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B737, en-route, northwest of Philadelphia PA USA, 2018

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Revision as of 15:02, 21 May 2018 by Content.Manager (talk | contribs) (Investigation)
Summary
On 14 April 2018, a sudden uncontained left engine failure occurred to a CFM56-7B powered Boeing 737-70 as it climbed through approximately FL 320 abeam Philadelphia. Ejected debris broke a cabin window causing rapid decompression and the death of a passenger seated nearby. Diversion to Philadelphia followed without further significant event. The same day, the Investigation, which is continuing, found that the failure was due to metal fatigue in a single fan blade causing it to shear from the hub. The engine manufacturer subsequently issued inspection requirements for similar engines and Airworthiness Directives based on this were immediately issued.
Event Details
When April 2018
Actual or Potential
Event Type
Airworthiness, Loss of Control
Day/Night Not Recorded
Flight Conditions Not Recorded
Flight Details
Aircraft BOEING 737-700
Operator Southwest Airlines
Domicile United States
Type of Flight Public Transport (Passenger)
Origin LaGuardia Airport
Intended Destination Dallas Love Field
Actual Destination Philadelphia International Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Climb
ICL / ENR
Location En-Route
Origin LaGuardia Airport
Destination Philadelphia International Airport
Location
Approx. 65 miles northwest of Philadelphia International Airport
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General
Tag(s) En-route Diversion
LOC
Tag(s) Airframe Structural Failure,
Loss of Engine Power,
Extreme Bank
EPR
Tag(s) Emergency Descent,
“Emergency” declaration
AW
System(s) Engine - General
Contributor(s) Corrosion/Disbonding/Fatigue,
Ejected Engine Failure Debris
Outcome
Damage or injury Yes
Aircraft damage Major
Injuries Few occupants
Fatalities Few occupants (1)
Causal Factor Group(s)
Group(s) Aircraft Technical
Investigation Type
Type Independent

Description

On 14 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 flight 1380 was climbing through approximately FL 320 and 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 pressurization. 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 nearby passenger 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, with the 5 crew members uninjured. After landing, substantial debris impact damage on the left side of the airframe was evident and some of this debris was subsequently found on the ground in the vicinity of the failure location.

Investigation

An Investigation is being 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 displayed together with the wind velocity in the area of descent, these allowed to identify areas where debris could be and then recovered. It was noted that the First Officer had been the designated [PF]] for the flight and that during the diversion, the Captain had taken over this role.

FDR data showed that the engine failure had occurred as the aircraft was climbing through approximately FL 325 twenty minutes after takeoff from La Guardia. It was accompanied by a sudden roll from wings level to 41° left wing down before this was checked and the aircraft quickly 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. Considerable vibration was also apparent in the passenger cabin. The flight crew both donned their oxygen masks, an emergency was declared to ATC and an emergency descent was initiated. Once that was complete, ATC provided radar vectors to Philadelphia. They 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 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 landing, which occurred on runway 27L 22 minutes after the engine failure, the aircraft cleared the runway and was stopped. The Emergency Services attended and the passengers were disembarked to buses for transit to the terminal.

Inspection of the failed engine found that most of the engine air intake 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 observed to be intact but had evidence of numerous impact marks. The fan case had not been penetrated but it did have a hole that corresponded to one of the fan blade impact marks and fan case tearing (see the illustration below).

Damage to the engine cowl (inboard). [Reproduced from the Investigation Update]

One fan blade, number 13 of 24, was missing having separated at its attachment to the hub. Two pieces of this fan blade were recovered from within the engine between the fan blades and the outlet guide vanes and they showed features consistent with metal fatigue on their convex side near the fan blade leading edge. All the other 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, the 23 remaining fan blades were removed from and subjected to an ultrasonic inspection which did not find any cracks.

It was established that the failed engine fan blades had accumulated in excess of 32,000 cycles since new and had last been overhauled in November 2012 - 10,712 cycles prior to the failure under investigation using visual and fluorescent penetrant inspections. It was noted that, in response to a previous fan blade failure with similar consequences which had occurred to another Southwest Airlines CFM 56-7B powered Boeing 737-700 in August 2016 which is still under investigation, eddy current inspections of fan blades had been added to the engine overhaul requirements.

Maintenance records for the failed engine show that its fan blades had been periodically lubricated as required per the AFM. The Investigation is now seeking to estimate the point at which the fatigue cracking in the failed blade began and assess its propagation so as to establish whether current methods used to detect such cracks are sufficient.

Collateral impact damage to the accident aircraft airframe attributable to the ejected debris was assessed as “significant” and was evident on the leading edge of the left wing (see the first illustration below), the side of the fuselage and the left horizontal stabilizer. Next to the row 14 window which had been broken, a large gouge impact mark which was consistent in shape to a recovered piece of the fan cowl and latching mechanism could be seen (see the second illustration below). No window, aircraft structure or engine material was found inside the cabin.

Left wing leading edge damage. [Reproduced from the Official Report]
The left side window at row14 with the ‘matching’ part of recovered engine fan cowl. [Reproduced from the Official Report]

Safety Action taken by the engine manufacturer CFM International on 20 April 2018 was noted as the issue of SB 72-1033 applicable to CFM 56-7B series engines recommending ultrasonic inspections of all fan blades on engines over 20,000 cycles and thereafter every 3000 cycles. This was used as the basis for the issue of FAA EAD 2018-09-15 which required a 30,000 cycles or above inspection and also, within 20 days of issue that all CFM56-7B engine fan blade configurations must be ultrasonically inspected for cracks per the CFM SB with replacement of any fan blade with crack indications before further flight. On equal date to the FAA AD, the EASA issued EAD 2018-0093E with the same blade inspection and blade inspection response actions to be performed.

An Investigative Update was issued by the NTSB on 3 May 2018 and this summary is based on information contained in it and the two on site briefings given by the NTSB Chairman on the day of the accident and the day following. The Final Report is not expected until 12-15 months after the event occurred.

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