From Engineering,Com
By Andrew Wheeler
It's every passenger's nightmare: flying miles above the Earth and an explosion occurs. You might check the loudness of it relative to your position on the plane. Air rushes, objects are flying around the cabin means there’s a hole somewhere…Oxygen masks drop from the ceiling, confirming your fears.
On Tuesday April 17th, 2018, this nightmare became real. A Boeing 737-700 flown by Southwest Airlines, with 144 passengers inside, two jet engines underneath, took off from LaGuardia (NY) for Dallas. About 20 minutes after takeoff, the flight reached cruising altitude of 32,000 ft and the captain turned off the seat belt sign. But, encountering turbulence, the crew requested seat belts back on, according to a passenger. Right after, the left engine seemed to explode and debris broke a window. A passenger was sucked into the window and was struck, half in and half out. Passengers tried to pull her back into the cabin, succeeding only after the plane performed a rapid descent and an emergency landing, but the damage was done. The passenger died from blunt force trauma to the head, neck and torso.
The Victim
Several rows back from the fan blades, in the window seat sat Albuquerque resident and mother of two Jennifer Riordan. There had been an announcement of turbulence, according to a passenger on the aisle seat of the same row, and both the middle seat and the aisle seat passengers had secured their seat belts. Riordan had secured her seat belt, too, say NTSB inspectors.
Cabin Pressure
The pilot, in this case former US Navy Pilot Tammy Jo Shults, set the cruise altitude on a cabin pressure controller during pre-flight procedures. After the wheels come off the ground during takeoff, the outflow valve began to close, which initiates pressurization of the cabin.
The 737-700 would take about 20 minutes to ascend to a cruising altitude of 30,000 feet. That is when flight SWA 1380 lost its blade. The cabin air pressure is about 10 pounds per square inch, which is equal to the air pressure at about 6000 feet above sea level.
The stronger the structure of an airplane, the more differential pressure it can tolerate. The average is 8 lbs. per square inch. When the cabin window of flight SWA 1380 was shattered, the pressurized air blew outward.
According to Federal Aviation Regulations, pilots begin to need oxygen when they fly above 12,500 feet for over 30 minutes without cabin pressurization. Passengers however, need to use oxygen continuously anywhere above 15,000 feet in altitude without cabin pressurization.
After the engine failure, Shults and her co-pilot descended to 10,000 feet in just five minutes, high enough to not hit anything, but low enough so that everyone aboard can breathe safely.
Engine Failures
Commercial airplanes have a variety of safeguards to protect the passengers during engine failures—the use of multiple engines, for example. Pilots are trained to cut off fuel to a failing engine and land a plane on the remaining engine.
A modern jet engine assembly has thousands of moving parts, many of them rotating at very high speeds. The fan blades, visible from the front, are the biggest. The largest of jet engines spin the fan blades at about 4,000 RPMs generating forces of up to 7,000g. Parts can exit the cowling at speeds of 1,000 ft/s, about the speed of a handgun bullet, but with considerably more mass.
A turbofan blade’s tendency to break off is well documented and has caused a number of tragic mishaps. A turbine blade part will destroy the engine, but the plane will most likely land. Passengers will have a very bad day, but will be able to walk out to kiss the ground. The Federal Aviation Administration (FAA) calls this a category 3 event. Category 4, the dreaded uncontained engine failure, occurs when the blade or debris exits the engine and fatally damages flight systems, rips into the aircraft cabin and/or causes a crash landing—or worse
Uncontained Engine Failure
A jet engine under development must show that it can withstand an uncontained engine failure. In the video below is what may be the first such test available to the public, test engineers hold their breath as a Rolls-Royce engine destined for the Airbus A380 spins up. An explosive device shoots a fan blade off its mounting, reducing the engine to a smoking ruin. In a protected facility, the engineers exhale and congratulate each other. Nothing shot out of the engine where it should not have.
The number of uncontained “gas turbine engine rotor failures” has been on a steady decline over the years, as reported by the FAA in a 1997 report, even though the number of miles flown has increased.
Protection rings surround the fan blades to contain them should they come off. In this and the previous Southwest failure, the containment rings seem to appear intact. However, in both cases, the inlet cowl was torn away and is completely missing. It is likely that pieces of the inlet cowl formed a cone of debris, with at least one piece striking the window of the airplane.
The fuselage is protected by a Kevlar band wrapped around it. It's wrapped in the same position aft on the plane as the fan blades. That’s why there’s no windows there [row 11], according to John Baker, PE, in EngTips, our online engineers forum.
In light of damage occurring from debris blown back from the engine in flight, it would seem the Kevlar bands, in line with the fan blades would only offer protection while the aircraft is on the ground, which is not when the blades are spinning their fastest and would be encountering their highest forces.
U.S. National Transportation Safety Board (NTSB) investigators at Philadelphia airport found that the No. 13 fan titanium alloy blade broke off near the disk hub. The disk hub was examined, and evidence of fatigue cracking was discovered.
According to NTSB Chairman Robert Sumwalt, the fan blade separated in two places, and that it appears the secondary failure of the turbofan engine was caused by this fatigue fracture. Sumwalt described the damage to the leading edge of the left wing, saying it “was banged up pretty good,” and that they could “see paint transfer.” The NTSB found no acrylic shards from the windows inside the airplane around row 14, where the victim was seated, and the window was broken.
Sumwalt spoke highly of the CFM56 engines and the entire Boeing 737 fleet, but indicated awareness about the similarity of the uncontained engine failure of the August 27thSouthwest Airlines flight of 2016.
Southwest
An August 2016 Southwest Airlines flight 3472 from New Orleans to Orlando also suffered an uncontained engine failure, also from a from a fan blade breaking off and also destroying the front cowling. It also made a foot-long tear in the wing. However, flight 3472 landed with all 104 passengers and crew intact.
After the 2016 incident, engine manufacturer CFM issued guidance protocol for ultrasonic inspection of specific high-time fan blades and the FAA released a proposed Airworthiness Directive to require engines that logged more than 15,000 cycles-in-service to undergo ultrasonic inspection in June of 2017.
According to Reuters, Southwest, along with other airlines, pushed back on CFM’s protocol, saying the engine manufacturer had “vastly understated” the cost and number of engines in operation and inspecting all engines in 12 months was not enough. Southwest also requested not all fan blades be inspected. The FAA proposed the testing be done in 18 months, to which the airlines agreed. However, there is little to indicate that the tests were done as proposed. Former NTSB chairman Mark Rosenker said, “There did not seem to be an urgency” at the FAA to complete the inspections.
MRO-Networks.com reports that Southwest was looking to cut maintenance costs by using parts from old engines in a 2012 report. It is not known if the fan blades were taken from old August 2016 engines.
Southwest CEO Gary Kelly said the engine had logged only 10,000 cycles since being overhauled. A cycle is one takeoff and one landing. The plane had been inspected Sunday. NTSB inspector Sumwalt said in him preliminary report that a crack appeared towards the inside of the fan blade and would not have been visible in a visual inspection.
The Engine
In August 2016, Southwest Airlines Flight 3472 suffered a similar failure, an uncontained engine failure with a CFM56 turbofan engine -- the same engine as last week's Flight 1380.
The CFM56 engine is the world’s bestselling jet engine, according to CFM, with over 30,000 delivered and powering both Boeing and Airbus planes. CFM is a joint venture composed of equal parts GE (US) and Safran (France).
Southwest announced in 2012 that it is phasing out the 737 "Classic" with the CFM56-3 engines starting in 2012 and finishing by 2017. Several parts on the CFM56-3 engines are interchangeable with the CFM56-7 which is used on Boeing 737-700. It is not knows if the turbine blades in either incident were from the older engines.
What Happens Next?
In the next two weeks, the FAA will issue an airworthiness directive requiring inspections of specified CFM56-7B turbofan engines. According to a statement released by the FAA on April 18th, “The directive will require an ultrasonic inspection of fan blades when they reach a certain number of takeoffs and landings. Any blades that fail the inspection will have to be replaced.”
In response to the recent tragedy, Southwest released a statement saying they would be accelerating ultrasonic inspections of CFM56 engine fan blades, which they said would take about 30 days. Southwest Airlines reported operating 693 737-700/800s as of December 31st, 2017.
