US safety investigators have warned over a safety issue with a vibration-reduction feature on the CFM International Leap-1B turbofans that power the Boeing 737 Max.
The issue can cause smoke to enter the cabin or cockpit.
The US National Transportation Safety Board (NTSB) is urging regulators, Boeing and CFM to take “urgent” steps to address the risk, which involves load reduction devices (LRDs) in the engine, an 18 June report states.
LRDs activate in cases of major engine problems – such as fan blade loss – but can also cause smoke to fill 737 Max cockpits or cabins.
The NTSB’s report calls on the Federal Aviation Administration and European Aviation Safety Agency to require airlines incorporate Leap-1B software modifications already being developed by Boeing and CFM. That update is intended to “prevent or limit the amount of smoke released into the cockpit or cabin after load-reduction device activation”.
The NTSB also asks the FAA, EASA and China’s aviation regulator to determine if similar risks are posed by LRDs on the Leap-1A and -1C variants, which power the Airbus A320neo family and Comac C919, respectively.
The proposals create another hiccup for Boeing and its 737 Max.
“These recommendations address the potential for smoke to enter the cockpit or cabin after activation of the load reduction device resulting from an engine blade failure or imbalance on CFM Leap-1B engines, and pilot awareness about what to do if this occurs,” the NTSB report says.
Asked to comment, the FAA says it “agrees with the NTSB recommendations”.
“We alerted operators that smoke could enter the flight deck following the activation of the load reduction device in the engines, as a result of a bird strike, and Boeing did as well,” the regulator says. “We advised operators to evaluate their procedures and crew training to ensure they address this potential issue. When the engine manufacturer develops a permanent mitigation, we will require operators to implement it within an appropriate timeframe.”
It previously evaluated the issue but determined last year that the concern did “not warrant immediate action”. The FAA on 18 June reiterated its previous decision to address the issue following “its standard rulemaking process”.
LRDs are designed to decouple the Leap’s fan from its supports to prevent further damage if the fan becomes significantly imbalanced. And LRDs allow engines to be lighter; without them, turbofans would need beefier components capable of supporting unbalanced fans.
The NTSB’s recommendations respond to a 20 December 2023 incident involving a Southwest Airlines 737 Max 8. That jet’s port-side Leap-1B ingested a bald eagle during take-off from New Orleans and “began to shake violently”, activating the LRD.
LRD activation allows oil from the Leap-1B’s sumps to contact hot engine parts, creating smoke. The smoke can be pumped by the 737 Max’s bleed air system into either the cockpit or cabin, depending on which of the two engines is affected. The left-hand engine’s bleed air system feeds the cockpit and the right-side’s feeds the cabin.
The Southwest jet’s pilots reported “acrid white smoke” in the cockpit and the captain said the instruments were difficult to see. The smoke quickly dissipated after the first officer pulled the engine fire switch.
A similar incident involved a Southwest 737 Max departing Havana on 5 March 2023 when the aircraft’s right-hand Leap-1B ingested a turkey vulture, also activating the LRD and causing smoke to fill the cabin.
In both cases, the flight crew landed the aircraft safely.
The NTSB’s report also urges the FAA to ensure airlines inform pilots about changes Boeing made to the 737 Max’s pilot manual and handbook in response to the issue.
Asked to comment, Boeing says it and CFM “have been working on a software design update” and that it supports the NTSB’s recommendations.
Boeing in late 2024 informed operators that it updated the pilot handbook and manual to include “enhanced safety descriptions and checklists to help flight crews more-quickly take appropriate actions”, Boeing adds.
CFM is “aligned with the NTSB’s recommendations and the work is already underway in close partnership with our airframers,” the engine maker says.
American Airlines and Southwest Airlines in February 2024 issued guidance to pilots specifying how to respond to LRD activations. Steps include shutting down the affected engine.
The Allied Pilots Association, which represents American’s pilots, has criticised the guidance, saying pilots must act quickly to avoid a smoke emergency and that rushing can lead to errors.
In deciding not to take action last year, the FAA overrode recommendations made in October 2024 by an FAA safety panel. That panel proposed that 737 Max pilots be instructed to take-off with the left Leap-1B’s bleed air system shut off – which would prevent smoke from entering the cockpit should that engine’s LRD activate.
Story updated on 19 June to include comments from the FAA.
