In the cockpit of Southwest flight 1380
On the 17th of April in 2018, Southwest Airlines flight 1330 experienced an engine failure while climbing to cruise altitude.
The aircraft was a Boeing 737-7H4 (registration N772SW) with two turbofan engines manufactured by CFM International. The CFM-7B24 engine has 24 fan blades installed on the fan disk.
The flight departed normally from LaGuardia Airport in New York with 144 passengers on board. The first officer was the Pilot Flying and the captain was the Pilot Monitoring. Both pilots were extremely well qualified and had previously flown in the military (the captain had been a US Navy fighter pilot and the first officer had been a pilot in the US Air Force).
Half an hour later, while climbing to the cruise at flight level 380 (38,000 feet) one of the 24 fan blades in the left engine fractured at its root as a result of a low-cycle fatigue crack.
In the cockpit, the flight crew heard a loud bang and the aircraft began to vibrate. The left engine fan speed decreased from 99.5% to 61.5% while the engine core speed decreased from 97.8% to 89.8%. At the same time, the 737 began to roll to the left.
The fan blade impacted the engine fan case and broke into pieces. Blade fragments flew forward and into the nacelle inlet (part of the structure that houses the engine). The fan case became deformed, damaging the inlet further until the inlet separated from the aircraft. The deformity also increased the load on the fan cowl. Cracks appeared on the fan cowl skin and frames and, in a matter of seconds, severed the three latch assemblies which connect the inboard and outboard halves of the fan cowl. Both fan cowl halves separated from the aircraft. One fan cowl was found in the inboard fan cowl aft latch keeper. Pieces of the engine nacelle were recovered from a field on the flight path.
One of the fan cowl fragments smashed into the left fuselage and a cabin window broke, causing a rapid depressurisation. The passenger seated closest to the window was partially pulled out of the aircraft. Cabin crew and passengers were able to drag the passenger back in; however after landing the passenger died of injuries sustained when she was pulled out of the window in the rapid depressurisation.
The cabin altitude warning horn sounded, which meant that the pressure in the cabin had exceeded 10,000 feet. Pressurised flights maintain a cabin altitude below 8,000 feet, so it seemed clear that they had suffered some sort of depressurisation event. Both pilots put on their oxygen masks.
The aircraft continued to roll to the left for a bank angle of 41.3°. The Pilot Flying began to roll the aircraft back to wings level and regained control of the aircraft attitude. The power to both engines was reduced to idle as the flight crew initiated an emergency descent.
The captain initially asked for information on the nearest airport, which was Harrisburg International Airport in Middletown, Pennsylvania (MDT). However, both pilots quickly agreed that their best option was Philadelphia International Airport, which was not significantly further had four runways, the longest of which was 2,000 feet longer than the single runway at Harrisburg, and better rescue and firefighting capabilities. The Aircraft Rescue and Firefighting (ARFF) rating ranges from A to E; Harrisburg, the closest airport, was categorised as B while Philadelphia was categorised as E.
After the flight was cleared direct to Philadelphia, the captain made a public announcement that they were diverting. As the aircraft descended through 17,000 feet, the captain declared an emergency and took over as Pilot Flying with the first officer filling the role of Pilot Monitoring as well as trying to find out what was happening in the cabin. I’ll look at that next week but, for the moment, let’s stay in the cockpit.
This edited recording of the captain working with air traffic controller makes for compelling listening:
As they began their extended final, the captain informed ATC that they needed a single channel and to stop changing frequencies. This was to reduce the load in the cockpit and also meant that she wasn’t explaining her requirements and the situation to new people. The approach controller agreed and told her he’d was just going to let her drive and would clear other traffic out of the way.
Captain: Okay, could you have the medical meet us there on the runway as we’ve got injured passengers.
Approach: Injured passengers. OK. And are you — is your airplane physically on fire?
Captain: No, it’s not on fire but part of it is missing.
Captain: They said there’s a hole and, um, someone went out.
Approach: I’m sorry, you said there’s a hole and somebody went out?
Approach: Southwest thirteen eighty…It doesn’t matter. We’ll work it out there. The airport just off to your right. Report it in sight please.
The first officer, in his role as Pilot Monitoring, went through the Engine Fire or Engine Severe Damage or Separation checklist.
The aircraft landed safely, 17 minutes from the start of the incident.
One passenger was fatally injured and eight passengers received minor injuries.
From the final report:
Metallurgical examinations of the fractured fan blade found that the crack had likely initiated before the fan blade set’s last overhaul in October 2012. At that time, the overhaul process included a fluorescent penetrant inspection (FPI) to detect cracks; however, the crack was not detected for unknown reasons. After an August 2016 FBO event involving another SWA 737-700 airplane equipped with CFM56-7B engines, which landed safely at Pensacola International Airport, Pensacola, Florida, CFM developed an eddy current inspection (ECI) procedure to be performed at overhaul (in addition to the FPI that was already required). An ECI has a higher sensitivity than an FPI and can detect cracks at or near the surface (unlike an FPI, which can only detect surface cracks).
The important point here is that Southwest Airlines flight 3472 had suffered an identical issue with a fractured fan blade almost two years before, in August 2016. CFM International had developed an on-wing ultrasonic inspection technique and issued a service directive for this inspection to take place as a one-off inspection of specific aircraft based on the engine serial number, the number of service cycles or the service time. The engines on N772SW did not match any of the three parameters and so the ultrasonic testing was not done.
After the accident, Southwest announced that they were scheduling ultrasonic inspections of all CFM engines in the fleet. These ultrasonic inspections of aircraft engines, which were done with every fan blade relubrication, identified fifteen blade cracks which had not previously been detected.
Since then, the FAA has issued a follow-up Airworthiness directive for all CFM56-7B engines to undergo repeated ultrasonic inspections for fan blades before they reach 20,000 cycles or within 113 days, whichever occurs later, and then to be repeated every 3,000 cycles. The fractured fan blade on Southwest flight 1380 had accumulated 32,636 cycles since new; the earlier fractures in August 2016 were on fan blades which had been through 38,152 cycles. The additional fifteen cracked fan blades discovered after the accident had an average of 33,000 cycles since new.
From the final report:
The National Transportation Safety Board (NTSB) 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.
The passengers on the flight were given $5,000 each and a $1,000 voucher for future travel with the airline. The captain of the aircraft, Tammie Jo Shults, and the crew were thanked for their heroism at a ceremony at the White House. Shults has written a book about her life and the incident called Nerves of Steel.
The aircraft is parked in Victorville, California for storage. When I was 16, a young man asked me to marry him and move to Victorville and live happily ever after. I declined. However, if I’d known then that a large boneyard facility with hundreds of commercial and military aircraft would end up located there, I might not have been quite so sure of myself.