The cowling is actually not very strong, it’s more for aerodynamics optimized inlet flow and reduced form drag. The shroud is what constrains the fan and the casing the remainder of the compressor and the turbine disks. The shroud did its job, but one or more of the turbine disks look like they let a blade or two go and punched through the casing and a fuel nozzle ring line...wish there was more vid to see how long those flames kept going.First thing that I thought of was "thankfully the cowling would have kept the blades inside..."
Secondary flames well outside the combustion chamber. I’m unassumadly hypothesizing a component release (aka compressor or turbine blade) that interacted unfavourably with a fuel distribution component (manifold or metering line) that caused a secondary flame event in the outer air plenum. The flames are visibly present in the bypass plenum, showing through the reverser vanes (the visible grid-like structure enveloped in flames). No way that is directly related to the deeply contained combustion chamber, which is positionaed significantly further aft in the engine than the video shows. Going carefully out on a limb here, I think it likely that the low-pressure section of the compressor failed and wasn’t fully contained and ruptured through the forward compressor casing and compromised some of the fuel distribution components, causing an atmospheric combustion event in the bypass plenum. Depending on where the affected fuel line was relative to the fuel control unit, it’s possible that an engine cut-off signal from the cockpit ceased fuel flow to the combustion chamber, but was still downstream from the rupture site, so fuel could still have flowed at low pressure ahead of the FCU cut-off, not high-pressure, but enough to allow the flames seen in the video. I don’t know the specific valving of a United 777-200 to know what fuel flow is stopped where when the throttle is closed or fire handle is pulled, but the pilots, flight attendants and passengers alike handled it well. Particular shout out to the passenger videoing the event, who kept the video focused on the event and didn’t drop his phone and didn’t fill the air with expletives...this is far above the standard Internet abilities of amateur videographers...BZ.It was like watching a transparent engine; the fan's turning up front and ignition is happening behind.