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Fatigue crack on Southwest CFM56-7B failed engine blade..NTSB

Fatigue crack on Southwest CFM56-7B failed engine blade..NTSB

by Kiriakos Tsakiridis -
Number of replies: 0

MAY 4, 2018

Incident

On April 17, 2018, Southwest Airlines flight 1380, a Boeing 737-700, N772SW, experienced a failure of the left CFM-56-7B engine and loss of engine inlet and cowling during climb about flight level 320.
Fragments from the engine inlet and cowling struck the wing and fuselage, resulting in a rapid depressurization after the loss of one passenger window. The flight crew conducted an emergency descent and diverted into Philadelphia Internati​onal Airport (KPHL), Pennsylvania.

Of the 144 passengers and five crewmembers onboard, one passenger received fatal injuries and eight passengers received minor injuries. The airplane sustained substantial damage.

Initial findings

Initial examination of the airplane revealed that the majority of the inlet cowl was missing, including the entire outer barrel, the aft bulkhead, and the inner barrel forward of the containment ring. The inlet cowl containment ring was intact but exhibited numerous impact witness marks. Examination of the fan case revealed no through-hole fragment exit penetrations; however, it did exhibit a breach hole that corresponded to one of the fan blade impact marks and fan case tearing.(See figure 1.)
 


The No.13 fan blade had separated at the root; the dovetail remained installed in the fan disk. Examination of the No. 13 fan blade dovetail exhibited features consistent with metal fatigue initiating at the convex side near the leading edge.

Two pieces of fan blade No. 13 were recovered within the engine between the fan blades and the outlet guide vanes. One piece was part of the blade airfoil root that mated with the dovetail that remained in the fan disk. The other piece, identified as another part of the airfoil, measured about 2 inches spanwise, appeared to be full width, was twisted, and weighed about 0.650 pound. All the remaining fan blades exhibited a combination of trailing edge airfoil hard body impact damage, trailing edge tears, and missing material.

No 13 Fan Blade examination

The No. 13 fan blade was examined further at the NTSB Materials Laboratory; Figure 2 shows a portion of the blade in detail. Fatigue fracture features emanated from multiple origins at the convex side and were centered about 0.568 inch aft of the leading edge face of the dovetail and were located 0.610 inch outboard of the root end face.
The origin area was located outboard of the dovetail contact face coating, and the visual condition of the coating appeared uniform with no evidence of spalls or disbonding. The fatigue region extended up to 0.483 inch deep through the thickness of the dovetail and was 2.232 inches long at the convex surface. Six crack arrest lines (not including the fatigue boundary) were observed within the fatigue region. The fracture surface was further examined using a scanning electron microscope, and striations consistent with low-cycle fatigue crack growth were observed.
 


Engine Condition and Maintenance Records

The accident engine fan blades had accumulated more than 32,000 engine cycles since new. Maintenance records indicated the accident engine fan blades had been periodically lubricated as required per the Boeing 737-600/700/800/900 Aircraft Maintenance Manual.

According to maintenance records, the fan blades from the accident engine were last overhauled 10,712 engine cycles before the accident. At the time of the last blade overhaul (November 2012), blades were inspected using visual and fluorescent penetrant inspections. After an August 27, 2016, accident in Pensacola, Florida, in which a fan blade fractured, eddy current inspections were incorporated into the overhaul process requirements.

In the time since the fan blade overhaul, the accident engine fan blade dovetails had been lubricated 6 times. At the time each of these fan blade lubrications occurred, the the fan blade dovetail was visually inspected as required for the fan blades installed in the accident engine.

On April 20, 2018, CFM International issued Service Bulletin 72-1033 applicable to CFM International CFM 56-7B-series engines recommending ultrasonic inspections of all fan blades on engines that have accumulated 20,000 engine cycles and subsequently at intervals not to exceed 3,000 engine cycles.

Impact on Airframe

The remainder of the accident airplane’s airframe exhibited significant impact damage to the leading edge of the left wing, left side of the fuselage, and left horizontal stabilizer. (See figure 3.) A large gouge impact mark, consistent in shape to a recovered portion of fan cowl and latching mechanism, was adjacent to the row 14 window (see figure 4; the window was entirely missing. No window, airplane structure, or engine material was found inside the cabin.



 


 http://newsinflight.com/2018/05/04/fatigue-crack-on-southwest-cfm56-7b-failed-engine-blade-ntsb/