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#21
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Looking at the few available photos... the busted window sure doesn't seem to be where it would be expected for a fan blade failure. It is several rows behind the visible damage to the engine and there doesn't appear to be any major damage to the wing in the vicinity of the window. I wonder if the window failed because of flexing of the plane's superstructure caused by the engine failure instead of because it was hit by shrapnel. Aluminum flexes, glass does not.
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#22
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ETA: I wondered about the lack of damage to the fuselage too. It is possible that the window was blown out of the frame by stress, but I'm not sure if that would result in the apparent injuries. Several passengers including the woman suffered lacerations. The woman could have suffered those from the window frame and the others from when they pulled her back in and/or tried to plug the window. Last edited by GenYus234; 18 April 2018 at 05:27 PM. |
#23
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On the other hand, the fan discs in the cowling are almost all in front of the wing. I'm not dismissing the possibility that a rapidly rotating piece being flung outward wouldn't be carried over the leading edge of the wing in the airflow and into the fuselage above it. I'm going to wait on the investigation, though. ~Psihala |
#24
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For those who are interested, the NTSB has a Youtube channel and they posted a "B-Roll" video of the damage. None of it shows the interior of the airplane or really the fuselage, but here 'tis anyway:
~Psihala |
#25
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It really doesn't look like the rotor failed. Instead it looks like the cowling failed and then the engine ingested parts of that, which damaged the first fan blade. (Or something pretty darn big hit the engine.) Lots of curious damage to the wing's leading edge well away from the engine.
I guess we'll know what happened in a couple years. |
#26
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Any jolt delivered by the engine hard enough to disrupt the fuselage structure to that degree would simply result in the engine tearing itself and its pylon off the wing. They used to design the pylons exactly for that unlikely event, with "fuse pins" of calibrated shear strength. But after the debacle of the 747 shear pin fatigue failure, I don't know if they still do that.
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#27
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I'm pretty sure that all modern airliners are reasonably stable around every axis. What I've heard is that Airbus tends towards narrower margins of static stability, owing to their confidence in their computer control systems. But even they are still in the positive range, and still controllable when system failures drop you into direct control modes. |
#28
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#29
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This is a terrible tragedy that could have been worse without such a competent crew (and passengers). But it ends almost five years with zero fatalities in commercial airlines. What an amazing safety record.
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#30
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It sounds utterly terrifying, but hearing the total calm in the voices of the cockpit crew while they communicated with the tower was reassuring. "Well, a piece of the aircraft is missing, so we'll be going a little slower."
On the other hand, the number of people in pictures wearing the oxygen masks only over their mouths is a little disheartening. I mean, even if you missed the safety demo, it seems pretty obvious to me that you'd want to breathe normally, you know, through your nose. Obviously it didn't cause any major problems -- as I understand it, the plane descended rapidly after the decompression, so I'm not sure how long the oxygen was needed for. |
#31
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Alaska 261 excepted, I think that most airliners have positive static stability in inverted flight, in the sense of having a positive pitch input force gradient throughout the operational speed range. I'd also bet that most airliners have enough pitch trim authority that you can trim them to fly hands-off inverted. I don't know about the roll axis, but that's a second-order issue. Yaw is a non-issue; vertical stabilizers don't care which way is up as long as they stay attached (an important conditional for airliners of the Airbus persuasion).
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#32
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*There was the runway overrun accident at Chicago Midway about a decade ago that killed a child in a car, though. As I understand it, the oxygen supplied to those masks is only enough to last for a couple of minutes, anyway. Standard procedure after a decompression is to descend to 10,000 feet as quickly as possible, and the oxygen masks are just to keep the passengers from passing out in the few minutes it takes to descend to where there's breathable air. Last edited by WildaBeast; 19 April 2018 at 06:07 AM. |
#33
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ETA: I didn't refresh before posting. . . . Thanks WildaBeast, that's what I was wondering about. |
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#35
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~Psihala |
#36
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No need to, the barrel roll was not inverted flight as I've been talking about it. That is, flight where the vertical forces acting on the aircraft are the reverse of normal flight. Note the opening paragraph:
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FETA: The Vomit Comet can maintain micro-gravity conditions for about 25 seconds. Super shorthand math would suggest that an inverted 1g dive would be shorter than that. Last edited by GenYus234; 19 April 2018 at 04:25 PM. |
#37
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I'm not doubting it.
~Psihala |
#38
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First off, my usual disclaimer: I am not an engineer, aero or otherwise. I have designed a racing sailplane (www.hpaircraft.com), two end-to-end aircraft flight control systems, and various bits for racing aircraft ranging from gliders to jets, but most of that is on the basis of the usual calculus sequence, one class in engineering statics, and consultation with real aero engineers where necessary. My understanding of stability and control is only a little bit above what any small airplane pilot or RC model builder has (shoutout to HP-24 project follower Martin Simons!). Basically, my rationale is this: In simple terms, the first-order driver of pitch stability is the relationship between the aircraft center of gravity and its neutral point. The neutral point is basically a longitudinal location where aerodynamic forces balance out. If the CG is aft of the neutral point, the airplane is longitudinally unstable. If the CG is forward of the neutral point, it's stable. The further the CG is ahead of the neutral point, the more stable the airplane is. The first order driver of the location of the neutral point is the airplane's planform, that is, its shape when viewed from above. Profile considerations such as the airfoil sections of the wing and tail, and of the pitching moments of those sections, have an effect, of course. But unless we're talking about tailless aircraft, those effects are pretty small in comparison to the planform. Here's a typical online neutral point calculator intended for light aircraft and models. Observe that it has no inputs whatsoever for profile characteristics, or even the fuselage. All it cares about are the sizes and shapes of the wing and tail, from which it derives the mean aerodynamic chord (MAC) and a bunch of stability parameters. Given that the airplane planform is the same when looking down from above or up from below, I think that it is a reasonable assumption that longitudinal stability doesn't really care which way is up. The conventional thinking (which is usually but not universally true) is that the tailplane presses down to counter the moment applied by the CG being ahead of the neutral point. The result is a balance of forces and an equilibrium. If the airplane is disrupted and pitches down, it will tend to speed up. As it speeds up, the wing produces more lift (upward) and the tail produces more lift (downward). The result is that the airplane pitches up and slows down. Likewise, if the airplane is disrupted so that it pitches up, it slows down, the wing produces less lift, the tail produces less down force, and the airplane pitches down and speeds back up. Of course, there are some additional considerations for sustained inverted flight. Like, does the airplane's pitch control (elevators, the movable surfaces on the trailing edge of the horizontal surface at the tail) have enough range of motion to sustain level flight? That is, if the airplane is upside down and the pilot presses the control yoke forward, can they raise the nose upwards (away from the ground) far enough to keep the airplane from overspeeding? On that point, my thinking is that you probably couldn't certificate a Part 25 airplane unless it had enough control authority to recover from unusual attitudes including excursions into the -1g flight regime. Turbulence happens, and all that. On that basis, I think there is a reasonable basis to assume that the pitch controls have enough authority for sustained -1g (inerted) flight. Bottom line: No, airliner makers are not going to go out of their way to design their airplanes for inverted flight. But the aerodynamic principles that determine whether an an airplane has longitudinal (pitch) stability don't care much whether the airplane is upside down or right side up. So if they design the airplane to be stable in normal flight (and they almost always do), it is probably stable the other way up as well. Quote:
As to the Boeing rep saying that the MD80 "...cannot sustain inverted flight," that is certainly true for longer values of sustained. Airplanes that are designed for sustained inverted flight have special provisions in their fuel and lubrication systems. The fuel pickups inside the fuel tanks (or at least the tank used for aerobatic flight) have "flop tubes" with weights so that they follows the fuel as it goes to the top of the tank when inverted or side of the tank in knife-edge. They usually use either fuel injection or special carburetors that work any way up. The engine lubrication systems are either dry-sump or have weighted valves so that they can draw oil from either the top or the bottom of the engine as needed. Airliners have none of these provisions, and to make things worse they are to a large extent dependent on hydraulic systems that might not be equipped to work upside down. So if you turn an airliner over, the engines will be starved for fuel and quit. They will also be starved for oil, but probably not until it is completely irrelevant. Most importantly the hydraulic systems will probably quit working as the fluid pickups at the bottoms of the reservoirs start sucking air. However, those things are largely irrelevant to issues of static stability. And as they relate to Alaska 261, even a very sketchy plan is sometimes better than no plan whatsoever. Pilots and engineers I talked with tend to agree that there's a substantially non-zero probability that if the tailplane had held for another minute or so, they might have pulled off an inverted ditching and saved at least a few of the passengers. --Bob K. |
#39
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The inescapable reason why commercial jets aren't and probably can't be flown upside down for an extended period probably has little to do with aerodynamics. The fuel systems and hydraulic fluid systems are almost certainly designed to be at least partly gravity fed.
Even a car engine wont operate for more than a few seconds upside down. Fuel and oil feed are both dependent on gravity to keep the fluid in the sump or tank reservoir around the uptake tube. |
#40
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And the toilets probably aren't designed to be turned upside down, either...
(especially the older style ones with the blue fluid in them, which as I understand were pretty much just glorified port-a-potties) |
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