The National Transportation Safety Board (NTSB) has announced that CFM International, in collaboration with Boeing, will upgrade the software of the 737 MAX engine, the LEAP-1B, following two bird strike incidents that resulted in a large amount of smoke entering the cockpit.
Cockpit filled with acrid white smoke
In a preliminary report provided by the NTSB, investigators described one of the two bird hit incidents. On December 20, 2023, a Southwest Airlines flight from Louis Armstrong New Orleans International Airport (MSY) was struck by a bird shortly after departure.
The airline’s 737 MAX 8, registered as N8830Q, was flying flight WN554 when the left-hand engine sustained a bird strike at roughly 1,000 feet (304.8 meters), causing vibration and loss of thrust on the number one engine, a CFM International LEAP-1B.
The NTSB reported that shortly after the bird strike, the left engine master caution fire warning light and engine first switch illuminated, and the fire bell rang in the cockpit.
In response, the captain, who was pilot flying (PF), requested the ‘Engine Fire or Engine Severe Damage checklist’ from the quick reference guide.
As the first officer, acting as pilot monitoring (PM), began going through the checklist, the cockpit filled with “acrid white smoke,” according to the NTSB. In response, the PM yelled out “masks,” and both pilots put on their masks, with the first officer returning to the QRH.
The pilots declared an emergency to Air Traffic Control (ATC) and requested that fire trucks be dispatched as soon as they returned to New Orleans.
The captain told the investigators that vision was limited and that he couldn’t see anything beyond the first officer, and that the smoke made it difficult to see his instrument panel.
“The captain further stated that, after the engine fire switch had been pulled, the smoke began to rapidly dissipate.”
The 737 MAX 8 eventually landed in New Orleans, pausing on the runway. Following an examination by the airport’s rescue and firefighting (ARFF) personnel, the aircraft taxied to the gate, where the 139 737 MAX 8 passengers successfully deplaned without injury.
The other bird strike, which resulted in smoke filling the 737 MAX interior, occurred in March in Cuba. The right engine absorbed the birds, forcing the airplane to return to Havana.
The NTSB stated that, while Southwest Airlines was not required to disclose either incident, after the Federal Aviation Administration (FAA) began investigating the two bird strikes and the resulting serious smoke episodes, it launched its own inquiry into the two flights.
Engine oil entering the cabin’s air supply
The NTSB stated that the LEAP-1B engines are equipped with a load reduction device (LRD), which is intended to reduce aircraft and engine damage when there is a large fan imbalance.
“The design was intended to enable the fan to be mechanically disconnected from the turbomachinery, thus reducing the severity of the vibration that is transmitted into the airframe.”
LRDs are mechanical features that do not require pilot input. When an LRD is engaged, engine oil enters the compressor upstream of the pneumatic bleed ports, which provide bleed air to the cabin and flight deck, according to the NTSB.
In a prior comment to Simple Flying, a CFM International official confirmed that LRDs have been widely utilized in the industry for over 20 years and have performed as intended.
“The LEAP engine meets FAA and EASA bird ingestion certification requirements. The birds in these incidents significantly exceeded the size and weight of regulatory bird ingestion certification standards; however, the engines still performed as designed during these events.”
According to the preliminary study, engine bleed air from the high-pressure compressor (HPC) flows from the engine to the pressure regulating shutoff valve (PRSOV), and then to the left and right air conditioning packs. When closed, the valve prevents bleed air from the injured engine from entering the plane.
“The valve closes automatically if the engine running relay is de-energized (the affected engine spools down below 62% minimum N2), the engine start switch is moved to cutoff, or the engine fire switch is pulled.”
However, the investigators pointed out that the left air conditioning pack, powered by the left engine that was struck by a bird in New Orleans in December 2023, feeds air to the cockpit.
In turn, the right pack supplies air to the cabin, while excess air from the left air conditioning pack is recirculated, blended in the mixing manifold, and distributed throughout the cabin.
In November, the FAA said that it would hold a Corrective Action Review Board (CARB) meeting to review the flight data and determine a course of action. The regulator stressed that this was not an imminent flight safety concern.
No immediate safety action
On November 26, the FAA determined that no immediate safety measures were required to address the bird strikes and subsequent smoke incidents. In a statement to Simple Flying, the regulator noted
“The CARB’s work included evaluating several internal FAA safety recommendations. Based on the available data, the CARB determined the issue does not warrant immediate action and the FAA will follow its standard rulemaking process to address it.”
The NTSB claimed that following the bird strike event in New Orleans, Boeing issued a Flight Crew Operations Manual (FCOM) bulletin and made improvements to the FCOM and the QRH.
“CFM International, in collaboration with Boeing, has begun work on a software design update.”
Finally, following two bird strike occurrences in 2023, Southwest Airlines implemented Boeing’s improved FCOM and QRH. The NTSB is continuing its probe.