NTSB Urges LEAP-1B Engine Fixes After Smoke Incidents

Smoke Risk in LEAP-1B Engines: NTSB Issues Urgent Safety Recommendations

On June 18, 2025, the National Transportation Safety Board (NTSB) issued an urgent safety recommendation concerning CFM International LEAP-1B engines. These engines, a cornerstone of modern commercial aviation, are used on Boeing 737 MAX aircraft and have logged over 1.5 million flight hours. The recommendation follows two serious incidents in which smoke entered the cockpit and cabin after bird strikes triggered a safety mechanism known as the Load Reduction Device (LRD).

The NTSB’s findings point to a potentially systemic issue in the design of the LRD, which, though intended to reduce engine damage during emergencies, can inadvertently allow hot oil to leak into the aircraft’s ventilation system. This has raised red flags across the aviation industry, prompting calls for immediate action from aircraft manufacturers, engine developers, and international aviation regulators.

Given the widespread use of LEAP engines across Airbus A320neo, Boeing 737 MAX, and COMAC C919 aircraft, the implications of this safety concern are far-reaching. This article delves into the technical background, incident analysis, and the broader impact of the NTSB’s recommendations on aviation safety and operations.

Understanding the Load Reduction Device and Its Risks

The Intended Function of the Load Reduction Device (LRD)

The Load Reduction Device (LRD) is a mechanical safety feature built into the LEAP-1B engine. It is designed to activate automatically during severe engine imbalances, such as those caused by fan blade failures or bird strikes. When triggered, the LRD decouples the fan from the engine core to minimize vibrations that could otherwise damage the airframe or engine mounts.

This system is a product of lessons learned from over a billion flight hours on the CFM56 engine family. Its primary goal is to enhance safety without requiring pilot intervention, especially during critical phases of flight like takeoff and landing. However, despite its safety-driven design, the LRD has introduced a new and unanticipated hazard.

Investigations revealed that LRD activation can dislodge oil-supply tubes or fracture engine sump flanges. This allows engine oil to enter areas of high temperature, where it vaporizes and is then carried into the aircraft’s ventilation system via bleed air ports, resulting in smoke in the cockpit and cabin.

“What was once considered a fail-safe mechanism now presents a potentially serious hazard under specific but foreseeable conditions,”, NTSB report, June 2025.

Case Studies: Southwest Airlines Incidents

Two Southwest Airlines flights in 2023 serve as case studies for the LRD-related smoke hazard. In December, Flight 554 departed from New Orleans when a bird strike led to LRD activation. Within seconds, thick white smoke filled the cockpit, impairing the pilot’s visibility. The crew managed to return safely, but the NTSB noted that the 10–15 second delay in manually shutting off the engine bleed valve was a critical vulnerability.

Earlier that year, in March, a similar incident occurred on Flight 392 departing from Havana. A bird strike on the right engine led to LRD activation and subsequent vapor fog entering the passenger cabin. Passengers reported a chemical-like odor and visible haze. The crew declared an emergency and returned without injuries, but the incident reinforced concerns about the LRD’s unintended consequences.

Both incidents highlighted that while the LRD effectively mitigated engine damage, it introduced a new risk by enabling smoke to enter occupied areas of the aircraft. The NTSB emphasized that bird strikes are not rare, occurring roughly every 2,000 flights globally, and must be accounted for in engine safety systems.

Technical Analysis and Design Implications

The NTSB’s technical analysis found that oil leakage into the high-pressure compressor, where temperatures can exceed 500°F, results in rapid vaporization. This vapor is then distributed through the aircraft’s ventilation system, which draws bleed air from the engine. The system’s reliance on manual intervention to shut off the bleed valve poses a safety concern, particularly during high workload periods like takeoff.

CFM and Boeing have proposed a two-phase solution. The short-term fix involves a software update that automatically closes the bleed valve upon LRD activation. The long-term solution includes redesigning the oil-supply tube anchorage and sump sealing mechanisms to prevent displacement during LRD events.

These solutions aim to eliminate the delay in pilot response and reduce the likelihood of smoke entering the aircraft. However, implementation will require regulatory approval, certification, and fleet-wide retrofitting, which could be both time-consuming and costly.

Industry Response and Broader Implications

Regulatory and Manufacturer Actions

Following the NTSB’s urgent recommendation, the Federal Aviation Administration (FAA), European Union Aviation Safety Agency (EASA), and the Civil Aviation Administration of China (CAAC) have been asked to evaluate the risk across all LEAP engine variants, including the LEAP-1A and LEAP-1C used on Airbus and COMAC aircraft, respectively.

Boeing has revised its Flight Crew Operations Manual (FCOM) and Quick Reference Handbook (QRH) to include new procedures for managing LRD-related smoke events. These revisions instruct pilots to immediately close the engine bleed valve and initiate emergency descent protocols if necessary.

Training simulators are being updated to include LRD-specific scenarios, and operators are being urged to brief flight crews on the new procedures. The NTSB stressed that awareness and preparedness are crucial, especially given that many pilots were previously unaware of the LRD’s potential to cause smoke ingress.

Economic and Operational Impact

Retrofitting the existing fleet of over 1,200 Boeing 737 MAX aircraft with the proposed software update is estimated to cost $150,000 per engine, amounting to approximately $360 million. Additional downtime during modifications could cost airlines an estimated $1.2 million per day in lost utilization.

Despite these costs, industry analysts suggest that proactive compliance with the NTSB’s recommendations may help mitigate reputational damage for both Boeing and CFM. The joint venture between GE Aerospace and Safran Aircraft Engines has committed to implementing the changes and supporting operators throughout the transition.

Beyond financial implications, the issue underscores the importance of comprehensive risk modeling in engine design. As aircraft systems become more complex, ensuring that safety features do not introduce new hazards is a critical challenge for manufacturers and regulators alike.

Future Directions and Safety Innovations

The LEAP engine’s LRD issue offers valuable lessons for future engine development. One potential area of innovation is the integration of real-time oil leak detection systems, which could provide early warnings before smoke enters the aircraft.

The incident also highlights the need for harmonized global regulations. As the aviation industry becomes increasingly interconnected, ensuring consistent safety standards across regions is essential. The International Civil Aviation Organization (ICAO) may play a key role in standardizing LRD-related procedures.

Looking ahead, resolving the LRD issue could set a new benchmark for fail-safe design in next-generation propulsion systems, including those powered by sustainable aviation fuels or hydrogen. The aviation industry must balance innovation with rigorous safety validation to maintain public trust and operational reliability.

Conclusion: A Turning Point in Engine Safety

The NTSB’s urgent recommendation concerning LEAP-1B engines marks a pivotal moment in aviation safety. While the LRD was designed to protect aircraft from structural damage, its unintended consequence, smoke ingress, revealed a critical vulnerability. The incidents involving Southwest Airlines flights underscore the importance of continuous monitoring, evaluation, and adaptation in aerospace engineering.

As regulatory agencies, manufacturers, and operators work together to address the issue, the aviation industry is reminded that even the most well-intentioned safety features require thorough testing under all plausible scenarios. The resolution of the LRD flaw will not only restore confidence in the LEAP engine family but also inform the design of future propulsion systems.

FAQ

What is the Load Reduction Device (LRD)?
The LRD is a mechanical safety feature in LEAP engines that decouples the fan from the engine core during severe imbalances to reduce vibration and prevent structural damage.

Why is smoke entering the cockpit and cabin?
When the LRD activates, it can dislodge oil-supply tubes, allowing oil to enter hot engine areas and vaporize. This vapor can then be circulated into the cockpit and cabin via the bleed air system.

What actions are being taken to resolve the issue?
Boeing and CFM are developing a software update to automatically close bleed valves upon LRD activation and are redesigning certain engine components. Regulatory agencies are evaluating similar risks in other LEAP engine variants.

Sources: NTSB Press Release, NTSB Investigation Report, NTSB Docket DCA24LA330