B737, en-route, northwest of Philadelphia PA USA, 2018
B737, en-route, northwest of Philadelphia PA USA, 2018
On 17 April 2018, sudden uncontained left engine failure occurred to a CFM56-7B powered Boeing 737-700 when climbing through approximately FL320. Consequent damage included a broken cabin window causing rapid decompression and a passenger fatality. Diversion to Philadelphia without further significant event then followed. A single fan blade was found to have failed due to undetected fatigue. The Investigation noted that the full consequences of blade failure had not been identified during engine / airframe type certification nor fully recognised during investigation of an identical blade failure event in 2016 which had occurred to another of the same operator s 737-700s.
On 17 April 2018, a Boeing 737-700 (N772SW) being operated by Southwest Airlines on a domestic passenger flight from New York La Guardia to Dallas Love Field as SWA1380 had just passed FL320 on the climb to FL380 in day VMC and was passing abeam Philadelphia when a sudden explosive noise was followed by significant airframe vibration, flight deck indications of a left engine failure and fire and of loss of cabin pressurisation. After following the prescribed immediate response procedures, the flight crew declared an emergency and diverted to Philadelphia. Fire indications ceased but a report from the cabin advised that a left side cabin window at row 14 had been broken by debris and a secured passenger seated nearby partially sucked out of it. That passenger was brought back fully into the cabin but subsequently died. Eight of the other 143 passengers sustained minor injuries but none of the five crew members were injured. After landing, substantial damage was evident on the left side of the airframe and to the left engine and related debris was subsequently found on the ground in the vicinity of the engine failure location.
An Investigation was carried out by the NTSB. Relevant data from the FDR and CVR were successfully downloaded. ATC radar recordings of engine debris falling to the ground were also available and, having been moderated with the recorded wind velocity in the area of descent, it was possible to identify areas where debris could be and some of it was recovered.
It was noted that the 56 year-old Captain had been employed as a pilot at Southwest for 24 years following service as a US Navy pilot and had a total of 11,715 flying hours which included 10,513 hours on type. She had been a 737 Captain since 2000. The 44 year-old First Officer had been employed as a pilot at Southwest for 10 years following service as a USAF pilot and had a total of 6,927 flying hours which included 6,927 hours on type. He had been the designated PF for the flight until the emergency occurred, at which point the Captain had taken over in accordance with Company procedures.
FDR data showed that the engine failure had occurred about half an hour after takeoff from La Guardia. It had been accompanied by a sudden roll from wings level to 41° left wing down before this was checked and the aircraft recovered to wings level. In the flight deck multiple warnings, alerts and failure indications had been displayed, including left engine fire and increased engine vibration and cabin pressurisation failure, the latter only later established as having been due to debris hitting and breaking one of the left side passenger cabin windows. The cabin crew subsequently reported that considerable noise and airframe vibration was being experienced in the passenger cabin. Following their immediate recovery of control, both pilots donned their oxygen masks, initiated an emergency descent and an emergency was declared to ATC. On request, ATC provided initial radar vectors towards Philadelphia and after coordination later cleared the flight there on a direct routing. ATC were advised that although the aircraft was being operated single engine, there was no longer an engine fire indication from the failed engine but that parts of it were missing and some passengers had been injured, one seriously. The fight crew reported detecting some ‘controllability problems’ and for this reason subsequently decided to make the landing with flap 5 set rather than the usual landing settings of flap 30 or 40. After the landing, which occurred on runway 27L some 17 minutes after the engine failure, the aircraft cleared the runway and was then stopped. The emergency services attended and confirmed that there was no continuing hazard and the passengers were subsequently disembarked to buses for transit to the terminal.
Inspection of the failed engine found that parts of the left engine nacelle air intake and fan cowl doors were missing including the entire outer barrel, the aft bulkhead and the inner barrel forward of the containment ring (see the illustration below). There was also considerable damage to the remaining still-attached parts of the left engine fan cowl doors which had opened.
Next to the row 14 left side window which had been broken, a large gouge impact mark which was consistent in shape with a recovered piece of one of the three fan cowl latches which secure the inner and outer fan cowl doors closed could be seen (see the illustration below). No window, aircraft structure or engine material was found inside the cabin.
Overall, collateral impact damage to the airframe attributable to the ejected debris was assessed as “significant” and was evident on the left wing leading edge (see the illustration below), the fuselage side and on the left horizontal stabiliser. It was also subsequently found that although the fan case had not been penetrated from the inside, it did have a hole that corresponded to a fan blade impact mark.
Why It Happened
It was found that one of the 24 fan blades (number 13) in the CFM56-7B left engine had caused the engine to fail when it fractured at its root, a failure referred to by the Investigation as a ‘Fan Blade Out’ (FBO) event. The cause of the fracture was identified as low cycle fatigue, the same cause as that which had previously been identified in an August 2016 (non injury) uncontained engine failure to a Southwest Boeing 737-700 with the same engine type installed for which a substantive report of the NTSB Investigation has still not been released. The fatigue crack in both cases had initiated in part of the blade root known as the dovetail which remained within a slot of the fan disc. The other 23 fan blades showed evidence of trailing edge impact damage, tears and missing material and some also had leading edge tip curl or distortion. Upon completion of the in-situ engine inspection in the current case, these remaining fan blades were removed from the hub and subjected to an ultrasonic inspection which did not find any cracks.
It was established that the failed engine fan blade set had accumulated in excess of 32,000 cycles since new and had last been overhauled in October 2012, 10,712 cycles prior to the failure under investigation. However, metallurgical examination of the failed blade indicated that “the crack had likely initiated before this overhaul”. At the time it was performed, the overhaul had included a Fluorescent Penetrant Inspection (FPI) which was intended to detect any cracks but had not done so for “unknown reasons”, although it was concluded that at this point, the crack would “most likely not have been detectable” using FPI. It was noted that, the crack had also gone undetected during a subsequent on-wing fan blade visual inspection which had been routinely carried out as part of an OEM-recommended fan blade re-lubrication aimed at keeping in-service fan blade loading within the predicted range and “preventing wear on the fan disc and the fan blade dovetail coating”.
The Investigation found evidence that some of the fragments of the failed blade had been ejected forwards into the intake and that impact of others with the fan case had caused local deformation in it over a short period of time which had then been propagated both around and forward/aft of the fan case. After reaching the aircraft structure via the inlet attach ring, this deformation had generated large loads that resulted in local damage to the intake and, together with the forward-travelling fan blade fragments, compromised the structural integrity of the intake and caused parts of it to depart the aircraft.
The extent of collateral damage consequent upon the failure of a single fan blade during the accident sequence was found to have exceeded the assumptions made during type certification dependent on the results of FBO debris containment modelling insofar as the fan blade fragments travelling forward of the fan case in the event under investigation had a trajectory angle that was greater than that observed during this work. The extent of intake damage caused by these blade fragments was also greater than that observed during the same certification assessments which had not adequately accounted for FBO-generated loads transmitted to the fan cowl through the radial restraint fitting and had resulted in them not being accounted for in the design of the fan cowl. It was, however, recognised that since certification of the CFM56-7B engine in December 1996 and the 737-700 airframe in December 1997, “new technologies and analytical methods have been developed that will better predict the interaction of the engine and airframe during an FBO event and the response of the inlet, fan cowl and associated aircraft structures”.
It was noted that after the similar 737-700 FBO event in August 2016, engine manufacturer CFM International had developed an Eddy Current Inspection (ECI) procedure to be carried out during overhaul of CFM56-7B engines in addition to the FPI already required. It was also noted that unlike an FPI, which can only detect surface cracks, an ECI has a higher sensitivity and can also detect cracks near to as well as at the surface. CFM had also responded to this earlier event by developing an on-wing ultrasonic inspection process which could be carried out at the same time as fan blade re-lubrication. It was noted that although the failed blade had not been subjected to these additional inspection processes - nor had it been required to be - where used they had identified 15 blade cracks on separate engines as of August 2019.
The Investigation noted that whilst the performance of appropriate Checklists according to SOPs is a critical part of safe flight operations, given the emergency situation aboard the accident flight, the performance by the experienced flight crew of most, but not all, of the items in the ‘Engine Fire or Engine Severe Damage or Separation’ Non-normal Checklist and the non-performance of the three other relevant Non-normal Checklists had nevertheless “allowed the crew to appropriately balance the procedural requirement of executing checklists with the high workload associated with maintaining aircraft control” whilst completing a safe and timely emergency descent to a landing at an appropriate multi-runway diversion airport.
It was noted in respect of the response to the event by the three cabin crew that although they were aware of the imminent landing, all of them sat on the cabin floor for the landing instead of in their assigned crew seats, contrary to operator SOPs. The fact that neither of the two cabin crew assigned to the forward cabin station were at that location was of specific concern although it was accepted that there had been no actual consequence. It was accepted that this situation had occurred on a full flight with no opportunity for the necessary re-seating of passengers and that this had been a factor in what had occurred. It was also observed that the applicable cabin crew manual “did not discuss any actions to take if no seats were available for a passenger who needed to be re-seated” and further noted that there appeared to be no specific FAA guidance on the subject.
The formally documented Findings of the Investigation included, but were not limited to, the following:
- This accident demonstrated the susceptibility of the fan cowl installed on (all) Boeing 737 NG-series aircraft to a fan-blade-out impact location near the radial restraint fitting and the effects of such an impact on the structural integrity of the fan cowl.
- Given the results of CFM’s engine Fan-Blade-Out (FBO) containment certification tests and Boeing’s subsequent structural analyses of the effects of an FBO event on the airframe, the post-FBO events that occurred during this accident could not have been predicted.
- The structural analysis modelling tools that currently exist to analyse a Fan-Blade-Out (FBO) event and predict the subsequent engine and airframe damage will allow aircraft manufacturers to better understand the interaction of the engine and airframe during an FBO event and the response of the inlet, fan cowl and associated structures in the aircraft’s normal operating envelope.
The Probable Cause of the accident was determined as “a low-cycle fatigue crack in the dovetail of fan blade No. 13, which resulted in the fan blade separating in flight and impacting the engine fan case at a location that was critical to the structural integrity and performance of the fan cowl structure (and which) led to the in-flight separation of fan cowl components, including the inboard fan cowl aft latch keeper, which struck the fuselage near a cabin window and caused the window to depart from the airplane, the cabin to rapidly depressurise and the passenger fatality”.
Safety Action taken in the immediate aftermath of the accident was noted as having been as follows:
- on 20 April 2018, CFM International issued SB 72-1033 applicable to all CFM 56-7B series engines recommending ultrasonic inspections of all fan blades on engines over 20,000 cycles and thereafter at 3000 cycle intervals.
- on 20 April 2018, the FAA issued EAD 2018-09-51 which required, within 20 days of receipt, a one-time ultrasonic inspection (USI) of all 24 fan blade dovetail concave and convex sides on CFM 56-7B engines with 30,000 or more flight cycles since new to detect cracking using the instructions provided in the CFM SB 72-1033 and if any unserviceable indication as specified in that SB is found, the affected fan blade is to be replaced before further flight.
- on 20 April 2018, the EASA issued EAD 2018-0093E with the same requirements for blade inspection and action on findings as the FAA EAD.
The following 7 Safety Recommendations were made upon completion of the Investigation:
- that the Federal Aviation Administration require Boeing to determine the critical fan blade impact location(s) on the CFM56-7B engine fan case and redesign the fan cowl structure on all Boeing 737 next-generation series airplanes to ensure the structural integrity of the fan cowl after a fan-blade-out event.
- that the Federal Aviation Administration, once the actions requested in the first Safety Recommendation are completed, require Boeing to install the redesigned fan cowl structure on new-production 737 next-generation-series airplanes.
- that the Federal Aviation Administration, once the actions requested in the first Safety Recommendation are completed, require operators of Boeing 737 next-generation-series airplanes to retrofit their airplanes with the redesigned fan cowl structure.
- that the Federal Aviation Administration expand the Title 14 Code of Federal Regulations Part 25 and 33 certification requirements to mandate that airplane and engine manufacturers work collaboratively to (1) analyse all critical fan blade impact locations for all engine operating conditions, the resulting fan blade fragmentation, and the effects of the fan-blade-out-generated loads on the nacelle structure and (2) develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.
- that the Federal Aviation Administration develop and issue guidance on ways that air carriers can mitigate hazards to passengers affected by an in-flight loss of seating capacity.
- that Southwest Airlines include the lessons learned from the accident involving Southwest Airlines flight 1380 in initial and recurrent flight attendant training, emphasising the importance of being secured in a jumpseat during emergency landings.
- that the European Aviation Safety Agency expand certification requirements for transport-category airplanes and aircraft engines to mandate that airplane and engine manufacturers work collaboratively to (1) analyse all critical fan blade impact locations for all engine operating conditions, the resulting fan blade fragmentation and the effects of fan-blade-out generated loads on the nacelle structure and (2) develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.
The Final Report was adopted by the NTSB on 19 November 2019 and subsequently released on 18 December 2019.