Southwest Airlines Flight 3472

The aircraft involved was a 16-year-old Boeing 737-7H4 (WL) (registration number N766SW), delivered to Southwest in May 2000.[2] The jet was subsequently returned to service and saw its last flight for Southwest in Nov 2019 to Greenwood, MS (GWO).[3]

The accident was investigated by the Federal Aviation Administration and the National Transportation Safety Board (NTSB). On September 12, 2016, the NTSB reported their initial findings.[4]

Initial findings from the examination of the airplane include:

• The left engine inlet separated from the engine during the flight. Debris from the engine inlet damaged the airplane fuselage, wing and empennage.
• A 5-inch by 16-inch hole was found in the left fuselage just above the left wing.
• No fan blade or inlet material was found in the hole and the passenger interior compartment was not penetrated.
• During the accident sequence, the airplane experienced a cabin de-pressurization.
• The aircraft maintenance records are being reviewed.

Initial findings from the engine examination include:

• One fan blade separated from the fan disk during the accident flight.
• The root of the separated fan blade remained in the fan hub; however, the remainder of the blade was not recovered.

Initial findings from the metallurgical examination conducted in the NTSB Materials Laboratory include:

• The fracture surface of the missing blade showed curving crack arrest lines consistent with fatigue crack growth. The fatigue crack region was 1.14-inches long and 0.217-inch deep.
• The center of the fatigue origin area was about 2.1 inches aft of the forward face of the blade root. No surface or material anomalies were noted during an examination of the fatigue crack origin using scanning electron microscopy and energy-dispersive x-ray spectroscopy.
• The blades are manufactured of a titanium alloy and the root contact face is coated with a copper-nickel-indium alloy.

Future investigative work by the NTSB will include 3-D measurements of the contact areas of all the blades, a non-destructive examination of the blade surfaces for cracks, and a review of the engine maintenance records.

Parties to the investigation include the Federal Aviation Administration, Southwest Airlines, the Southwest Airlines Pilots Association, and CFM International, which is a joint venture between GE Aviation (US) and Safran Aircraft Engines (France). The French Bureau of Investigation and Analysis for Civil Aviation Safety has also appointed an accredited representative who is supported by a technical advisor from Safran Aircraft Engines.

On March 30, 2020, the NTSB determined the probable cause of the accident as follows: "A low-cycle fatigue crack in the dovetail of fan blade No. 23, which resulted in the fan blade separating in flight and impacting the fan case. This impact caused the fan blade to fracture into fragments that traveled farther than expected into the inlet, which compromised the structural integrity of the inlet and led to the in-flight separation of inlet components. A portion of the inlet struck the fuselage and created a hole, causing the cabin to depressurize."[5]

• Southwest Airlines Flight 1380, a 2018 accident involving the same airline with an engine failure with a similar aircraft and engine causing one fatality.