Disclaimer:
This article is for educational purposes only. Airlines and flight crews should follow official guidance from competent authorities (FAA, EASA, etc.) and the aircraft/engine manufacturers regarding safety procedures and procedures. The information contained here in does not constitute official advice, instructions or recommendation of any form or type.
Introduction
The Boeing 737 MAX and Airbus A320neo families are powered by CFM International’s LEAP engines (LEAP-1B on the MAX and LEAP-1A on the NEO). These engines include a safety Load Reduction Device (LRD) – essentially a mechanical fuse (shear-pin assembly) in the fan section – that automatically activates under extreme fan unbalance (such as a bird strike or blade failure) to protect the engine structure . When an LRD “fuses,” it severs the load path between the fan and the shaft. However, activation also creates a path for engine oil to leak from the sump and supply lines into the engine core airflow. The leaked oil is then ingested into the engine compressor and contaminates the bleed air used by the Environmental Control System (ECS) . In practice, this can result in dense smoke or fumes entering the cabin and/or cockpit, posing a serious in-flight hazard. Both the NTSB and EASA have identified recent cases where bird strikes on 737 MAX LEAP-1B engines triggered the LRD and filled the aircraft with smoke.
Recent smoke events on 737 MAX
In 2023 the industry saw two serious 737 MAX events. On 5 March 2023, a Southwest Airlines 737-8 (MAX 8) took off from Havana and suffered a right-engine bird strike shortly after departure. The crew declared an emergency, shut down the engine, and returned safely, but passengers and crew reported very heavy smoke in the cabin. One passenger was hurt during evacuation. On 20 December 2023, another Southwest 737-8 departing New Orleans suffered a left-engine bird strike and subsequent in-flight shutdown. The flight deck was filled with “acrid white smoke” so thick the captain could barely see instruments. Both crews donned oxygen masks and returned safely; no serious injuries occurred . The NTSB notes these were triggered by the LEAP-1B’s LRD, which caused oil to leak into the engine core and bleed air. These incidents are under investigation (NTSB DCA24LA330, AIR-25-03), and NTSB has already issued urgent safety recommendations to address them.
Airbus A320neo aircraft (LEAP-1A) have not yet experienced an in-service LRD-activation smoke event. Nonetheless, regulators and manufacturers are monitoring the issue. The NTSB has formally urged that CFM LEAP-1A and LEAP-1C (the C919 engine) be evaluated for the same LRD smoke risk . In practice, A320neo crews can mitigate smoke similarly by following Airbus QRH procedures; EASA notes that “A320neo-family pilots can secure the affected engine and mitigate smoke contamination by following quick-reference procedures for engine failure and smoke”
Smoke hazards and dynamics
EASA has published a Safety Information Bulletin (SIB 2025-06) after these events, describing the hazard: when an LRD activates, “oil leaks into the core compressor upstream of the aircraft bleed ports, contaminating the bleed air delivered to the Environmental Control System (ECS)” . This oil-filled airflow can generate dense smoke or fumes that spread into the cabin and flight deck. EASA highlights two main hazards:
- smoke/fumes in the flight deck can reduce visibility and impair crew performance in a high-workload situation, even risking temporary incapacitation ; and
- smoke in the cabin or cockpit can cause injury or distress to passengers/crew . In short, even a contained engine failure becomes more dangerous if toxic oil smoke fills the interior.
Air distribution differs by aircraft. In a 737 MAX’s normal bleed configuration, the cockpit normally sources bleed from the left engine and the cabin uses bleed from both engines. Thus an LRD event on the left engine tends to pump smoke into both cockpit and cabin, whereas a right-engine event may primarily smoke the cabin (the flight deck might be spared short-term) . Airbus A320s have a different ECS layout, but the principle is the same: contamination will enter wherever the affected engine supplies bleed. The duration of the smoke depends on how quickly the bleed is isolated. If the damaged engine auto-shuts or is shut down, the engine’s bleed valve will close and stop the flow. However, if the engine continues at high core speed, the bleed remains open. EASA stresses that in that case the crew must promptly act to isolate the bleed from the affected engine (by shutting down the engine or manually closing the bleed). Otherwise smoke will continue flooding the cabin.
Crew Procedures and Mitigations
Both Boeing and Airbus have procedures to deal with catastrophic engine damage and smoke. Boeing’s 737 MAX QRH includes an “Engine Fire/Severe Damage or Separation” checklist and a “Smoke, Fire or Fumes” checklist. EASA reports that applying these QRH actions will allow 737 MAX crews to isolate the engine and mitigate smoke . In practice, pilots treat LRD-triggered smoke like an engine fire: pull the affected engine’s fire handle (closing bleed valves and fuel) and shut down the engine. Boeing has even issued a Flight Crew Operations Manual Bulletin directing crews to run the engine fire checklist first when engine trouble and smoke occur simultaneously . The QRH “Smoke, Fire or Fumes” procedure on the MAX then vents any remaining smoke and prepares oxygen masks as needed.
Airbus A320neo QRH/ECAM procedures cover engine failure and smoke similarly. The QRH calls for “Engine (X) Failure” actions and uses the SMOKE/FUMES/AVNCS SMOKE procedures (with or without ECAM prompts) to isolate the engine and vents . In practice, A320 pilots will follow the engine shutdown steps (including pulling the fire handle for the affected engine) and then run the smoke procedure, which typically closes bleed valves, switches packs, and turns on recirculation fans or ram air to clear smoke. EASA explicitly confirms that following the quick-reference handbook procedures on an A320neo “can secure the affected engine and mitigate the smoke contamination” after LRD activation.
Immediate best practices: In any suspected LRD smoke event, crews should don oxygen masks immediately (crew members), coordinate with cabin crew, and complete the QRH checklists. Key steps include: shutting down the affected engine, turning off that engine’s bleed air (fire handle pull), and switching the ECS to use fresh air (ram air or remaining engine bleed). Oxygen masks and smoke goggles (flight attendants for cabin) are essential for crew performance. NTSB notes that in one incident the smoke cleared rapidly only after the first officer pulled the fire handle as directed . Pilots should also consider an expedited descent to lower altitude (and emergency landing) if smoke persists. Importantly, once the engine is secured or bleed isolated, the smoke stops. The toxic cloud will dissipate, as seen in the New Orleans case where smoke rapidly cleared after isolation.
Regulatory Actions and Recommendations
Responding to these cases, both U.S. and European regulators have issued advisories. The NTSB’s June 2025 investigation report (DCA24LA330) made urgent safety recommendations. Key points: NTSB found that while pilot action can mitigate smoke, a software modification (in development by CFM/Boeing) would accelerate bleed shutoff, “minimizing the quantity of smoke and reducing the crew’s workload.” It urged the FAA to inform operators and crews of the hazard and the new procedures. Specifically, Safety Recommendation A-25-10 (Urgent) tells FAA to ensure all pilots flying LEAP-1B equipped aircraft are aware of the potential for smoke after LRD activation, and of Boeing’s updated QRH/FCOM procedures . It also recommended that FAA/EASA require a software update (once certified) on all LEAP-1B engines to prevent or limit smoke . Additionally, the NTSB formally asked U.S., European, and Chinese regulators to check LEAP-1A and LEAP-1C for the same vulnerability and mandate fixes if needed.
The NTSB press release also highlighted that Boeing has already issued a bulletin alerting operators that LRD activation can put smoke in the cockpit . Boeing told the NTSB that it and CFM are working on a software design update. On June 18, 2025, the NTSB went public with an urgent recommendation and released its 11-page interim report AIR-25-03 . Around the same time, U.S. news reports noted the FAA agreed to require any permanent mitigation once available, and encouraged airlines to reinforce training and procedures now . In short, regulators are treating this as a safety issue requiring crew awareness, training, and eventually technical fixes.
The European Union Aviation Safety Agency’s SIB (July 2025) takes a similar approach on the A320neo/737 MAX fleets. Although it is “information only,” it recommends that airlines identify which aircraft have the LRD-equipped LEAP engines, brief crews, and incorporate the scenario into training. EASA explicitly advises operators:
- Identify LRD-equipped LEAP engines in the fleet and brief flight crews accordingly.
- In a severe engine damage or vibration event, crews should anticipate possible LRD activation and dense smoke.
- Emphasize this scenario in conversion and recurrent training so crews understand the challenge and proper mitigation.
These align with NTSB’s urging that pilots be “fully aware of the potential for smoke in the cockpit if the load reduction device is activated” during critical flight phases . Both Boeing and the airlines are now reiterating procedures about shutting off engine bleed immediately when an engine is severely damaged. Notably, Boeing moved shutting off bleed (via fire handle) higher up in the checklist sequence, reflecting lessons learned.
Best Practices for Operators and Crews
To summarize, operators and pilots should treat an LRD-triggered engine failure as a combined fire/ smoke emergency. Best practices include:
- Fleet Awareness: Keep track of which aircraft have LRD-equipped LEAP engines. Make sure dispatchers and crews know which plane has this capability.
- Crew Briefing: Include LRD smoke events in pre-flight briefings and bulletins. Emphasize that certain bird-strike or failure events can cause smoke in the cabin/cockpit even after an engine is shut down.
Training & Checklists: Incorporate the LRD scenario into simulator training. Pilots should practice immediately pulling the engine fire handle and shutting down the affected engine if severe vibration or engine failure occurs with smoke. Follow the QRH/ECAM Engine Fire/Severe.
Damage and Smoke/Fumes procedures promptly. Ensure crews understand that bleed isolation (fire handle) may be needed sooner than in older procedures.
- Use of Masks: Don oxygen masks at the first sign of toxic smoke in the cockpit. Cabin crew should don smoke goggles and follow their smoke/fume protocols.
- System Management: Turn off the damaged engine’s bleed air immediately (via engine fire handle) so the ECS stops pumping smoke into the cabin. Switch packs/ventilation to use fresh/ram air if possible.
- Communication: Inform ATC and prepare for an emergency landing as soon as practicable; squawk 7700 and declare emergency. Descend if needed for safer cabin environment.
- Documentation: After any such event, thoroughly document and report it to regulators (FAA/EASA/NTSB) so they can include it in safety analyses.
Adherence to official guidance is crucial. The NTSB and EASA remind us these recommendations are interim measures – final technical fixes (like the CFM/Boeing software mod) will be mandated once validated . Until then, human factors (awareness and response) are the primary defense.
Conclusion
LRD activation in modern LEAP turbofan engines is a rare but serious condition that can produce dense smoke in an aircraft cabin or cockpit. The Boeing 737 MAX cases in 2023 highlighted how a once obscure
safety device (the Load Reduction Device) can inadvertently compromise bleed-air quality. Aviation authorities and manufacturers are responding with crew advisories, checklist updates, and engineering solutions.
For airlines and crews, the key is preparedness: knowing which engines have LRDs, recognizing the symptom (sudden engine damage plus smoke), and executing the emergency checklists without hesitation. Following the updated QRH/ECAM procedures – engine shutdown, bleed isolation, and smoke evacuation – has been shown to stop the contamination promptly. In all cases, safety of flight and occupants comes first: if smoke appears after an engine failure, handle it with the highest priority procedures and land as soon as practicable.
The CFM LEAP engine LRD issue underscores the importance of continuous safety evaluation in aviation. While the technical solution timeline extends to 2026, immediate procedural and training enhancements provide interim risk mitigation. This case study reinforces that aviation safety results from the collective vigilance of manufacturers, operators, regulators, and flight crews working together to identify and address emerging challenges.
As the industry awaits the software modifications, enhanced pilot awareness and proper emergency procedure execution remain the primary defenses against this newly understood risk. The collaborative international response to this issue exemplifies aviation safety culture at its finest, ensuring that lessons learned benefit the global aviation community.
References:
