(CNN)Just look at the left engine of Southwest Flight 1380, and it's obvious that something terrible befell that machine.
Southwest emergency landing puts focus on engine safety
Whatever happened inside that complicated system of fuel and fire and whirling components, it resulted in the decompression of the cabin at about 32,000 feet and, later, the death of a passenger after she nearly flew out a window.
When pilots Tuesday safely diverted the Boeing 737-700 -- with 149 passengers and crew members on board -- to Philadelphia from its New York-to-Dallas route, the metal surrounding the engine was in tatters.
As federal investigators begin to piece together what happened, the flying public undoubtedly is asking: What about the thousands of other jet engines in service right now?
The Southwest jet engine showed evidence of "metal fatigue," National Transportation Safety Board Chairman Robert Sumwalt told CNN's "New Day" on Wednesday. "What we want to find out is why was this not detected ahead of time."
"This was an internal metal fatigue area," Sumwalt continued. "So careful (visual) maintenance inspection from outside the fan blade would not have detected it, more than likely."
In 2016, the same airline and the same type of jet experienced an engine failure en route from New Orleans to Orlando, forcing the plane to land in Pensacola, Florida.
Engine debris damaged the jet's fuselage, wing and tail, according to the NTSB. The passenger compartment wasn't penetrated, but the cabin did decompress. Part of a fan blade had separated from the engine, and inspectors found evidence of a crack in the fan blade, which they said was consistent with metal fatigue.
"Nobody got injured, but it was hauntingly parallel to what happened yesterday," CNN aviation analyst Miles O'Brien said.
Manufacturers regularly inspect engines for hidden cracks using X-ray machines or ultrasound devices -- the same kind of technology doctors use to check the health of expectant mothers.
"The engines are the most closely monitored piece on the airplane," said John Goglia, a former NTSB board member. "Every engine manufacturer and airline monitors them very, very closely."
But engine maintenance and inspection protocols vary by airline, Goglia said, adding that airlines, in consultation with engine manufacturers, must get their maintenance protocols approved by the Federal Aviation Administration.
Southwest Airlines said in an email Wednesday that workers did a visual inspection of the engine that failed. An ultrasonic inspection of engine was scheduled to take place this December.
CFM International, which made the engine that apparently failed, issued a service bulletin in June 2017, recommending that operators who had planes with 15,000 or more cycles on their engines since their last inspection should perform ultrasonic tests on the fan blades. An engine cycle lasts from when an engine starts until it shuts down.
The FAA followed with a proposed rule in August calling for inspection of certain fan blades on CFM56 turbofan engines. Southwest in October 2017 sent a letter to the FAA saying it needed 18 months to complete the inspections.
The FAA's proposed rule was never put into effect, said Mary Schiavo, a former inspector general with the Department of Transportation, which oversees the FAA, and it's unclear whether Southwest ever began the inspections based on the FAA proposal. CNN emailed Southwest, but the airline said it didn't have answers to that question and did not know how many engines could be affected.
The FAA said Wednesday night that it will issue an airworthiness directive in the next two weeks requiring inspections of certain CFM56-7B engines. The directive will require an ultrasonic inspection of fan blades when they reach a fixed number of cycles.
Sumwalt said Southwest CEO Gary Kelly told him on Tuesday night that "Southwest will begin an aggressive ultrasonic inspection campaign for their entire fleet." That covers more than 700 737s, including more than 500 737-700s.
"The accelerated inspections are being performed out of an abundance of caution and are expected to be completed over the next 30 days," according to a statement on Southwest's website.
Schiavo called for a "surgical grounding" -- "a very strategic grounding of certain engines on certain planes to get this testing done immediately."
CFM did not respond to CNN's request for comment. In a statement on its website, the company said: "The CFM56-7B engine powering this aircraft has compiled an outstanding safety and reliability record since entering revenues service in 1997 while powering more than 6,700 aircraft worldwide. The engine family has accumulated more than 350 million flight hours as one of the most reliable and popular jet engines in airline history."
One of the most infamous cases of engine failure -- a worst-case scenario -- involved United Flight 232, a triple-engine jetliner carrying 11 crew and 285 passengers from Denver to Chicago in 1989.
A fan disk came apart inside the engine, cutting through the plane's hydraulic system and disabling all flight controls. The crew was forced to use its remaining two engines to steer the plane to an emergency crash landing at the airport in Sioux City, Iowa. The fiery crash killed 111 people.
After using volunteers to comb the surrounding countryside, inspectors found the fan disk buried in a field. It showed a half-inch fatigue crack. Further investigation showed that the crack existed at the time of the engine's previous inspection but wasn't detected. The NTSB said the probable cause of the wreck was "inadequate consideration" of "human factors" and "limitations in the inspection and quality control procedures used by United Airlines."
As for Tuesday's incident, "The NTSB and the FAA are going to be looking very closely at the sequence of events that led up to this accident," Gogila said. "It's going to take some time to determine what happened, unless it's really obvious. There have been occasions when the NTSB and the FAA have moved almost instantaneously, but that's pretty rare."
The NTSB is focusing on preventive measures, Sumwalt said. "We want to look at procedures to discover this before it is catastrophic."