I'm sure you've all heard about the airplane that ditched in the Hudson last week. (Just 30 blocks from my office!) When it happened, after we found out more about what caused the plane to ditch, I wondered how long it would take before the 911 Truthers came up with a conspiracy theory about it.
Not long. Via SkepticBlog comes news of a conspiracy theorist claiming that the ditching doesn't make any sense. Brian Dunning at SkepticBlog does a good
job explaining what's so stupid about this, but there were two things about
it that I thought were particularly interesting from the point of view of a math and computer science geek.
First, as long-time readers know, one of the things that I've harped on many times is just how bad we are at understanding numbers and scale. The conspiracy theorist provides a great example of this. He asks:
Also in this picture are dozens and dozens of "survivors"... it's hard to count because the picture is so fuzzy and distant (a common theme... distant and fuzzy pics). How can more than 20 people stand on a wing and not collapse it?
How can 20 people stand on the wing of a plane and not have it snap off?
How does a plane fly? The aerodynamic shape of the wing creates a differential in air pressure between the bottom and top of the wing, which produces lift. What that means is that the air flowing over the wings pushes the wings up, and the wings lift the airplane. The wings are capable of lifting the entire airplane: all of the people on the plane, the metal hull, cargo, engines, fuel. The weight of a completely empty A320 (the type of plane in this crash) is 42 metric tons, or just a little under 100,000 pounds. Fully loaded (fuel, passengers, and cargo), it's something over 60 metric tons - or about 130,000 pounds.
The wings of the plane routinely lift 60 metric tons. The force on the wings at takeoff is equivalent to a dramatically higher static load than that. And the wings are built and tested with a large safety margin to
be able to withstand significantly higher forces than they'd ever encounter in real circumstances.
How much do 100 people standing on a wing weigh? Assume an average of 200 pounds each - which is almost certainly much too high - and you're looking
at around 20,000 pounds.
Of course, there's a different standing on the wings. The upward force of lift is uniformly distributed over the wings. In the pictures, people are pretty well distributed - they're not bunched towards the end. But even if they were, the weight of the people on the wing wouldn't be enough to break it. After all - in routine operating conditions, the lift is uniform. In turbulent conditions, you get huge torquing forces on the wings. When's the last time you heard of an airplane's wing snapping off in turbulence?
And all of that is ignoring the fact that the plane was, at the time, floating on water. The water was supporting the wings - meaning that the wings could handle even more weight.
The idea that people standing on the wings of a plane floating on the water should have broken the wings is just ridiculous, when you consider how
their weight compares to the kinds of forces that the wing routinely encounters.
The second thing comes from a totally different direction, and involves
image manipulation. Our conspiracy theorist believe that the photographs of the plane have been heavily photoshopped.
Why?
Basically, because when you zoom in on the photos, you can see extreme
pixellation along the edges of the plane. When you edit an image, particularly if you're not particularly good at it, you get edge artifacts - that is, pixellated jags where you joined two images together.
Once again, though, we hit an issue where understanding what scale
means leads to errors.
One of the causes of the artifacts in photoshop is the fact that the
resolution of the boundary of the region that you cut and pasted doesn't match
the resolution of the actual image - so you get artifacts. The jags are
always there in the image. But they become more visible, because the
resolution mismatch brought along some pixels from the original context of the
clipped section, and those contrast more against context into which you've
inserted the clipped section. Photoshopping didn't create the jags;
it just made them more visible. The other cause of artifacts in photoshopped images is resolution mismatch - the resolution of the source image doesn't match the resolution of the image into which it's clipped - so when you look at the boundary, you can artifacts produced by that mismatch.
Take a normal digital image, and scale it up, and you'll see those jagged
artifacts exactly the same way that you would in a bad photoshop. The image
is, ultimately, pixellated. When you get to a high enough scale, you'll see
jagged artifacts of that pixellation. Those artifacts will be more visible in places where there's more contrast between the colors on either side of the boundary. Like, say, the contrast of the airline logo against the white background of the airplane's hull.
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Having read the post that's included in your link, my first question for the loony bird is, "Why?" What does he think anyone has to gain by faking the ditching of an aircraft like that? Most conspiracies have a decent motive as part of the story, but this one is a reach at best.
What would be the point of such a conspiracy? To make the airline look bad? To give the pilot a heroic image? To remove any current restrictions on goose hunting?
It's my understanding that all the unused jet fuel also helped keep the plane afloat (and, of course, provided extra support for the wings and the people standing on them).
First, with the fuel bladders full, water thereby could not seep into the wings.
Second, jet fuel has a specific gravity of around 0.7. That means that it was providing significant buoyancy.
A nice visual demonstration of how impressively strong the wings for a passenger aircraft can be is the the Boeing 777 wing load test.
At least now I know what the Canadian geese do in the winter ;-p
Obligatory XKCD link: http://imgs.xkcd.com/comics/photoshops.png
It can't get more stupid than that!
No, not really. While in (relative) motion a whole bunch of aspects of the wing (and other parts of the aircraft) direct air downwards and the reaction to that lifts the plane upwards.
This is probably the best explanation of lift.
But it is also true that you can find the same lift force if you integrate the pressure over the entire wetted area of an aircraft and then consider the vertical component of that result - e.g. it is possible to find lift with no knowledge of the velocity field.
Within 30 blocks of your office? That clinches it. You must have been involved in the conspiracy! (What conspiracy there was. It's not clear, is it?)
You can always tell the conspiracy theorists. They begin by saying "I'm not a conspiracy theorist, but..."
And then the idiocy commences.
"How can 20 people stand on a wing and not collapse it?" How stupid can you be? Flight 1549 was an Airbus A320. The empty weight of such a craft is 93,500 lbs. Maximum take off weight is 169,000 lbs? Do you really think that the 4,000 or so lbs (20 people x 200 lbs) on the surface of a wing, supported by the bouyancy of the water beneath it is really going to have any effect at all?
Why no video? I've been in four or five car crashes in my life, and seen at least a dozen more. None are on video. Your cell phone, even if it is video equipped is not likely to be too useful: user interfaces on such things generally require multiple button pushes, and response is typically very slow.
But here's the other thing: there is video!
http://www.youtube.com/watch?v=fwIe-e7Apkc
and
http://www.youtube.com/watch?v=FIS-aKJMY3E
Apparently also your skeptic doesn't understand what a coincidence is. A coincidence implies that two things which are independently unlikely occurred. Coincidences aren't rare events, or lucky events.
But here's the real thing that blows my mind: what is the actual counterclaim? That this plane _didn't_ crash? That it didn't have an engine failure? That it never was in the water? Really? What does the author think happened, and why was it compelling enough to get the 150 people on board, the five crew, the likely hundreds of witnesses, the rescue crews, and the FAA all to concoct this elaborate "hoax"?
"I am not a conspiracy theorist" indeed.
Keith Braithwaite is correct; see http://www.allstar.fiu.edu/aero/airflylvl3.htm The Bernoulli lift is minor, serving only to keep the boundary layer of air attached to the wing to prevent turbulence and thus, drag.
True, but trivial.
About wing strength, in the early 1980s I crewed several races with a Can-Am team. These cars have rear wings which were made out of aluminum. One of the things you do is push the car around the pits by pushing on the wing. The first time we were going to move the car, I asked the guys if there was any special place to push on the wing to not damage it. They laughed and said if you can dent that thing by pushing on it, it'd collapse at racing speeds. They were right; it was like a steel girder.
Great blog! I can't believe the lack of common sense with the "Skeptologists" and other 911 looneys.
Maybe you could say a bit about the 9/11 thing.
I mean I saw a lot of coverage during the last years - some claiming the official story, some doubting it, some even suggesting an inside-job etc.
Same goes with literature, even though most newspapers and TV news seems to favor the official story.
Of course there are a lot of lunatics stating plainly foul things (like seeing the devil in the smoke,...) but some pointing to things you can't just ignore.
My problem is: I can't verify most of these claims (no matter if the come from the sceptics or the truthers) so maybe you can enlighten me with an more or less unbiased article?
There is no mispelling on "wetted" - this is a technical term in fluid mechanics:
http://en.wikipedia.org/wiki/Wetted_area
My point is just that the high pressure / low pressure explanation for the lift generated by an airfoil, while not the best, isn't wrong.
Every time I watch any documentary about airplanes, I always freak out just little when people start walking on the wings; and then remember that the weight of a person is pretty small percentage of the load these have to handle in the air. I think it might be due to an optical illusion that makes them look so thin and so we naturally think "weak".
And the thing about the photos, it wouldn't be they were zooming in on a jpeg were they? Jpeg's really muck up straight lines and depending on the compression ratio can produce a lot of artifacts right around the line with virtually none elsewhere.
Re #13:
There's not much to say about 9/11. It's been studied to death. The horrible events that day were witnessed in person by thousands of people. It's completely clear what happened, and anyone who denies that is, to be blunt, an incredible idiot.
The so-called arguments that it wasn't caused by planes crashing into the building are beyond stupid. At best, they're a mass of silly, naive bullshit; at worst, they're vile malicious slander against the people being blamed.
On the naive side, you've got things like "The shape of the hole in the building after the plane crashed into it wasn't shaped like a plane". Yeah, and the real world behaves exactly like road-runner cartoons; when something crashed through a rigid surface at high speed, it leaves a perfect profile. Or "jet fuel doesn't burn that hot". Yeah, try visiting the inside of a jet engine... Fires in enclosed spaces do behave differently that fires in open-spaces.
On the slanderous side, you've got things like the claims it was done by the Israelis, and the proof was that Jews didn't go to work that day. Only problem? Plenty of Jews died. I knew a few, because they went to my synagogue.
RE #15:
Good point; I forgot about compression artifacts!
In a file with color contrasts, image compression will create artifacts immediately around the location of the color change. Areas that are relatively uniform in color (like, say, the surface of the river) will appear sharp; but at the point of transition, you'll get a collection of artifacts. The exact frequency and shape of the artifacts will depend greatly on what kind of image compression you use.
It's yet another problem with armchair analysis of this sort of thing: you're working with "evidence" that is highly compromised by the communication medium that you used to obtain it. You're not getting direct observation. You're not even getting copies of direct observations. You're getting something that's been highly processed to make it feasible for millions of people to view/download it.
It's like the 9/11 Truthers who sit with screenshots of TV images of the plane crashing into the second tower: you can't make reliable observations using a source that's been so heavily mangled. Those images were first captured by a TV camera - which does not produce a particularly high-quality image. Then it was scaled and broadcast for television - which is a very-low quality image. Then it was screen-captured - which introduces errors into the image. Then they obsessively analyze it - scaling the image up beyond the actual maximum resolution of the real image. Standard analog TV resolution is, at best, around 350x250 pixels. That's not much at all.
@C. König
I do not want to distract this thread. I also have a healthy respect for Mark.
So I am just going to post one link, that's it. I won't discuss this any further here, there's appropriate threads for those discussions, not this one.
The 9-11 mysteries movie (1 hour 30 mins)
http://tinyurl.com/dnfssd
Just watching this will pretty well cover all their arguments
If we're talking about JPG compression specifically, it works more or less as follows:
The image is reencoded from RGB to YCbCr space: one component for luminance (a black and white image, if you will) and two for chrominance. The chrominance is usually downsampled, since there's a lot of information we can discard with very little perceptual effect.
The image is then split into 8x8 pixel tiles, each of which is then transformed into a frequency domain using the Discrete Cosine Transform, and the higher frequencies with smallest weights are then discarded. The worst offenders, perceptually, are the boundaries between the tiles.
Okay, we have something bad from a 9/11 Truther. Can anyone find anything bad in a defense of the official 9/11 story? How about this:
> Why couldn't ATC find the hijacked flights? When the
> hijackers turned off the planes' transponders, which
> broadcast identifying signals, ATC had to search 4500
> identical radar blips crisscrossing some of the
> country's busiest air corridors.
http://www.popularmechanics.com/technology/military_law/1227842.html?pa…
(This should be trivial. There are two howlers in one sentence.)
Close, but not entirely accurate: JPEG artefacts are caused by JPEG compression, which is, of course, near-ubiquitously used for photos, but need not be. I do a lot of work with engravings, and routinely am able to keep to lossless formats throughout the entire job.
This does not, of course, affect the argument as applied here, it's more of a technical inaccuracy.