The Boeing 737-700's engines are designed to contain its parts and not shed them outside the engine cowl. (The cowl is the surrounding enclosure, analogous to an automobile's hood.) The current speculation is that
metal fatigue from one of the numerous fan blades that rotate at incredibly high RPM was the culprit.
It's a mystery why the fan blade was not contained in the engine after it broke off.
The fan blades are the first stage in the process of creating thrust in a jet engine. Which means that a piece of metal was sent smashing through almost the entire machine, decimating more engine parts along its path, similar to the result of throwing a pen through a typical cooling house fan. The entire engine became unbalanced, shaking itself into pieces.
A fan blade crack is almost invisible, impossible for a pilot or aviation mechanic to detect via a visual inspection on the ground. Fan blades are attached to discs that spin around a central shaft in various sections called stages, most of them not visible even when peering directly into a jet engine. Mechanics use an "ultrasonic test," which requires special instrumentation. Southwest's procedures for inspection will be reviewed by the National Transportation Safety Board.
The triumph amid the tragedy in this event is the response of the crew. An engine failure is a scenario that airline pilots practice for as a routine on every recurrent training session. That being said, the Southwest pilots were faced with multiple emergencies. Not only did an engine failure occur, but so did an explosive pressurization event as a result of the cabin being punctured by shrapnel. Air was being expelled rapidly from the airplane.
In addition, the leading edge of the left wing was peppered with pieces of engine parts, which most likely created some type of abnormal flying characteristics. Fortunately, the characteristics did not prevent a successful landing in Philadelphia. As a precaution against potential flight control issues, the crew landed with fewer flaps than on a normal approach, resulting in a higher speed for landing.
What would have been happening in the cockpit during this emergency? The pilots most likely heard an explosion of sorts and then felt a vibration. They would have immediately checked their engine instrumentation and realized they had a problem. It's important to note that the wings, and thus the engines, are located beyond the pilots' field of vision from the cockpit, so it's impossible for a pilot to see or know exactly what occurred. A call from a flight attendant in the cabin would have confirmed the specific nature of the problem.
In this circumstance, judging by the fact that the captain's voice was heard on the radio, the copilot's duties would have been to begin the severe damage checklist, which involves, among other things, shutting down all fuel to the engine. The captain would have been flying the airplane and talking on the radio with air traffic control, declaring an emergency. This is a trained division of duties between pilots, with the non-flying crewmember focusing on checklists while the other crewmember focuses on flying the airplane.
When the pilots realized that the catastrophic engine failure was accompanied by a pressurization issue, they would have begun the memory items involved with that particular checklist. The priority with an explosive pressurization event is for the crew to don their oxygen masks and then rapidly bring the airplane down to an initial habitable breathing altitude of 10,000 feet, using flight control devices on top of the wings called speed brakes, which are designed to reduce lift.
Speed brakes are often used as part of normal operations for descents, deployed in smaller increments. They do create some buffeting vibration, which may have been magnified for the passengers on Flight 1380, as some indicated.
With a rapid loss of cabin pressure, the oxygen masks deploy automatically. As a side note regarding passenger oxygen masks, remember the briefing from the flight attendant that instructs you to "place the mask around your nose and mouth?" Do it, please. Photos of the cabin on Flight 1380 indicate most passengers were just holding the yellow cup to their mouth. You can't get the full benefit of breathing without using your nose. Enough said.
The calm, cool and collected voice of the captain is a testament to her experience and her training. Both pilots deserve accolades for their performance, as do the flight attendants handling the trauma in the cabin. Yes, this was a tragedy, but it was triumphantly handled by professional aviators. Kudos to all involved, including the passengers valiantly assisting the woman who unfortunately did not make it, and the air traffic controllers that expedited the emergency to a successful landing.
And finally, credit will be due shortly to the parties in the NTSB investigation. They will follow the clues to find the reason for this event, to prevent it from ever happening again.