NTSB says United engine failure caused by metal fatigue

After a preliminary onsite exam, the NTSB says the engine failure on a United aircraft on 20 February was likely caused by metal fatigue. Two fan blades of the Boeing 777-200’s right-hand Pratt & Whitney PW4077 engine were severed, one at the root and one in mid-span. The aircraft with the tail number N772UA had just departed from Denver International airport and was bound for Honolulu as UA328. The fan blade that was severed at the root “indicates damage consistent with metal fatigue,” NTSB chairman Robert Sumwalt says Monday. “This piece is being flown on a private jet tonight to a Pratt & Whitney lab where it will be examined tomorrow under the supervision of NTSB investigators.” A second fan blade, which looks from photographs to have been severed mid-span, “is consistent with being struck with the other fan blade” in what the NTSB calls “overload damage”. “It probably got hit as the other piece was separated,” Sumwalt says. The engine has 22 fan blades. One blade was “embedded in the engine containment ring at about the one o’clock position.” Sumwalt adds. The other was recovered from a soccer field in Broomfield, Colorado. The aircraft rained debris on neighbourhoods below as it returned to Denver for an emergency landing. Sumwalt added that the safety organisation will be examining the blades for “crack arrest marks”, which he compared to marks in the sand similar to when a tide comes in or goes out on a beach, in order to determine how long the blades had been damaged.<br/>