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Photo of Boeing 737 NG from BoeingThe FAA proposed a new Airworthiness Directive for certain Boeing 737 NG models on Tuesday in response to a deadly depressurization incident in 2018. With an incident in 2016 and 2018, with one death as a result, the FAA found that the two fan blade-out (FBO) events resulted in the separation of the engine inlet cowl and fan cowl, damaging the fuselage. This caused a loss of pressurization and an emergency descent among other cracks and damages in the fuselage, exhaust nozzle, stabilizer and/or window. The AD will require an inspection to check for affected parts and any necessary installations, as well as a revision of the existing maintenance or inspection program to incorporate new airworthiness limitations. The comment period for the proposed AD is open until Jan. 26, 2024.
On Aug. 27, 2016 a Boeing model 737-700 series powered by a CFM56-7B engine experienced failure in the left engine due to the FBO but the plane landed successfully. The second incident was on April 17, 2018 on another Boeing 737-700 series plane powered by the CFM56-7B engine. In the second incident, the FBO resulted in the release of the fan cowl parts and the engine cowling departed the plane. The fan cowl parts damaged the fuselage and caused a loss of pressurization and forced an emergency descent. While the plane landed safely, one passenger died and several others were injured. This was the first U.S. passenger airline accident fatality since 2009. According to The New York Times, a window shattered and passengers said one passenger, Jennifer Riordan, was partially sucked out of the aircraft. Unfortunately, the Riordan was taken to the hospital and pronounced dead. Passengers reported the frenzied moments trying to pull her back into the plane and seeing to her serious injuries while they strapped oxygen masks to their faces, with one brave passenger using his back to try and block the opening in the plane during the descent. Despite the emergent situation, the pilots landed the plane smoothly. Seven other passengers were injured during the event.
After the 2018 incident, the NTSB reported that the low-cycle fatigue crack in the fan blade dovetail was initiated due to higher-than-expected dovetail stresses under normal operating loads, which was likely not detectable during the fluorescent penetrant inspection during the fan blade set's previous overhaul and visual inspections at the time of the fan blade relubrications. The fan blade fragments traveled forward on the fan case and along with the displacement wave created by the impact, caused damage that compromised the structural integrity of the inlet and caused parts of the inlet to leave the plane. The impact of the inboard fan cowl with the fuselage near the cabin window caused the window to depart the plane, causing the rapid depressurization event in the cabin and the death of a passenger.
In the incident two years prior, Southwest stated that the Captain made the decision to divert the flight due to a mechanical issue and landed without issue at the Pensacola International Airport. There were no injuries to the passengers or crew members and the NTSB was notified. The final NTSB report indicated the same failures seen in the 2018 incident and a portion of the inlet struck the fuselage and created a hole, leading to a depressurization event. In the final report, the NTSB said that 19 other cracked fan blades on CFM56-7B engines had been identified as of January 2020 and those fan blades had accumulated an average of 33,000 cycles since new when the cracks were detected.
In response to these two notable incidents, the FAA issued two ADs for the CFM International Model CFM56-7B engines. An emergency AD was issued on May 23, 2018 requiring the one-time ultrasonic inspection of the concave and convex sides of the fan blade dovetail. An amendment was issued on Dec. 26, 2018 requiring the initial and repetitive inspections of the concave and convex sides of the fan blade dovetail to detect cracking and replace any blades found. The FAA issued an AD to address fan blade failure due to cracking, which could result in an engine in-flight shutdown, uncontained release of debris, damage to the engine, damage to the plane and possible plane decompression. The December AD addressed the failure of the fan blade which could result in the engine inlet cowl disintegrating and debris penetrating the fuselage, causing loss of pressurization and causing an emergency descent.
Since the ADs were issued the FAA has found that additional rulemaking was necessary to reduce the probability of unsecured nacelle components if an engine fan blade failure occurred. Exemption No. 19212, dated July 13, 2022 indicated that Boeing had developed modifications to the inlet cowl, fan cowl and exhaust nozzle that must be completed by July 31, 2028. Boeing petitioned and the FAA agreed to amend Exemption No. 19212 to 19212A, adding a requirement that solutions to address potential maintenance errors must be incorporated before Dec. 31, 2029. To implement these design changes, the FAA must first issue rulemaking to address the unsafe conditions.
The new proposed AD would address the conditions related to the primary exhaust nozzle that was a result of the FBO events. During an FBO event, the primary exhaust nozzles that are not strengthened could leave the engine and potentially damage a stabilizer or strike the fuselage or window. If this condition is not addressed, this could result in a loss of control of the airplane, or a rapid decompression and hazard to window-seated passengers aft of the wing. The FAA said these unsafe conditions are likely to exist or develop on other products of the same design.
The FAA noted that a previous Boeing Special Attention Requirements Bulletin inadvertently excluded Boeing Model 737-600, -700, -700C, -800, -900 and -900ER series with certain line numbers. These planes are affected by the same unsafe conditions and this AD would require the accomplishments of the applicable actions in the bulletin. The FAA confirmed with Boeing that the instructions of the Special Bulletin apply to the expanded group of airplanes. A Boeing bulletin dated Sept. 1 identifies the "System Airworthiness Limitation No. 2 - Fan Blade Out Conditions," and "System Airworthiness Limitation No. 3 - Fan Blade Out Conditions" as airworthiness limitations to be incorporated. The FAA has also determined that "System Airworthiness Limitation NO. 4—Engine Nacelle Maintenance Errors" must be incorporated into the AD, specifically providing the limitation to mandate solutions to maintenance errors that must be completed by Dec. 31, 2029.
An estimated 1,215 planes on the U.S. registry will be affected. The estimated cost for U.S. operators to inspect for affected part numbers or maintenance records is $206,550. The option for a bridge bracket installation would cost up to $79,163,325 but not all planes will have the affected primary exhaust nozzle so the fleet cost will be significantly lower. The agency also estimated that it would take an average of 90 work hours per operator to revise existing maintenance or inspection programs, but this number will vary from operator to operator. The estimated cost per operator is about $7,650.Read more about the AD and submit a formal comment
This AD will apply to Boeing Models 737-600, -700, -700C, -800, -900 and -900ER as well as the planes identified in Boeing Special Attention Requirements Bulletin 737-78-1106 RB dated Sept. 1. The airplanes have the line numbers 1245, 1614, 1810, 1839, 1885, 1934, 1979, 1991, 2080, 2157, 2232, 2822, 3017, 3189 and 3319. Comments on the proposed AD are due by Jan. 26, 2024.