Accidents Investigation Branch (AAIB) classified the event as a serious incident, launching an extensive investigation. Subsequent examinations revealed significant corrosion in the rudder’s rear quadrant shaft support bearings, a condition exacerbated by trapped moisture in the rear bay of the aircraft.
Aircraft and Maintenance History
Aircraft
The ATR-72-200 is a twin-engine turboprop aircraft widely used for short-haul passenger and cargo operations. The rudder system, integral to the aircraft’s directional control, includes components such as the rudder damper, rear quadrant shaft, and support bearings, all of which are critical for its functionality.
Maintenance History
• Previous Issues: In January 2023, the aircraft had been grounded due to electrical issues.
• Rudder Stiffness Reports: Over the month preceding the incident, three reports of rudder stiffness had been documented by different flight crews. Despite extensive maintenance interventions, including the replacement of multiple components, the root cause remained undiagnosed.
• Service Bulletin (SB) Oversight: ATR issued a Service Bulletin (SB 72-27-1020) in 1993 recommending corrosion-resistant bearings for the rudder system, but it had not been implemented on this aircraft.
Sequence of Events
1. Pre-Flight Observations:
• During the pre-flight control check, the crew noted significant stiffness in the rudder system but decided to proceed with the flight, relying on recent maintenance assurances.
2. In-Flight Complications:
• Rudder stiffness became critical during the landing flare, rendering the rudder pedals nearly immovable. The crew relied on nosewheel steering to maintain directional control after touchdown.
3. Post-Landing Findings:
• Upon inspection, maintenance personnel confirmed that the rudder was almost immovable, both when operated via the cockpit pedals and manually.
Investigation Findings
Primary Cause
• Corrosion of Rudder Rear Quadrant Shaft Bearings:
The investigation identified severe corrosion in the two support bearings on the rudder’s rear quadrant shaft. The No. 2 bearing was completely seized, while the No. 1 bearing exhibited significant wear and corrosion.
Contributing Factors
1. Moisture Ingress:
• Moisture accumulation in the rear bay was identified as a key contributor to the corrosion. Sources included:
• Degraded sealant on the vertical stabiliser.
• Residual de-icing fluid mixed with water.
• Inadequate drainage due to a missing drain hole in the rear bay door frame.
2. Over-Torqued Components:
• An over-torqued rudder damper attachment bolt contributed to additional stiffness in the rudder system.
3. Missed Maintenance Opportunities:
• Routine inspections failed to identify the deteriorating condition of the bearings. Visual checks did not include disassembly or manual rotation of the rear quadrant shaft, which might have revealed the problem earlier.
4. Operational Decision:
• The crew’s decision to proceed with the flight despite detecting rudder stiffness during pre-flight checks eliminated an opportunity to address the issue on the ground.
Safety Risks and Potential Consequences
1. Loss of Directional Control:
• Complete rudder immobility during critical phases, such as landing or go-around, increases the risk of runway excursions or loss of control.
2. Systemic Oversight:
• Failure to implement the 1993 Service Bulletin contributed to the persistence of legacy components prone to corrosion.
3. Operational Challenges:
• Flight crews faced increased workload and safety risks due to the unexpected failure during landing.
Recommendations
1. Mandatory Implementation of Service Bulletins:
• Airlines operating older ATR models should prioritise compliance with SB 72-27-1020 to replace steel bearings with corrosion-resistant alternatives.
2. Enhanced Maintenance Protocols:
• Visual inspections should include manual rotation of critical components, such as the rudder rear quadrant shaft, to detect latent issues.
• Rear bay drainage systems should be reviewed and repaired to prevent moisture accumulation.
3. Improved Crew Training:
• Flight crews should be trained to prioritise safety over operational continuity when abnormalities are detected during pre-flight checks.
4. Regulatory Oversight:
• Aviation authorities should monitor compliance with manufacturer-recommended Service Bulletins and enforce periodic audits of maintenance practices.
5. Moisture Mitigation Measures:
• Operators should enhance sealing and drainage systems in aircraft areas prone to moisture ingress.
Safety Lessons
This incident underscores the importance of proactive maintenance and adherence to manufacturer recommendations to prevent component degradation. It also highlights the critical role of pre-flight control checks in identifying and addressing potential safety risks before takeoff.
Disclaimer
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Our investigation reports are based on all the evidence and facts we have at the time of writing and posting. We apologise if any details are missed or are not fully accurate.
