Boeing has indicated it has completed redesign work and retrofit instructions on specific aircraft as requested by the Federal Aviation Administration (FAA). The manufacturer has filed paperwork indicating it met the FAA's July 31st deadline for submitting design changes and issuing service bulletins related to redesigning its 737 Next Generation nacelles.

What is being fixed?

Two Southwest Airlines Boeing 737-700s were the subject of fan blade out (FBO) events that led to unexpected aircraft damage caused by pieces of the engine escaping from the structure surrounding the engine (the nacelle), striking the aircraft when a fan blade broke and hit the nacelle in a different way than expected by Boeing, regulators, and the engine's maker CFM International. The word nacelle is derived from a French term for a small boat.

Airplane engines are designed to contain their parts in the event of failure and can ingest significant amounts of foreign objects and debris. In the most recent Southwest incident, one of the 24 flan blades broke off and traveled forward, striking part of the engine cowling, which then broke off and impacted the side of the airplane. The company said in its most recent filing that the required work was completed ahead of schedule:

“By July 31, 2023, Boeing will have submitted all design changes to the FAA and will have released service bulletins to provide inlet modifications, fan cowl modifications, fan cowl support beam modifications, and exhaust structures modifications for retrofit of these design changes.

Photo: Denver International Airport

Boeing further requested an extension of the previously granted exemption to allow Boeing to work with operators and the FAA to address undetected maintenance errors and ensure full compliance. The manufacturer asked for 17 months to analyze the in-service incidents and develop a risk-mitigation strategy after becoming aware of a limited number of incidents in which operator maintenance errors could have contributed to fan cowls and the integrated drive generator doors departing during takeoff or landing.

The company hopes the extension will allow Boeing adequate time to develop appropriate solutions to address the potential for these maintenance errors and have "scheduled discussions with the FAA." ​​​​

What caused the problem?

On two separate occasions, individual fan blades, of which there are twenty-four, broke off and struck the inlet, causing significant damage. In the case of Southwest Airlines flight WN1380, safety regulators concluded the current design of the engine nacelle was not resilient enough to withstand a fractured fan blade hitting it at the bottom of the engine. Initial certification testing involved scenarios where the detached blades strike at the top of the engine, which would be more likely.

Photo: NTSB

The FAA declined to comment and referred all questions to Boeing when reached by Simple Flying, as it is still ongoing.

Parts of the engine cowling detaching are uncommon and not usually severe. Last August, part of the engine cowling tore off an Alaska Airlines Boeing 737-900ER, which made a safe emergency landing at Seattle International Airport.

Source: Aviation Week