Source: Air Accidents Investigation Branch
Report: AAIB Investigation to Leonardo AW139 G-CIMU
Publication date: 21 May 2026
Official URL: AAIB report – Leonardo AW139 G-CIMU
Executive Summary
The UK AAIB published an investigation on 21 May 2026 concerning a serious degradation discovered in a tail rotor duplex bearing on a Leonardo AW139 operated as G-CIMU.
According to the report, the bearing deterioration had reached a condition where continued operation could potentially have resulted in a loss of tail rotor control and therefore loss of helicopter yaw authority.
Although investigators could not determine a definitive root cause, the event highlights a critical latent failure mode affecting the tail rotor pitch control system.
Tail Rotor Pitch Control: Why the Bearing Matters
The AW139 tail rotor controls:
- anti-torque compensation,
- yaw control,
- directional stability during hover and low-speed flight.
Tail rotor thrust depends on the pitch angle of the tail rotor blades. Pilot pedal inputs are transmitted through a mechanical control chain including:
- actuator shafts,
- control linkages,
- and the duplex bearing involved in the investigation.
If the duplex bearing degrades:
- degradation of the duplex bearing may lead to misalignment within the tail rotor pitch control transmission mechanism,
- pitch control precision may be lost,
- and the tail rotor may no longer generate the commanded thrust.
This creates a potentially catastrophic loss-of-control scenario, particularly during hover or low-speed operations where tail rotor authority is critical.
AAIB Engineering Findings
The investigation identified:
- severe internal degradation of the duplex bearing,
- evidence of progressive wear over time,
- and secondary mechanical damage within the control assembly.
The AAIB concluded that:
- continued operation could likely have resulted in loss of tail rotor effectiveness,
- while the degradation mechanism itself may remain difficult to detect through conventional inspections alone.
The report therefore classifies the issue as a high-severity, low-detectability latent failure mode.
AAIB Recommendations
The AAIB recommended:
- broader analysis of removed bearings across the fleet,
- improved HUMS (Health and Usage Monitoring System) trend analysis,
- enhanced vibration monitoring capability,
- and earlier detection of tail rotor control anomalies.
The objective is to strengthen condition-based maintenance and predictive monitoring of critical rotating components.
AAIB vs EASA: Different Responsibilities
An important distinction is that the Air Accidents Investigation Branch investigates occurrences and issues safety recommendations, but does not impose operational maintenance requirements.
Actions such as:
- mandatory inspection intervals,
- component life limits,
- Airworthiness Directives (ADs),
- or temporary operational limitations or mandatory inspection requirements affecting part of the fleet,
would normally be issued by:
- European Union Aviation Safety Agency (EASA)
- and/or Leonardo Helicopters, the OEM (Original Equipment Manufacturer).
What the Report Does — and Does Not — Specify
The AAIB report confirms:
- the existence of a serious tail rotor duplex bearing degradation risk,
- and the need for increased condition monitoring.
However, as of the publication date:
- no revised inspection interval,
- no grounding directive,
- and no mandatory immobilization period
had yet been publicly released by EASA or Leonardo.
This means the report identifies the hazard, but the operational airworthiness response remains the responsibility of regulators and the OEM.
Potential Fleet Impact
The AW139 is widely used worldwide for:
- offshore transport,
- EMS/HEMS (Emergency Medical Services / Helicopter Emergency Medical Services),
- SAR (Search and Rescue),
- government and parapublic missions.
The potentially affected fleet is therefore estimated to include several hundred to more than one thousand helicopters worldwide.
Possible operational consequences may include:
- fleet inspection campaigns,
- expanded HUMS surveillance,
- more frequent bearing inspections,
- precautionary component removals,
- and potential reduction of bearing life limits.
At this stage, however no official inspection interval (25 FH, 50 FH, 100 FH, etc.) has been formally published in the AAIB material.
Operational Availability Impact
Depending on future EASA or Leonardo actions, operators could face:
| Potential Action | Estimated Downtime |
| Initial fleet inspection | Several hours to 1 day |
| Detailed bearing inspection | 1–2 maintenance shifts |
| Preventive bearing replacement | 1–5 days |
The most likely immediate impact is increased engineering workload and reinforced HUMS monitoring rather than large-scale grounding.
Conclusion
The AAIB investigation into AW139 G-CIMU highlights a potentially critical degradation mechanism affecting the tail rotor pitch control system through failure of a duplex bearing.
While the report does not itself impose mandatory maintenance actions, it clearly identifies a major airworthiness concern and reinforces the growing importance of:
- HUMS analytics,
- vibration monitoring,
- predictive maintenance,
- and condition-based inspection strategies in modern helicopter fleets.
Further operational and maintenance measures are expected to be defined by EASA and national airworthiness authorities.
Source: Air Accidents Investigation Branch
