Boeing 737-7H4 accident at Philadelphia, Pennsylvania, 17 Apr 2018

Accident narrative

On April 17, 2018, about 1103 eastern daylight time, Southwest Airlines flight 1380, a Boeing 737-7H4, experienced a left engine failure while climbing through flight level 320. Portions of the left engine inlet and fan cowl separated from the airplane. One fan cowl fragment impacted the left-side fuselage near a cabin window, causing the window to depart the airplane and resulting in a rapid depressurization. The flight crew conducted an emergency descent and diverted to Philadelphia International Airport in Philadelphia, Pennsylvania, landing safely about 17 minutes after the engine failure. Of the 144 passengers and five crewmembers aboard, one passenger received fatal injuries and eight passengers received minor injuries. The airplane was substantially damaged.

### The flight

The regularly scheduled domestic passenger flight had departed from LaGuardia Airport in Queens, New York, about 30 minutes earlier, destined for Dallas Love Field in Dallas, Texas.

The captain, 56, had accumulated about 11,715 hours of total flight experience, including 10,513 hours in the Boeing 737. The first officer, 44, had accumulated about 9,508 hours of total flight experience, including 6,927 hours in the 737. Three flight attendants were aboard. The first officer was the pilot flying, and the captain was the pilot monitoring.

### The sequence of events

Shortly after the airplane passed through flight level 320, the cockpit voice recorder recorded the sound of increased background noise. Flight data recorder data showed that the left engine's fan and core speeds decreased and vibration parameters increased. The flight crewmembers reported hearing a loud bang and feeling significant airplane vibration. The airplane immediately began an uncommanded roll to the left, reaching a maximum of 41.3 degrees before the first officer rolled the airplane back to wings level.

At 1103:39, the cabin altitude warning horn sounded, indicating the cabin altitude had exceeded 10,000 feet. Power in the right engine was reduced to idle, consistent with the start of an emergency descent, and power in the left engine was also reduced to idle. The left engine fuel cutoff parameter transitioned to cutoff at 1104:09, and the engine began to windmill.

The cockpit voice recorder recorded sounds consistent with the flight crew's use of oxygen masks at 1104:49. The captain transmitted to the air traffic controller that the flight had an engine fire and was descending. The controller asked which airport the crew wanted to divert to and suggested Harrisburg International Airport. The captain requested a vector for the closest airport, then stated, "we're looking at ah Philly." The first officer later stated that he had looked at a map, determined Philadelphia was a close suitable airport, and pointed it out to the captain. The airplane reached a peak descent rate of 5,228 feet per minute.

At 1105:52, the controller cleared the airplane direct to Philadelphia. The captain made a public address announcement informing the cabin crew and passengers of the diversion. The captain later clarified to the controller that there was no fire but they were single engine, and requested emergency vehicles be ready on the captain's side.

At 1109:30, the captain took the controls and became the pilot flying. The first officer assumed the duties of pilot monitoring and began the Engine Fire or Engine Severe Damage or Separation checklist. The flight crew performed most of the items on this checklist but did not initiate three other relevant non-normal checklists. The first officer asked about running checklists, and the captain responded, "nope just keep going." The captain later stated she flew slower than the maximum operating speed to reduce the severity of the airframe vibration.

At 1114:14, the captain requested a long final approach to allow time to accomplish checklists. Meanwhile, the first officer contacted the flight attendants via the interphone. A flight attendant informed him that a window was open and somebody was out the window. The first officer informed the captain about the injured passenger, and the captain decided to expedite the approach, stating "let's get it turned in."

The captain stated she wanted to use 5 degrees of flaps for the landing because she did not know how controllable the airplane would be, reporting "lots of drag" on the flight controls. The flight crew performed the Before Landing checklist. In the cabin, the flight attendants commanded passengers to keep their heads down. The airplane landed on runway 27L at 1120:30 at a speed of about 171 knots.

### Events in the cabin

Following the engine failure, the flight attendants donned oxygen masks and portable oxygen bottles and moved through the cabin. When a flight attendant reached row 14, she saw that the head, upper torso, and arms of the passenger in seat 14A had been pulled outside the airplane through the missing window. With the assistance of two passengers, the passenger was pulled back into the airplane. Medically qualified passengers administered CPR.

The flight was full, with no open cabin seats remaining. To allow the injured passenger to receive medical care, the two other passengers from row 14 relocated to the aft galley. One sat on the flight attendant aft jumpseat, and the other sat on the cabin floor.

Although the flight attendants were aware of the imminent landing, none was in her assigned jumpseat. Flight attendant A sat on the aisle floor near row 4 or 5, and seated passengers held her down. Flight attendant B sat on the floor in the aft galley with seated passengers holding her down. Flight attendant C also sat on the floor in the aft galley.

### What the investigation found

The airplane was equipped with two CFM International CFM56-7B24 turbofan engines. The left engine failure occurred when fan blade No. 13 fractured at its root, with the dovetail remaining within a slot of the fan disk.

The separated fan blade impacted the engine fan case and fractured into multiple fragments. Some fragments traveled forward of the engine and into the inlet. The impact also caused the fan case to deform locally, creating a displacement wave that traveled around the fan case and generated large loads in the inlet attach ring. The forward-traveling fragments and the displacement wave compromised the structural integrity of the inlet, causing portions of it to depart the airplane.

The impact of the separated fan blade with the fan case also imparted significant loads into the fan cowl through the radial restraint fitting, located at the bottom of the inboard fan cowl. These loads caused cracks to form in the fan cowl skin and frames, severing the three latch assemblies that joined the inboard and outboard halves. Large portions of both fan cowl halves separated and departed the airplane.

One recovered part was the inboard fan cowl aft latch keeper. The left side of the fuselage near the missing cabin window had impact damage and witness marks consistent with the size and shape of the inboard fan cowl aft latch keeper and surrounding structure. The Board concluded that the impact of this latch keeper with the fuselage caused the window to depart the airplane.

Metallurgical examinations found that the fan blade fractured due to a low-cycle fatigue crack that had likely initiated before the fan blade set's last overhaul in October 2012. During that overhaul, a fluorescent penetrant inspection was performed, but the crack was not detected. The crack was also not detected during subsequent on-wing visual inspections conducted during fan blade relubrications. The Board concluded the crack initiated because of higher-than-expected dovetail stresses under normal operating loads and was most likely not detectable during these inspections.

Following an August 2016 fan-blade-out event involving another Southwest Airlines 737-700, CFM developed an eddy current inspection procedure to be performed at overhaul, which has a higher sensitivity than a fluorescent penetrant inspection. CFM also developed an on-wing ultrasonic inspection technique that could be performed during fan blade relubrication. The Board concluded that the requirement to perform an eddy current inspection at the time of overhaul and an ultrasonic inspection at the time of relubrication should enable cracked fan blades to be detected and removed from service before reaching a critical size.

The investigation noted that during the CFM56-7B engine certification tests in 1996, the selected fan blade release position was at twelve o'clock. During the accident, the blade impacted the fan case near the six o'clock position. Postaccident analyses found that the fan cowl structure is more sensitive and susceptible to failure when an impact occurs near the six o'clock position because of the proximity to the radial restraint fitting. The forward-traveling fragments had a trajectory angle greater than observed during certification tests, and the resulting inlet damage and fan cowl stresses were greater than accounted for in the certification analyses. The Board concluded that given the results of the certification tests and subsequent structural analyses, the post-accident events could not have been predicted.

Regarding the flight crew's actions, the Board concluded that the crew's performance of most items on the Engine Fire checklist and the nonperformance of three other relevant checklists allowed the crew to appropriately balance the procedural requirement of executing checklists with the high workload associated with maintaining airplane control and accomplishing a safe and timely descent and landing. The Board also found the decision to land at Philadelphia was appropriate given the airplane's location, the emergency circumstances, and the airport's multiple runways and firefighting capabilities.

The Board concluded that although not a factor in the outcome, the flight attendants should have been properly restrained in their assigned jumpseats in case an emergency evacuation was necessary. The Board also noted a need for guidance on mitigating hazards to passengers affected by an in-flight loss of seating capacity, as the airline's manual did not discuss actions to take if no seats were available for a passenger who needed to be reseated.

### Probable cause

The National Transportation Safety Board determines that the probable cause of this accident was a low-cycle fatigue crack in the dovetail of fan blade No. 13, which resulted in the fan blade separating in flight and impacting the engine fan case at a location that was critical to the structural integrity and performance of the fan cowl structure. This impact led to the in-flight separation of fan cowl components, including the inboard fan cowl aft latch keeper, which struck the fuselage near a cabin window and caused the window to depart from the airplane, the cabin to rapidly depressurize, and the passenger fatality.