Southwest Airlines flight experiences engine explosion but makes a safe landing in Philadelphia... 5

[JohnRBaker]

Southwest flight makes emergency landing

https://www.cnn.com/us/live-news/southwest-flight-emergency/

John R. Baker, P.E. (ret)
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[JohnRBaker]

What surprised me is how far back it was where the window was damaged, as seen in the this photo of the plane:

This would indicate that perhaps it was not the main turbine that failed or at least it was not the only part of the engine that was damaged. Generally speaking, if a problem occurs with the main turbine, like a blade breaking or coming loose, they will fly directly outward in a radial direction, which is why there are NO windows directly inline with the main turbine, which can be seen in the photo below between the "w" and "e" in the "Southwest" logo on the side of the plane.

In place of the missing window, the fuselage is wrapped with Kevlar.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[cranky108]

According to your diagram, the missing window lines up with the wing door. Sort of a secondary question is how far back does the engine go from the wing, and does the back of the engine line up with that window?

If so, does that indicate something from the rear of the engine, like a end turbine blade?
Not as likely as a forward turbine blade, but still possible.

 

[Spartan5]

One dead after being partially sucked out of the window. Keep that seat belt on, folks.

 

[Strong]

More likely scenario is that a piece of the engine nacelle broke and went over the top of the wing on a curved trajectory. Not a good idea to choose a window seat on SW!

Walt

 

[JohnRBaker]

The emergency doors over the wings are further back than where the missing window is located, and the engine is actually mounted such that most of it is not under the wing but rather ahead of it. And the actual 'jet engine' only occupies about one-half the length of the engine housing and that's mostly in the rear half. The front half is where the so-called 'fan' is located and it's that part of the engine where the largest blades are found and why there is that Kevlar shield around the fuselage.

As for SW and this particular type of engine, they're not the only airline using them. In fact, my wife and I are flying home tomorrow morning, from Detroit back to SoCal, on American Airlines. And we will be flying in a Boeing 737-800, the same model as the SW jet involved in today's incident, and they are using the same engines, the CFM56-7B.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[3DDave]

Blown or pushed, not sucked. No one says a champagne cork is sucked from the bottle.

pprune.org is showing photos that indicate the fan is generally intact, but the last one, in Aug 2016, was attributed to a single fan blade separation. Apparently the damage to a woman passenger was from shrapnel penetrating the plane. Reports are that some passengers were trying to block the hole from the missing window.

Previous accident analysis

http://aerossurance.com/safety-management/uncontained-cfm56-failure-b737/

 

[JohnRBaker]

For reference purposes, here's a cross-section of a CFM56-7B engine showing how it's positioned inside the engine housing:

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[JohnRBaker]

Correction: The Southwest airliner involved in today's incident was a Boeing 737-700, not an -800. The -700 is about 20 feet shorter than an -800.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[Spartan5]

JohnR: Possible over-pressure of the cowl anti-ice system?

3DDave: No one says that liquid is being blown up a straw either *shrug*

 

[3DDave]

That's because an external system is acting to locally lower the pressure on the soda straw. Since the pressure outside the plane at any altitude is essentially no different whether the plane is there or not there's no suction. It's pressurization inside the plane that does all the pushing. In addition, once the plane pressurization is relieved, so is the push.

The same thoughts about over pressure of deicing were brought up the last time this happened. But it was still a fan blade, separated at the root that got chucked forward and then out. In March of this year the French/Europeans issued an order to inspect all fan blades for this engine type within the next 9 months.

 

[Spartan5]

3DDave I get it now. The next time I am draining an aquarium via gravity and a fish is suddenly drawn into the orifice when it swims too close to it, I'll be sure to explain to everyone how it just got blown out of the tank. Thanks for setting me straight

 

[hokie66]

Push or pull, however you want to look at it, it is the differential that matters.

 

[Compositepro]

The picture of the actual plane shows that the missing window is the fourth one back from the exit door over the wing.

 

[IRstuff]

I think John was referring to the as-designed skipped window, as opposed to the broken window.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[hokie66]

The missing window look like it is behind the wing. If there is a place engine parts are supposed to impact the fuselage, they missed.

 

[SwinnyGG]

If there is a place engine parts are supposed to impact the fuselage, they missed.

I'm sure part of the NTSB's findings will include suggested modifications to the Rapid Unplanned Engine Disassembly Component Collection Structure (RUEDCCS) of all future airliner designs.

Ok maybe not.

Wonder if what struck the window was a chunk of cowling.. My suspicion is that any part of the rotating assembly would leave the engine with enough force that it would impact the airframe in plane with where it had just been, or maybe just a little aft. A big piece of cowling would have its trajectory and relative speed influenced by airflow a lot more.

 

[Compositepro]

Jet engines have a Kevlar containment shroud inside the nacelle (cowling). In the photo the nacelle is now missing and the yellow containment shroud is visible. Propeller planes have the protection only on the fuselage for protection against detached blades. I have seen the specialized loom that is used to weave the fabric that is used for the shroud. The fabric is wound onto a spool that has the same barrel shape as the completed shroud. Test video here:

https://www.youtube.com/watch?v=j973645y5AA

 

[3DDave]

When a fan blade separates it is under both radial load from rotation and axial load from thrust production. It was theorized that there is a combination of the two that sends the blade forward from the fan-disk and out through the inlet. Others think that other blades just bash the loose one out the front. There is often extra damage to the fan and not enough fragments of the inlet to be certain about those few milliseconds. The blades aren't especially dense for the projected area they have and I expect they can deflect as they tear through the cowling. The last time a blade exited it left a considerable dent about mid-chord, just below a window, leading to a smaller leak. I think it was the blade as nothing in the inlet seems substantial enough to do the damage and that it just happened to shatter the window in an unfortunate trajectory.

There was an incident some years ago where a fan blade walked forwards out of the fan disk due to a resonant condition; This was in spite of a retainer and an estimated 113kips radial load. Since it was out-of-plane with the disk it lined up with a window when it went through the engine case; a sleeping, unbelted passenger was extruded out the window opening. I believe it was an 8.7 psi differential with a 16 * 10.6 window, so the passengers struggling to hold him needed to hold with nearly 1500 lbf; they were unsuccessful and I believe his body was never found.

https://www.fss.aero/accident-reports/look.php?report_key=124

Much more damage was done to the aircraft as the entire fan disk departed due to imbalance and numerous blades severed controls and fuel tank.

A person's chance is much better if they aren't a cork.

https://en.wikipedia.org/wiki/British_Airways_Flight_5390

 

[Sparweb]

Although it seems pretty obvious who was fatally injured, it's not necessarily true that the passenger partially sucked out the window is the person who died, since they were pulled back inside and (presumably) offered oxygen. I will wait for clarification from the airline or NTSB, rather than jump to that conclusion. But of course it's pretty likely that's who it was. Also not clear at the moment is what led to the passenger's fatal injuries. If the assumption above is actually correct, there is still the question of whether debris or decompression dealt the fatal blow.

I once did a detailed decompression analysis on a jetliner, and I think it is safe to say that the decompression in a 737 fuselage, escaping through a window, would take just a few seconds to completely exhaust the cabin pressure down to the ambient pressure, but that's probably obvious to anyone. At 32,000 feet altitude, the atmospheric pressure is 4.4 psi, absolute (30 kPa,abs). The cabin differential pressure in a 737 is about 7.5 psi,gauge. The pressure differential would drop to about 50% in the first second of decompression, if the window was ruptured but unobstructed.

It's a pretty horrible event no matter where you sit, but I would not have expected the debris to take out a window that far aft from the engine nacelle.
There is design guidance from the FAA on this matter. Some of you may find it informative: FAA Advisory Circular 20-128A, Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor and Fan Blade Failures.

Also studied by the US Navy: DOT/FAA/AR-04/16 Uncontained Engine Debris Analysis / Damage Assessment Model.

Note the Spread Angle ranges presented in Table 3-2 of the Navy report. The victim window in the SW 737 is at the extreme aft range of the predicted debris spread angle. It makes me wonder if some other event has occurred as a result of, or in conjunction with, this engine failure. The possibility is very small: Occam's razor says that it was the fan debris that ruptured the window, of course.


STF