The fan blade that failed on a Southwest Airlines Co. plane last month, killing a passenger, had been inspected seven times since late 2012 but without the sophisticated technology airlines are now under orders to use.

The inspections relied on visual observations and there were no reports of cracks, according to a preliminary report issued Thursday by the National Transportation Safety Board.

The investigation of the April 17 Southwest flight is one of the most significant in years, since it raises questions about the safety of the CFM56-7B engine, one of the world’s most popular. Before engines like this get approval from aviation regulators, manufacturers must demonstrate that failed fan blades won’t trigger extensive damage if they break loose.

Since a similar fan blade failure on another Southwest plane in 2016, engine maker CFM International Inc. has urged airlines to use either ultrasound or electric-current tests designed to find cracks beneath the surface.

The Federal Aviation Administration has issued two orders since April’s accident on Flight 1380 for the more sophisticated testing of fan blades, touching off a rush by airlines to examine thousands of the blades. The orders call for the inspection of blades that have made at least 20,000 flights by the end of August. If a blade’s number of flights isn’t known, it must also be tested by the same deadline.

Failed Blade’s History

The failed blade had made more than 32,000 flights, according to the NTSB.

Southwest says it expects to complete all the inspections, even those with fewer than 20,000 flights, of its fleet in coming weeks.

The most recent extensive inspection of the failed blade occurred in November 2012, using a dye designed to highlight small cracks on the surface. The engine had made 10,712 cycles — starting and stopping the engine, usually associated with a flight — since then, according to the NTSB.

For the six inspections since 2012, the blades were lubricated and mechanics examined them visually for flaws.

Southwest said in a statement on Thursday that the company was continuing “to cooperate fully with the National Transportation Safety Board’s (NTSB) ongoing investigation into Flight 1380.”

CFM is a joint venture between General Electric Co. and France’s Safran SA.

A passenger, Jennifer Riordan, who was a vice president at Wells Fargo & Co. in New Mexico, was killed after being partly sucked out of the shattered window. The plane, a Boeing Co. 737-700 bound for Dallas from New York’s LaGuardia airport, made an emergency landing in Philadelphia.

While there have been fatal crashes involving cargo haulers, foreign airlines and charter operators, it was the first death of a passenger on a U.S. airline since 2009.

Questions Remain

The preliminary report doesn’t say why the crack grew large enough for the blade to snap off or whether use of more sophisticated testing before the accident could have prevented the failure.

When the blade broke loose — the result of a crack that gradually expanded under the stresses of multiple flights — it bounced forward and collided with the relatively unprotected engine inlet, tearing it apart and sending shrapnel into the fuselage and wing.

Investigators may have identified the part that shattered the window. A metal piece that included a latching mechanism, which was recovered after the accident, matches “a large gouge impact mark” adjacent to the row 14 window that broke, NTSB said.

An examination of the failed blade under an electron microscope showed evidence of “low-cycle fatigue,” the NTSB said. That suggests that the crack grew relatively quickly, but investigators didn’t provide any timeline.

All the remaining blades on the failed engine were removed and tested using an ultrasonic device that can detect flaws beneath the titanium surface, but no additional cracks were found, NTSB said.

The engine’s containment shield, designed to prevent the kind of widespread damage that occurred in the accident, did block blade fragments from escaping. However, the majority of material at the front of the engine known as the “inlet cowl” was missing.

The report also includes new information about what happened on the plane in the moments after the failure. The 737 banked more than 40 degrees to the left. The pilots recovered, but had to fight the controls. “The flight crew reported that the airplane exhibited handling difficulties throughout the remainder of the flight,” the NTSB said.

The plane’s three flight attendants told investigators that they donned oxygen masks after the plane lost pressure and began moving through the cabin to assist passengers. That’s when they noticed that Riordan had been sucked through the window.

“As they moved toward the mid-cabin, they found the passenger in row 14 partially out of the window and attempted to pull her into the cabin,” the report said. “Two male passengers helped and were able to bring the passenger in.”