N766SW

On August 27, 2016, about 0922 central daylight time, Southwest Airlines (SWA) flight 3472, a Boeing 737-7H4, N766SW, experienced a left engine failure while climbing through flight level 310 en route to the flight's assigned cruise altitude. Portions of the left engine inlet departed the airplane, and fragments from the inlet impacted the left side of the fuselage, creating a hole. The airplane depressurized, and the flight crew declared an emergency and diverted to Pensacola International Airport (PNS), Pensacola, Florida, where the airplane made an uneventful single-engine landing. The 2 pilots, 3 flight attendants, and 99 passengers were not injured. The airplane was substantially damaged. The regularly scheduled passenger flight was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 121 from Louis Armstrong New Orleans International Airport, New Orleans, Louisiana, to Orlando International Airport, Orlando, Florida.

The flight was the first of the day for the airplane and the flight crewmembers. According to the cockpit voice recorder (CVR), the left engine started at 0903:21, and the right engine started 1 minute later. Air traffic control (ATC) cleared the flight for takeoff at 0907:08. The takeoff and climb were uneventful until 0921:45, when the CVR recorded a "bang" sound followed by the sound of a decrease in engine rpm. The flight data recorder (FDR) showed that the airplane's altitude was 31,259 ft. FDR data also showed that, by 0921:49, the left engine fan speed had decreased from 99% to 39% rpm.

At 0921:52, the captain asked the first officer to declare an emergency. Three seconds later, the first officer contacted ATC to declare an emergency and told ATC that the airplane was descending. At 0922:06, the flight crew completed the SWA Engine Fire or Engine Severe Damage or Separation checklist. At 0922:17, the CVR recorded a sound similar to the cabin altitude warning horn; FDR data showed that the cabin altitude warning parameter had transitioned from "No Warn" to "Warn" 1 second later. (The parameter remained at "Warn" until the FDR data ended at 0929:37 due to a loss of electrical power to the FDR.) At 0922:18, the left engine fan speed had further decreased to 33% rpm.

At 0922:43, ATC told the flight crew that PNS was about 70 miles ahead of the airplane's position. At 0922:51, the CVR recorded the sound of the flight crewmembers using their oxygen masks. Two seconds later, ATC cleared the airplane to flight level 180.

At 0926:47, the captain communicated with the cabin crewmembers about the engine failure and instructed them to secure the cabin. At 0929:22, the flight crewmembers commented about high vibration levels and indicated that they were going to keep the airplane's speed up. The FDR recording ended at 0929:37; the last recorded data (1 second earlier) showed that the airplane was at an altitude of about 17,400 ft and that the left engine fan speed was 17% rpm.

At 0931:38, the flight crew told the cabin crew that there would not be an evacuation. At 0932:10, the CVR recorded the flight crewmembers removing their oxygen masks. At 0935:17, the flight crew made another comment about high vibration levels. At 0937:39, ATC told the flight crew to expect the instrument landing system (ILS) runway 17 approach. Fifteen seconds later, the flight crew selected flaps 1 and 5 to assess the airplane's handling. At 0938:19 and 0938:30, the flight crew selected autobrake level 3 and flaps 15, respectively.

At 0939:10, ATC cleared the airplane for the ILS runway 17 approach. Twelve seconds later, the flight crew completed the descent and approach checklists. At 0940:01, the flight crew reported that the airport was in sight; 12 seconds later, the crew lowered the landing gear. At 0941:40, the flight crew again commented about the vibration. Eleven seconds later, the CVR recorded the sound of the 500-ft automated callout. At 0942:30, the CVR recorded the sound of the airplane touching down on the runway, about 21 minutes after the left engine failure occurred.

This report will discuss that the left engine failure occurred when a fan blade fractured and exited the engine fan case, which is referred to as a fan-blade-out (FBO) event. An FBO event consists of four phases (the impact phase, the engine surge phase, the engine rundown phase, and the windmilling phase) during which the airplane structure is subjected to various loads.

AIRCRAFT INFORMATION

The airplane was equipped with two CFM International CFM56-7B24 turbofan engines, with one engine mounted under each wing. CFM International was established in 1974 as a partnership between General Electric Aviation (GE), a US manufacturer, and Safran Aircraft Engines (Safran), a French manufacturer formerly known as Snecma. (GE manufactured the CF6 engine, and Snecma manufactured the M56 engine; those engine designations were combined to form the new company and engine names.) The CFM56-7B24 engine is one of several CFM56-7B models (other models are the -7B18, -7B20, -7B22, -7B26, and -7B27) installed in Boeing 737 next-generation-series airplanes (the 737-600, -700, -800, and -900). The CFM56-7B references throughout this report apply to all of the engine models.

The inlet, which is part of the Boeing 737-700 airframe, is an aerodynamic fairing that guides air into and around an engine. The inlet is attached to the front of each engine's fan case. The inlet is part of the nacelle, which houses the engine. (Other nacelle components include the fan cowl and the thrust reverser.) The CFM56-7B engine and the Boeing 737-700 inlet are further described in the sections below.

Engine

The CFM56-7B is a high-bypass, dual-rotor, axial-flow turbofan engine. A single-stage high-pressure turbine (HPT) drives the nine-stage high-pressure compressor (HPC). A four-stage low-pressure turbine (LPT) drives the booster assembly, which comprises the engine fan and three-stage low-pressure compressor. The engine rotates clockwise (aft looking forward).

The engine consists of three major assemblies: the fan, engine core, and LPT. GE is responsible for manufacturing the HPC, combustion chamber, and HPT (collectively referred to as the engine core). Safran is responsible for manufacturing the LPT and the booster assembly. Both companies assemble the engines; those assembled by GE are identified by an even engine serial number (ESN) prefix (for example, 874), and those assembled by Safran are identified by an odd ESN prefix (for example, 875). The ESN of the accident engine, 874112, showed that GE assembled the engine.

The fan and booster assembly comprises the front and aft spinner cones, fan disk, fan blades, booster rotor, booster vanes, and associated hardware. The fan disk, which is secured to the booster, has 24 fan blade slots. In accordance with the instructions in Boeing's 737-600/700/800/900 Aircraft Maintenance Manual, the fan blades are numbered sequentially (1 through 24) in the counterclockwise direction (forward looking aft).

Each CFM56-7B fan blade had a chord (width) of about 11 inches at its widest point. The nominal weight of each fan blade is about 10.83 pounds. The fan blades are made of a titanium alloy (known as Ti-6-4), and the dovetail part of the fan blade, which slides into the fan disk, has a copper-nickel-indium coating to protect the blade from wear and provide a better fit with the fan disk (compared with no coating). Before the application of the fan blade coating, the entire blade, including the dovetail, is shot-peened to increase the fatigue strength of the material and reduce surface tensile stresses that can lead to cracking. (Shot-peening is a process that adds a compressive residual stress surface layer to material, and residual stress is the stress that is present in solid material in the absence of external forces.)

A spacer is installed under each fan blade root primarily to limit fan blade radial (outward) movement. The spacer also ensures that axial (longitudinal) loads would be transmitted to the fan blade axial retention feature if an FBO event or bird ingestion were to occur. A platform is installed on both sides of each fan blade to provide a smooth aerodynamic flow path between the blades. A shim is installed over each fan blade dovetail to prevent fretting (wear) of the fan disk pressure faces and improve lubrication durability, which reduces the amount of stress on the fan blade dovetail and the fan disk pressure faces. The shims for fan blade Nos. 22 through 24 had a newer design configuration than the other shims installed in the fan assembly, which had the previous design configuration. The newer design configuration for the shims was introduced in February 2008 to improve reliability. Both shim configurations are authorized for CFM56-7B fan blades, and both configurations are interchangeable.

The fan disk and spacers are manufactured from a titanium alloy (Ti-6-4). Both shim configurations are manufactured from a nickel-chromium-iron alloy (alloy 718). The platforms are manufactured from an aluminum alloy. Figure 1 shows the CFM56-7B engine fan assembly.

Source: CFM

Figure 1. Fan assembly.

The fan frame assembly is the main forward support for the installation of the engine to the airframe and includes the fan frame, the fan case, and the fan outlet guide vanes. The fan case, which is made of an aluminum alloy, was designed to provide fan blade radial containment if an FBO event were to occur and transmit FBO loads to the fan frame and the inlet. Although the fan case provides the primary FBO radial containment protection, the inlet, which is attached to the fan case A1 flange, provides additional FBO protection. Figure 2 shows a cross-section of the engine and airframe components.

Source: CFM

Figure 2. Cross-section of engine and airframe components.

Inlet

The inlet is attached to the engine at the fan case A1 flange with 24 bolted assemblies (fast...