At 10:43am on April 17, Southwest Airlines Flight 1380 took off from New York’s LaGuardia Airport bound for Love Field in Dallas with five crew members and 143 passengers. But just over 20 minutes to the plane, at an altitude of about 30,000 feet somewhere above Hershey, Pennsylvania, the left engine of Boeing 737-700 exploded, blowing off its cowling. Debris smashed a passenger window, injuring a woman sitting next to it (who was almost thrown out of the broken window). With the cabin rapidly decompressing and an engine gone, the crew immediately began a descent, turning to Philadelphia for an emergency landing.
American commercial airline travel has been remarkably safe over the past decade: it has been more than 9 years since the last fatality on a US airline. Globally, 2017 was the safest year for air travel on record. Such an event—a fatality caused by a garbage truck—is extremely rare; the last engine debris killed an American commercial airliner is the year 1996.
But this is not a random act of God. What happened in flight 1380 appears to have been an “uncontained engine failure” in the front part of the left engine—the failure of one of the debris sprayer’s air blades tore through the walls of the the engine (hence the “lack of content) “part). This is not the first time that a Southwest plane has experienced such a failure: another Southwest 737-700 using the same engine (CFM56-7B) occurred in August of 2016 , without fatalities. And a United Airlines 777 flying from San Francisco to Hawaii suffered an engine failure caused by a broken blade last month, although the failure was contained and the plane landed safely.
— Joe Marcus (@joeasaprap) April 17, 2018
A cylinder of spinning sword
Turbofan engines are the standard propulsion plant for commercial aircraft. They are basically two devices in one. At the core of a turborecent is a turboairplane, which compresses and heats air to push it through turbines and out the back of the engine for push. But instead of using the rotational energy created by the turbines to compress the air, the engine also rotates a large volume of air in front of the engine that acts like a ducted propeller—pushing the air not only into the jet engine but also into the “bypass” channel in core engine environment for extra push. As a result, while turbofan engines may not get the raw performance of a turbojet, they are more efficient loads at moderate speed. Most turbofan engines today are “high bypass, low specific thrust”—meaning, they generate more of their thrust from the turbojet at their base than from the bypass thrust from the fan.
The engine used by Southwest’s 737-700s, the CFM56, is the world’s most popular high-bypass turbofan engine. Built by CFM International, a joint venture of Safran Aircraft Engines of France and General Electric Aviation, the CFM56 family of engines has been in service since 1974; there are more than 30,000 of them in service with more than 500 different airships, and, in total, CFM56 engines have more than 800 million flight hours, with surprisingly few major failures. The engine used by Southwest’s 737-700s—the CFM56-7B—is in use on 6,700 flights and has seen more than 350 million cumulative flight hours,
But a number of factors can cause even the most reliable engine to fail. With turbofans, the most frequent causes are maintenance errors and metal fatigue.
As the engine gets older, the blades (scimitar-shaped, hollow, and made of titanium alloy) can become glassy and start to break. Other reasons can also slow down—such as an engine failure on July 6, 1996 onboard. Delta Flight 1288. In that incident, an engine on a McDonnell-Douglas MD-88 blew through the engine bay and into the plane’s passenger compartment, killing two passengers. Inspection of the engine after a previous overhaul failed to find a large crack in the air port.
And when a fan is spinning at about 8,000 revolutions per second, it can become a deadly projectile—either traveling outward through the engine, or back into the engine’s fuselage, causing even a small amount of shrapnel. between the compressor rotors and the full-on engine explosion.
While any broken blade in an airplane engine would be destructive to the engine itself, the fan’s position outside of a proper engine and its large size and impact also make the airplane dangerous if it fails. A free-flying blade can bounce around inside the blade, smashing other blades of air, creating a hail of debris that the sword spins. The engine needs to be very resistant to damage from air failure to prevent the blades from flying out and hitting the body of the aircraft – ensuring that the failure is an “inherent failure.”
But in some cases, an air blade breaks free with enough kinetic energy that it becomes a large caliber bullet, shattering other air blades and passing through the engine housing. In the worst cases, such as the crash of United Airlines flight 232 in Sioux City in 1989, flying pieces of air can be cut by other major aircraft systems. In the same year, a blade failure on a British Midland Airlines 92—a 737 using an earlier version of the CFM56 engine—led to accident in Kegworth, Englandof which 47 died and 74 seriously injured.
Fortunately, more recent incidents with uncontained mechanical failures have not been on a UAL 232 scale. While the Delta incident in 1996 killed a mother and child and injured five others, there have been no fatalities in engine failure incidents since then. Most recently, in August 2016, in an incident similar to this week’s, Southwest Airlines Flight 3472 from New Orleans to Orlando suffered an uncontained failure of its left engine, blowing into the engine door – The result is passenger photos similar to those sent by the passengers of Flight 1380. As with this week’s accident, the plane experienced a collapse in the cabin, but not because of the blown window – hole 5 -inch by 16-inch is found in the fuselage just above the left wing, but there is no airfoil. or engine cowling material managed to penetrate into the passenger compartment.
The missing fan blade from Flight 3472 was never found, and The NTSB investigation of the incident is still ongoing. But the remaining surface cracks show signs of fatigue crack growth.
It is highly likely that the engine test of Southwest flight 1380 will find the same telltale signs of engine fatigue. The engines on the plane – “Next Generation” 737-700 with registration number N772S, put into service in 2000 – have been used for 40,000 flights, and about 10,000 of those are since the aircraft’s last major overhaul. While that’s not unusual for Southwest, which has one of the fastest growing fleets of airlines in the industry, it’s likely that Southwest will end up increasing the frequency of mechanical repairs as a result of this accident.
But remember: air travel is still much safer than driving.