Boeing is urged to redesign 737 engine fan cowl after fatal accidentDavid Wagman | November 19, 2019
Federal safety officials recommended that Boeing redesign the engine covers on its next-generation series 737 aircraft to better withstand the impact of engine failures.
The National Transportation Safety Board (NTSB) determined that a fractured fan blade from a CFM International CFM-56-7B engine, powering a Southwest Airlines Boeing 737-700, led to the engine inlet and fan cowl separating and subsequently damaging the fuselage, resulting in a rapid cabin depressurization.
The accident occurred on a flight from New York's LaGuardia Airport to Dallas as the plane was climbing through 32,000 ft. The engine failure led to an emergency landing at Philadelphia International Airport.
The NTSB said that a fan blade had fractured at its dovetail in what is called a "fan blade out" (FBO) event. In the chain of events that resulted, portions of the inlet and fan cowl separated from the airplane, a fan cowl fragment struck the airplane’s fuselage near a cabin window, the window separated from the airplane and the cabin rapidly depressurized. One passenger died as a result of the accident, and eight others sustained minor injuries.
The airplane was a Boeing 737-700, and the engine was a CFM56-7B. When the accident occurred, the NTSB was already investigating an engine failure resulting from an FBO event involving a CFM56-7B engine on another Southwest Airlines Boeing 737-700 in August 2016.
Both accidents resulted in a series of inspections from the engine manufacturer, CFM, and from the Federal Aviation Administration, intended to detect fatigue cracking.
Point of impact
The NTSB said the Philadelphia FBO event highlighted the critical location for an FBO impact on the engine case. During that accident, the fan blade struck the fan case at a location that was critical to the structural integrity of the fan cowl.
"This discovery puts manufacturers and aircraft operators in a position to take actions that can ensure the structural integrity of the fan cowl if an FBO event does occur," said Robert Sumwalt, chairman of the NTSB. The NTSB also noted that other airframe/engine combinations might have "critical fan blade impact locations" meaning that an impact at those locations could affect nacelle components, including the inlet and fan cowl.
The CFM56-7B engines were produced by CFM International, a 50/50 joint company of GE and Safran Aircraft Engines of France. CFM sent a team of technical representatives to the site to assist the NTSB in its investigation.
GE produces the high-pressure compressor, combustor and high-pressure turbine; SNECMA manufactures the fan, gearbox, exhaust and the low-pressure turbine; and some components are made by Avio of Italy. The engines are assembled by GE in Evendale, Ohio, and by SNECMA in Villaroche, France.
The fan blades on the accident engine were overhauled almost 11,000 engine cycles before the accident. During that November 2012 overhaul, the blades were fluorescent penetrant and visually inspected, the NTSB said.
The NTSB identified at least two critical safety issues as a result of this accident investigation:
• The need to ensure the structural integrity of the fan cowl on Boeing 737 next-generation (NG)-series airplanes after an FBO event involving CFM56-7B engines. It said that the separated fan blade hit the fan case at the six o’clock position (in other words, at the bottom of the engine). During the CFM56-7B engine FBO containment certification tests, the CFM-selected fan blade release position was at twelve o’clock.
Boeing’s post-accident analyses found that the fan cowl structure is more sensitive and more susceptible to failure when a separated fan blade hits the fan case near the six o’clock position because of the proximity of this fan blade impact location to the radial restraint fitting (at the bottom of the inboard fan cowl). The NTSB said it is important that the interaction of the fan case, radial restraint fitting and fan cowl during an FBO event "be well understood to preclude a failure of the fan cowl structure on Boeing 737NG-series airplanes."
• The need to determine whether other airframe/engine combinations have any critical fan blade impact locations and how an impact at those locations could affect nacelle components. The NTSB said the investigation revealed the concept of a critical location for an FBO impact and its effect on the structural integrity of the nacelle and its components. Other engine/airframe combinations may also be sensitive to the location of an FBO impact and have unintended load paths and/or loads that are greater than those accounted for in structural analyses, the NTSB said.
No Federal Aviation Administration (FAA) regulation under 14 CFR Part 25, Airworthiness Standards: Transport Category Airplanes, currently requires manufacturers to account for critical FBO impact locations in all engine operating conditions, the NTSB said. The corresponding European Aviation Safety Agency regulations also do not include this requirement.
An initial examination of the airplane involved revealed that most 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 showed numerous impact 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.
According to the preliminary investigation, the No. 13 fan blade had separated at the root; the dovetail remained installed in the fan disk. The NTSB examination of the No. 13 fan blade dovetail showed 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; it was about 12 in span-wise and full width and weighed about 6.825 lb. The other piece, identified as another part of the airfoil, measured about 2 in span-wise, appeared to be full width, was twisted and weighed about 0.650 lb.
All the remaining fan blades exhibited a combination of trailing edge airfoil hard body impact damage, trailing edge tears and missing material. Some also exhibited airfoil leading edge tip curl or distortion.
After the general engine inspection was completed, the remaining fan blades were removed from the fan disk and an ultrasonic inspection was performed. No cracks were identified on the remaining blades, the NTSB said.
The No. 13 fan blade was examined further at the NTSB Materials Laboratory. Fatigue fracture features emanated from multiple origins at the convex side and were centered about 0.568 in aft of the leading edge face of the dovetail and were located 0.610 in 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 in deep through the thickness of the dovetail and was 2.232 in 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.
The NTSB said that the accident engine fan blades had accumulated more than 32,000 engine cycles since their original manufacture. 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.
After an Aug. 27, 2016, accident in Pensacola, Florida, in which a fan blade fractured, eddy current inspections were incorporated into the overhaul process requirements, the NTSB said. The NTSB reported a Boeing 737-700, operating as Southwest Airlines flight 3472, experienced an "uncontained engine failure and cabin depressurization" as it climbed after takeoff. That plane landed without further incident and no passengers were injured.
In the time since the fan blade overhaul, the Philadelphia accident engine fan blade dovetails had been lubricated six times. At the time each of these fan blade lubrications occurred, the fan blade dovetail was visually inspected as required.
In April 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 had accumulated 20,000 engine cycles, and subsequently at intervals not to exceed 3,000 engine cycles.
Also on April 20, the Federal Aviation Administration issued emergency AD (EAD) 2018-09-15 based on the CFM International service bulletin. The EAD required CFM56-7B engine fleet fan blade inspections for engines with 30,000 or greater cycles.
The NTSB said that the rest of the Southwest Airlines jet airframe exhibited significant impact damage to the leading edge of the left wing, left side of the fuselage and left horizontal stabilizer. 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 and the window was entirely missing. No window, airplane structure or engine material was found inside the cabin.
Change in pressure and a steep left roll
During interviews with the NTSB, the flight crew stated the climb from LaGuardia was normal with no indications of any problems; the first officer was the pilot flying and the captain was the pilot monitoring.
They reported experiencing a sudden change in cabin pressure, aircraft yaw, cockpit alarms and a “gray puff of smoke.” They donned their oxygen masks and the first officer began a descent. Flight data recorder (FDR) data showed that the left engine parameters all dropped simultaneously, vibration increased and within 5 seconds the cabin altitude alert activated.
The FDR also indicated that the airplane rolled left to about 40 degrees before the flight crew was able to counter the roll with control inputs. The flight crew reported that the airplane exhibited handling difficulties throughout the remainder of the flight.
The captain took over flying duties and the first officer began running emergency checklists. The captain requested a diversion from the air traffic controller; she first requested the nearest airport but decided on Philadelphia. The controller provided vectors to the airport with no delay.
The flight crew reported initial communications difficulties because of the loud sounds, distraction and wearing masks, but as the airplane descended the communications improved. The captain initially was planning on a long final approach to make sure they completed all the checklists, but when they learned of the passenger injuries, she decided to shorten the approach and expedite landing.