A Boeing 737-700 was airborne out of New Orleans International Airport (Louisiana, USA) on this day in 2016, with a crew of 5 and 99 passengers onboard, en route to Orlando International Airport (Florida, USA).
The damaged engine and inlet (© NTSB)
It was the first flight of the day, for both the crew and the aircraft. The take-off and initial climb had been uneventful until the aircraft climbed through flight level 310 to reach its cruise flight level. At that time a loud bang was heard, the aircraft yawed and the parameters for the left-hand ( #1) engine had changed. The fan speed (N1) had dropped from 99% to 39% and strong vibrations were felt throughout the aircraft. An emergency was declared and the Engine Fire or Engine Severe Damage or Separation checklist.was completed by the crew. Shortly after completing the checklist the Cabin Altitude Warning sounded. The crew donned their oxygen masks and an emergency decent was initiated.
Fanblade installation (Source; CFM)
As the aircraft the fan speed continued to winddown, however the vibrations remained high. A diversion to Pensacola (Florida, USA) was initiated. ATC informed the crew to expect a ILS (Instrument Landing System) approach to runway 17. Early in the approach to Pensecola the crew configured the aircraft step by step for landing to assess the aircraft handling properties, flaps and slats were selected in steps, and autobrake was selected to level 3. A stabelised single engine approach was flown and a single engine landing was performed, 21 minutes after the start of the emergency.
The engine in flight (source; www.B737.org.uk © unknown)
The aircraft received substantial damage, none of the occupants of the aircraft were injured. The accident was investigated by the National Transportation Safety, Board (NTSB), in their investigation report the damage to the aircraft is given as;
16" x 5"puncture in the left fuselage (Causing the depressurization of the cabin)
Numerous impacts on the left side of the fuselage
Numerous impacts on the left wing
Numerous impacts on the left horizontal stabiliser
Left engine inlet missing for the large part
Sheared rivets on the aft bulkhead of the inlet
Fanblade #23 had failed at the blade root, and was not found
All remaining 23 fanblades sustained hard body impact damage
Wing and fuselage damage (left), fuselage tear (right, under yellow tape on left picture) (© NTSB)
The National Transportation Safety Board determines the probable cause(s) of this accident to be:
"A low-cycle fatigue crack in the dovetail of fan blade No. 23, which resulted in the fan blade separating in flight and impacting the fan case. This impact caused the fan blade to fracture into fragments that traveled farther than expected into the inlet, which ompromised the structural integrity of the inlet and led to the in-flight separation of inlet components. A portion of the inlet struck the fuselage and created a hole, causing the cabin to depressurize."
The engine manufacturer developed an amended inspection method fo the CFM56-7B engine fanblades. The full NTSB report, on which this blog is based, is available for readers'reference by clicking on the .pdf file below.