An engine-related concern involving Boeing’s 737 Max has caught the attention of both the National Transportation Safety Board (NTSB) and a Federal Aviation Administration panel, which has reportedly recommended new pilot procedures and a change to software that manages the engine bleed air system.
The concern involves a “load reduction device” (LRD) in the 737 Max’s CFM International Leap-1B turbofans. Designed to activate in cases of severe engine damage, the LRD can result in toxic smoke filling the cockpit or cabin, according to sources.
Such circumstances occurred on two Southwest Airlines 737 Max 8s during flights last year, after the jets’ engines ingested large birds.
Pilots are now expressing concern about the LRD system, which seems to be at the root of the issue, and say they were unaware until recently of its existence.
“The NTSB has opened an investigation into the Southwest Airlines left-engine bird strike and subsequent smoke-in-cockpit event that occurred in a Boeing 737-8 near New Orleans, Louisiana” on 20 December 2023, the NTSB says on 22 November.
That jet’s Leap-1B ingested a female bald eagle weighing 4.6-6.4kg (10.2-14lb), according to a CFM document dated 21 February and viewed by FlightGlobal.
The other similar event occurred on 5 March 2023, when a Southwest 737 Max’s right-side engine ingested a turkey vulture shortly after take-off from Havana, sending smoke into that aircraft’s cabin. The pilots of both Southwest jets safely landed the aircraft.
The 737 Max’s left-side Leap-1B supplies air – also called bleed air – through an air conditioning system to the cockpit, while the right-side Leap sends air to the cabin.
In recent days, news outlets reported that the FAA’s Office of Accident Investigation & Prevention (AVP), in response to those incidents, issued a memo on 28 October calling on 737 Max pilots to adopt new procedures to address the concern. It also reportedly recommended that the LRD system be redesigned.
The FAA did not respond to a request for comment.
It is unclear if the FAA is considering requiring a redesign.
The LRD system is composed of “mechanically weakened” structural components that fail by design if an engine’s fan becomes significantly imbalanced, which can occur due to lost fan blades or other significant damage, which can be caused by ingesting large birds. In such circumstances, the LRD “decouples” the fan from its supports.
LRDs allow engines to be lighter; without them, turbofans would need heavier fan frames, pylons and other components to support an unbalanced fan.
The Southwest incidents revealed that LRD activation led oil from the Leaps’ sumps to contact hot engine parts, creating thick and toxic smoke that the air systems pumped into one jet’s cockpit, and the other’s cabin.
“These engine failures occurred during a critical phase of flight and were further compounded by the immediate presence of smoke and fumes… entering either the flightdeck or the cabin through the air conditioning system,” 737 Max operator American Airlines told pilots in a 14 November memo viewed by FlightGlobal.
Dennis Tajer with the Allied Pilots Association, which represents pilots at American Airlines, says the pilots of the December 2023 Southwest flight, having donned oxygen masks, “had about 10 inches of visibility in the cockpit to land the plane. It was that bad”.
Tajer calls LRDs “an important safety feature on the engine that we recently learned about”.
Asked to comment, CFM says, “Load reduction devices have been widely used across the industry for more than 20 years and operated as designed. We are collaborating with FAA, EASA and Boeing to determine if there are any learnings from recent events”. The engine maker, co-owned by GE Aerospace and Safran Aircraft Engines, calls LRDs “proven” technologies “designed to prevent a hazard to the airplane”. It says Leaps meet FAA and EASA bird-ingestion requirements.
Boeing says it is “working with the authorities that are investigating these incidents. We continue to follow regulatory processes to properly address potential issues and ensure the continued safety of the global fleet”.
The US airframer also notes that in February it issued an “Operations Manual Bulletin” informing 737 Max customers about “potential impacts to the engine, and appropriate operating procedures”.
According to a source familiar with the matter, the 28 October memo from the FAA’s AVP office says release of oil is an expected consequence of LRD activation. Leap-1Bs do have valves that close automatically in such circumstances, stopping air flow. But the valves take close to a minute to shut – too long to prevent the smoke.
To address the risk, the AVP’s memo recommends an engineering fix that would involve making the automatic bleed air valve close faster, the source notes.
In the meantime, the AVP urges 737 Max pilots be instructed to take-off with the left Leap-1B’s bleed air system shut off (which would prevent cockpit smoke), or with both engines’ bleed air disabled. Pilots are familiar with “engine bleed-off take-offs”, which they sometimes perform to get better engine performance when taking off from high-altitude airports such as Mexico City.
Southwest and American in February notified pilots about the engine-failure-smoke risk. The bulletins told pilots to consider an event involving an engine failure and smoke in the cockpit to be an instance of “severe engine damage” – which would necessitate they complete a checklist calling for the affected engine to be shut down.
Tajer finds that solution concerning, noting that smoke caused by an LRD activation will permeate the cockpit before pilots can shut off the engine. He adds that pilots should not be rushed when shutting down a powerplant, as past crashes have resulted from the wrong engine being disabled.
“We are trained to take our time… By the time I get to the fire handle, we are learning, the oil has already done its smoke ingestion onto the airplane, and there is probably nothing left,” he says.
Story updated on 23 November in the first paragraph to note that the recommended FAA changes relate to the engine bleed air system.
