Investigation Summary
The accident was triggered by the fatigue failure of fan blade No. 23 in the left (No. 1) engine. The blade separated at the dovetail, the point where it attaches to the fan hub. Unlike typical contained failures where the fan case stops debris, the fragments from this blade traveled forward and outward with sufficient energy to compromise the structural integrity of the inlet cowl. The cowl subsequently separated from the aircraft in flight. Debris from the cowl struck the fuselage, creating a 5-inch by 16-inch hole just above the left wing root, which led to the immediate loss of cabin pressure. The aircraft also sustained damage to the wing, empennage, and horizontal stabilizer. The NTSB metallurgical analysis identified a fatigue crack approximately 1.14 inches long and 0.217 inches deep at the blade root, which had not been detected during previous maintenance inspections.
Final Conclusions
The NTSB determined the probable cause was a low-cycle fatigue crack in the dovetail of fan blade No. 23. This crack led to the blade's separation and subsequent impact with the fan case. The resulting fragments damaged the inlet cowl to the point of structural failure. The separation of the inlet cowl caused debris to strike the fuselage, puncturing the pressure vessel and causing depressurization. The investigation was concluded on March 30, 2020, with the release of the Final Report (DCA16FA217).
Video Analysis
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Airframe & Maintenance
Crew Experience
Captain
The captain had been with Southwest Airlines since 2008.
First Officer
The first officer had been with Southwest Airlines since 2007.
Systems & Failure Modes
CFM56-7B24 Engine
The high-bypass turbofan engine experienced a fan blade separation at the hub, leading to an uncontained failure of the inlet cowl.
Inlet Cowl
The aerodynamic structure at the front of the engine which failed and separated after being struck by fan blade fragments.
Cabin Pressurization System
The system failed to maintain pressure after debris from the engine inlet punctured the fuselage skin.
Interesting Facts
- 01The aircraft was 16 years old at the time of the accident.
- 02A 5-inch by 16-inch hole was found in the fuselage above the left wing.
- 03The fatigue crack was located 2.1 inches aft of the forward face of the blade root.
- 04No passengers or crew members were injured during the event.
- 05The aircraft was repaired and returned to service following the accident.
Safety Actions & Advisories
FAA Airworthiness Directive NPRM
Following the investigation, the FAA proposed new mandates for ultrasonic inspections of CFM56-7B fan blades to detect fatigue cracks that are not visible during standard visual inspections.