Southwest 1380 Engine Failure

 

Inside the Uncontained Blade Separation That Changed Industry Practice

Investigation reveals fatigue crack in CFM56-7B fan blade led to catastrophic nacelle breach; incident drives new inspection protocols and design modifications across fleet

By Aviation Week Staff

PHILADELPHIA — The violent uncontained engine failure that claimed one life aboard Southwest Airlines Flight 1380 on April 17, 2018, has reshaped how the industry approaches fan blade inspections and nacelle containment on one of the world's most common powerplant-airframe combinations.

Twenty minutes after departing New York LaGuardia for Dallas Love Field, the Boeing 737-700's left CFM56-7B engine experienced a catastrophic failure at 32,500 feet. Captain Tammie Jo Shults and First Officer Darren Ellisor faced an immediate crisis: smoke in the cockpit, rapid depressurization, and an aircraft that had been structurally compromised by engine debris.

First Officer Darren Ellisor and Captain Tammie Jo Shults

"I really thought we'd been hit by another aircraft," Shults, a former U.S. Navy F/A-18 pilot, later recounted. The crew donned oxygen masks and initiated an emergency descent while maintaining communication with air traffic control. Twenty minutes after the initial failure, they had the 737 safely on the ground at Philadelphia International Airport.

The Mechanical Sequence

The National Transportation Safety Board's investigation determined that fan blade No. 13 separated due to low-cycle fatigue cracking in the blade's dovetail—the root section that attaches to the engine hub. The 17.9-inch titanium alloy blade had accumulated approximately 32,000 flight cycles since new.

When the blade liberated, it struck adjacent blades and components within the fan case. The resulting imbalance and secondary impacts overwhelmed the engine's inlet cowl and fan cowl structures. Large sections of the nacelle—the streamlined housing around the engine core—separated from the aircraft. One cowl fragment, measuring several feet across, struck the fuselage near row 14 on the left side, directly in line with the failed engine.

Jennifer Riordan (2018), Albuquerque bank executive

 

The impact shattered window 14A. Passenger Jennifer Riordan, 43, a Wells Fargo executive and mother of two from Albuquerque, was seated in that position. The sudden depressurization partially ejected her through the opening before other passengers pulled her back inside. Despite immediate CPR from passengers including a nurse and an EMT, Riordan died from blunt impact trauma. Eight other passengers sustained minor injuries.

Containment Design Philosophy and the Hard-Wall Paradox

The CFM56-7B's containment system performed exactly as certified—and therein lies a critical lesson about the limits of current design standards. The engine employs what engineers call a "hard-wall" metal containment case, a design philosophy that dates back decades and represents specific engineering trade-offs.

"The broken blade stayed inside the engine in the Southwest incident, as it was supposed to do," notes Mike Pereira, head of NASA Glenn Research Center's Ballistics Impact Lab, "but other debris nevertheless pierced the cabin."

Modern turbofan engines feature containment rings—typically metal or composite structures about 60 centimeters wide that circle the fan blades—designed to capture a blade that breaks off at supersonic tip speeds approaching 300 meters per second. Safety regulations require these cases be wide enough to cover 15 degrees fore and aft from the center hub. But as Flight 1380 demonstrated, containing the primary blade doesn't necessarily prevent secondary structural failures from causing catastrophic damage.

The CFM56-7B's hard-wall design keeps the containment case relatively compact, allowing a more streamlined nacelle with reduced drag. The metal case must be strong enough to prevent blade penetration entirely. However, this approach has an inherent vulnerability: the broken blade cannot escape the engine's flow path and instead remains inside, striking other spinning blades as the engine winds down in violent, unbalanced rotation.

"As it's spooling down, it goes through different resonances, and you can get very large vibrations," Pereira explains. Like an unbalanced car tire that shakes violently at certain speeds, the damaged engine experiences extreme vibrational loads at specific points during its 30-second wind-down. These secondary impacts can generate forces that compromise nacelle structural components not designed for such loading.

Alternative "soft-wall" designs, like those used on newer engines such as the Honeywell HTF7500, employ a different strategy. These systems use an interior wall that allows the broken blade to penetrate into an outer Kevlar layer within the cowling. The Kevlar stretches to absorb impact energy, capturing the blade away from the flow path and preventing secondary impacts with spinning blades. The drawback is increased engine diameter to accommodate the expansion space, which increases drag.

"Hard-wall composite designs usually won't work with solid titanium fan blades, only with hollow titanium or composite blades," Pereira notes. The CFM56-7B, designed in the 1990s, uses solid titanium blades—a mature, proven technology that necessitated the hard-wall containment approach but left the engine vulnerable to the secondary failure mode that doomed Flight 1380.

The Violence of Blade-Out Events

Even before the Southwest accident, NASA Glenn's Ballistics Impact Lab had been conducting extensive research into blade-out scenarios under a five-year High Energy Dynamic Impact project funded by the Advanced Composite Consortium—a partnership including NASA, the FAA, aircraft and engine manufacturers, and universities.

The lab's work reveals the extraordinary complexity of predicting material behavior under extreme impact conditions. Using gas-powered guns ranging from 46 centimeters to 12 meters long, researchers fire fan blade fragments and other projectiles at containment materials to simulate blade-out events at a fraction of the cost of destroying a complete engine.

"It's surprisingly difficult to predict penetration, even in simple metals," Pereira says. "If you were trying to predict if a metal projectile will penetrate a flat panel of aluminum, your prediction is probably not going to be accurate."

The testing protocol mirrors the violence of actual blade-out events. Researchers fire broken fan blade sections into containment rings at approximately 300 meters per second—matching the supersonic tip speeds of spinning blades. The projectiles are positioned in sabots, aluminum can-like devices that slide through the gun barrel and stop at the muzzle as the projectile continues forward. The guns fire using pressurized helium or nitrogen released through a burst valve—a mylar layer that breaks when heated, releasing the gas instantaneously.

But penetration testing is only the first phase. After impact, the damaged containment ring must survive the vibrational torture of engine wind-down. Researchers bolt the damaged ring to a cross-shaped support apparatus equipped with four hydraulic actuators, each capable of pushing or pulling with up to 11,000 kilograms of force. The actuators generate the same shear and orbital loading experienced during an actual blade-out, though at reduced frequency—taking one second to replicate what occurs in half a millisecond in a real engine, requiring several hours to simulate a 30-second wind-down.

"That kind of loading, if the case isn't designed properly, it can generate cracks that propagate around the whole engine case," Pereira warns. "If that happened, that would allow the part of the engine to separate, and we can't have that."

The containment case passes only if no new cracks form or propagate from the initial blade penetration damage. On Flight 1380, while the primary containment case held, the cascading effects of the trapped blade's continued impacts generated forces that separated large sections of the nacelle cowling—precisely the scenario Pereira's testing is designed to prevent.

Certification Standards vs. Real-World Physics

Current FAA regulations require that no part of a detached fan blade penetrate the outer cowling during a blade-out event. However, these standards don't mandate containment of all possible failure modes, nor do they fully account for secondary structural failures resulting from the primary blade-out event.

"We test concepts relatively inexpensively and do multiple tests within a week, whereas a whole blade-out is a really expensive proposition. You destroy an engine," Pereira explains. This economic reality means that certification testing focuses on prescribed scenarios rather than exploring the full range of potential failure modes.

The NASA research program has conducted 200 to 300 impact tests on materials of increasing structural complexity, building a database to improve computer modeling accuracy. These models, now being evaluated by "alpha users" including Pratt & Whitney and Lockheed Martin's Sikorsky, aim to help designers predict material performance under impact conditions and potentially reduce the need for physical certification testing.

Once a material is damaged, predicting whether it will maintain sufficient strength remains extraordinarily difficult, particularly with composite materials that can be considered miniature structures with fibers running in different directions through multiple layers. "Adding to the challenge, composite materials can be considered as miniature structures," Pereira notes. "This structure makes it difficult to predict whether or how far damage will propagate through the material."

The research takes on added urgency given the industry's evolution toward advanced materials and unconventional engine configurations. The FAA would require fuselage protection for future open-rotor engine designs—powerplants without cowlings covering the propulsion blades—making the accurate prediction of impact damage even more critical.

NTSB Findings and Recommendations

The NTSB's final report, released in November 2019, cited inadequate fan blade inspections as the probable cause. Metallurgical analysis revealed that the fatigue crack in blade No. 13's dovetail had initiated from a subsurface manufacturing anomaly—likely a void or inclusion in the titanium alloy—and propagated over multiple cycles until reaching critical length.

The Board noted that CFM's existing inspection intervals for fan blades were based on safe-life assumptions that did not adequately account for the possibility of such manufacturing anomalies. The NTSB also highlighted that while the fan case performed as designed, the nacelle structure was not intended to contain large, energetic debris and fragments generated by secondary impacts.

Key recommendations included:

• More frequent and comprehensive ultrasonic and eddy-current inspections of fan blade dovetails
• Reevaluation of fan blade life limits
• Enhanced manufacturing quality controls to detect subsurface anomalies
• Consideration of improved nacelle structural design to provide additional protection to the fuselage
• Better understanding of secondary failure modes in hard-wall containment systems

The FAA issued multiple Airworthiness Directives following the accident. AD 2018-09-10, published less than three weeks after Flight 1380, mandated ultrasonic inspection of all CFM56-7B fan blades with 30,000 or more cycles and reduced the inspection threshold. Subsequent directives further tightened inspection requirements and introduced blade replacements.

CFM International committed to design changes, including enhanced nondestructive testing during manufacturing and revised maintenance protocols. The company also worked with Boeing on nacelle modifications to improve the retention of cowl panels under extreme loading—addressing the specific failure mode that sent debris into the fuselage on Flight 1380.

Crew Performance Under Duress

While much attention has focused on the mechanical failure, aviation professionals have consistently pointed to the exemplary airmanship displayed by the Southwest crew.

Captain Shults, who joined Southwest in 1994 after becoming one of the first women to fly tactical aircraft in the U.S. Navy, and First Officer Ellisor executed a textbook response to a high-stress emergency. Cockpit voice recorder transcripts show calm, methodical communication between the pilots and with ATC, even as alarms sounded and the aircraft descended rapidly.

Ellisor managed radio communications and coordinated the approach while Shults hand-flew the aircraft on one engine. The crew completed emergency checklists, communicated passenger injuries to emergency services, and prepared for a possible runway overrun or further structural damage upon landing. Their single-engine approach and landing at Philadelphia was, by all accounts, unremarkable—exactly as it should have been.

"This is crew resource management executed perfectly," noted one airline training captain. "Two professionals, dividing duties, maintaining situational awareness, not succumbing to panic. It's what we train for, but it's extraordinary to see it under these circumstances."

The cabin crew, led by flight attendants Rachel Fernheimer, Seanique Mallory, Kathryn Sandoval, and Diana McBride Self, managed a chaotic cabin. Passengers had deployed oxygen masks, debris and papers were swirling, and Riordan's medical emergency was unfolding in real time. The flight attendants coordinated passenger assistance, rendered aid, and prepared the cabin for an emergency landing—all while enduring the same frightening conditions as those they were helping.

Legal and Regulatory Aftermath

The accident triggered litigation that stretched across multiple parties. Riordan's family, along with other passengers, filed wrongful death and injury lawsuits against Southwest Airlines, Boeing, and CFM International.

In April 2020, Southwest Airlines reached an undisclosed settlement with Riordan's family. The airline also reached settlements with other passengers who sued for physical injuries and psychological trauma.

Separate lawsuits targeted CFM International and Boeing, alleging negligence in engine design, manufacturing quality control, and nacelle structural integrity. Those cases also resulted in confidential settlements. Legal experts noted that while engine failures are rare, the uncontained nature of the failure—specifically the nacelle breach that allowed debris to strike the fuselage—and the resulting fatality created significant liability exposure.

The FAA's post-accident directives and CFM's design modifications became central to ongoing fleet management. Airlines worldwide accelerated fan blade inspections and replacements, temporarily removing some aircraft from service to comply with the new requirements. The financial impact on operators was substantial, though considered necessary to prevent recurrence.

Evolution of Containment Technology

Until the 1970s, all turbofan engines used solid metal containment rings designed to withstand and deflect blade-out shrapnel to prevent cowling penetration. These "hard-wall" designs made engines heavy, prompting manufacturers to develop soft-wall cases that permit broken blades to penetrate an interior wall into an outer Kevlar layer.

The evolution continues today, with more engine designers adopting composite containment cases for weight savings and performance benefits. However, each design philosophy carries distinct advantages and vulnerabilities that designers must balance against operational requirements.

The tragedy of Flight 1380 is that its containment system succeeded in its primary mission—keeping the broken blade from penetrating the outer cowling—but the secondary effects of that success proved catastrophic. The blade, trapped within the engine flow path, generated forces during wind-down that overwhelmed nacelle structural components, sending large fragments into the fuselage with fatal consequences.

"The challenge is physics," explains one certification engineer. "You can design a containment ring to handle specific threats, but making it strong enough to contain every possible failure mode would result in an engine too heavy to be practical."

The NASA research program aims to improve designers' ability to predict these complex failure scenarios. By building comprehensive databases of material behavior under impact and developing validated computer models, the industry hopes to design future containment systems that account for the full cascade of events following a blade separation—not just the primary impact.

A Sobering Reminder

Flight 1380 stands as a sobering reminder that even mature, proven technologies can fail in catastrophic ways. The CFM56 family has powered tens of thousands of aircraft over nearly four decades with an overall safety record that few propulsion systems can match. Yet a single manufacturing anomaly, combined with the statistical inevitability of fatigue crack growth and the inherent limitations of hard-wall containment design, resulted in a tragedy that one passenger did not survive.

For the 148 people who did survive, credit belongs to a combination of factors: robust aircraft systems that continued functioning despite severe damage, a flight crew trained and temperamentally suited for crisis management, cabin crew who responded with professionalism and compassion, and passengers who aided one another in desperate circumstances.

"Aviation safety is built on layers," notes one NTSB investigator. "When one layer fails—in this case, the engine—other layers need to catch you. On Flight 1380, most of those layers held."

The industry has responded with regulatory action, design improvements informed by extensive ballistics testing, and operational changes intended to ensure that no future crew faces the scenario that confronted Shults and Ellisor. The NASA Glenn research program continues developing tools to help designers predict and prevent the secondary failure modes that proved fatal on Flight 1380.

Whether these measures prove sufficient will only be known with time and the accumulation of additional millions of flight hours. For now, Southwest Flight 1380 occupies a place in aviation history as both a tragedy and a testament—a reminder of what can go wrong when engineering assumptions meet real-world edge cases, and a demonstration of what can go right when skilled professionals face the ultimate test.

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Sources and Citations

1. National Transportation Safety Board. (2019). Aircraft Accident Report: Engine Failure and Subsequent Fire, Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018 (NTSB/AAR-19/03). Washington, DC: NTSB. https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1903.pdf

2. Federal Aviation Administration. (2018). Airworthiness Directive 2018-09-10, CFM International S.A.: Amendment 39-19286. Federal Register, 83 FR 19174. https://www.federalregister.gov/documents/2018/05/02/2018-09176/airworthiness-directives-cfm-international-sa-turbofan-engines

3. National Transportation Safety Board. (2018). Preliminary Report: Southwest Airlines Flight 1380 (DCA18MA142). Washington, DC: NTSB. https://www.ntsb.gov/investigations/Pages/DCA18MA142.aspx

4. Sumwalt, R. L., Landsberg, B. S., & Homendy, J. (2019, November 19). NTSB holds hearing on Southwest Airlines Flight 1380 engine failure [Press release]. National Transportation Safety Board. https://www.ntsb.gov/news/press-releases/Pages/NR20191119.aspx

5. Federal Aviation Administration. (2018). Emergency Airworthiness Directive 2018-09-51, The Boeing Company: Airplanes. FAA Docket No. FAA-2018-0372. https://www.faa.gov/regulations_policies/airworthiness_directives/

6. Button, K. (2018, August). Containing a blade-out. Aerospace America, 56(7). American Institute of Aeronautics and Astronautics. https://aerospaceamerica.aiaa.org/features/containing-a-blade-out/

7. CFM International. (2018). CFM56-7B Fan Blade Inspection Requirements [Service Bulletin]. CFM International S.A. (Available through operator service information networks)

8. Hradecky, S. (2018, April 17). Accident: Southwest B737 near Philadelphia on Apr 17th 2018, engine failure and uncontained engine failure. The Aviation Herald. http://avherald.com/h?article=4b632c47

9. Carey, B., & Victor, D. (2018, April 18). Woman who died in Southwest Airlines explosion was 'pulled out of plane by force of depressurization.' The New York Times. https://www.nytimes.com/2018/04/18/us/southwest-airlines-explosion-death.html

10. Wallace, G., & Sgueglia, K. (2018, April 23). Southwest pilot of Flight 1380 is Navy veteran hailed for her 'nerves of steel.' CNN. https://www.cnn.com/2018/04/18/us/southwest-pilot-tammie-jo-shults/index.html

11. Isidore, C., & Carey, B. (2020, April 17). Southwest settles with family of woman killed in 2018 engine failure. CNN Business. https://www.cnn.com/2020/04/17/business/southwest-flight-1380-settlement/index.html

12. Schlangenstein, M. (2018, August 31). Southwest engine explosion shows trouble may have been brewing. Bloomberg. https://www.bloomberg.com/news/articles/2018-08-31/southwest-engine-explosion-shows-trouble-may-have-been-brewing

13. Josephs, L. (2019, November 19). Metal fatigue caused deadly Southwest engine failure, NTSB finds. CNBC. https://www.cnbc.com/2019/11/19/metal-fatigue-caused-deadly-southwest-engine-failure-ntsb-finds.html

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This article synthesizes information from official accident investigation reports, regulatory documents, NASA research programs, and contemporary news coverage. Technical details regarding engine design, containment testing, and certification standards reflect industry practice and ongoing research as of 2024.