National Transportation Safety Board investigators have found evidence of fan blade metal fatigue in the Pratt & Whitney PW4000-112 engine that failed in flight on February 20, raining nacelle debris over a mile-long area of a Denver suburb and leading to the grounding of 69 Boeing 777-200s still in service.

Briefing reporters Monday night from Washington, D.C., NTSB chairman Robert Sumwalt said that one of the blades that fractured at its root contained what he called beach marks or crack arrest marks, visible to the naked eye, that can indicate how many cycles the airplane flew before the fatal crack finally propagated to the point of the blade breaking off. The NTSB has flown the piece to Pratt & Whitney headquarters in Connecticut, where engineers began a more detailed inspection on Tuesday.

“Once you put it under a scanning electron microscope, you can actually count the number of cycles since the initiation of that fatigue,” said Sumwalt.

The NTSB’s on-site inspection found one blade fractured at the root, an adjacent blade fractured at about mid-span, and a portion of one embedded in the containment ring. The remainder of the blades showed damage to the tips and leading edges.

Sumwalt said that although investigators found one piece of a fan blade on a soccer field in Broomfield, Colorado, the NTSB does not consider this a case of uncontained failure because the pieces did not breach the containment ring surrounding the engine.

“From a practical point of view from the flying public, it really doesn’t matter,” he commented. “It doesn’t change anything as to whether it was contained or not. It was still an event that we don’t like to see.”

Sumwalt also reported no structural damage to the airframe, even though images showed a large hole in a composite faring adjacent to the engine. Sumwalt wouldn’t speculate about what created the hole, noting only that a piece of metal from the nacelle could fairly easily puncture the fairing.

NTSB officials have sent the airplane’s flight data recorder and cockpit voice recorder to Washington, D.C., for analysis, which Sumwalt said would aid in determining why the fire that broke out did not extinguish after pilots cut the fuel supply. “We do have indications that the fuel to the engine was turned off,” said Sumwalt. “So we will be looking to see exactly what may have continued to propagate a fire.”