The solution to these puzzles first occurred to me as I was pondering the "One Obvious Mask", shown on the left. (This is also known as the Spiegel video.) There is, I must admit, a very obvious straight line division where the plane appears to disappear into the building. Is this just a badly done video mask added to give the illusion that the plane entered the building, with no visible damage to the wall? That is the puzzle.
First notice a couple other things about this animation sequence. The sun-lit side of the tower on the right half of this image is facing East, and it looks relatively clear and bright with columns and windows shimmering in the morning sun. And the shady South side is on the left half where the plane is impacting, and it looks very dark with almost no visibility of any columns or windows.
So I thought to myself, hmm... If there is shade on that side of the building, where is the edge of the shadow where the plane first enters? (We might refer to this edge as the "plane" of the shadow, but that would be confusing.)
In other words, as the plane approaches the tower, first it is in full sunlight, and then it must cross into the shade of the tower at some point before disappearing into the wall. If that sharp edge we see on the right wing is not the edge of the shadow, then where is the shadow's edge? There is no other indication of a shadow's edge on the plane, so that edge we see must be it.
But where exactly does the plane pass through the wall? This intersection with the wall is not visible at all in this sequence, and that must be explained also. Why can't we see it?
First, let's figure out where the sun is and how thick the shadow must be? How much of a gap is there between the edge of the shadow and the wall? What is the angle of the sun at that time of day?
We have a good visual indicator of where the sun is by looking at the North side of the same tower shown in the two images on the right, taken moments after the impact when something rather dense popped out casting a shadow. Notice the long thin shadow stretching across the face, angling down a little. (The image on the right side doesn't seem to show this same shadow. See The Missing Shadow for that puzzle, which I am not addressing here.)
We don't need precise numbers at this point, though I would appreciate it if someone wants to look them up. (Found this: "Fortunately we do know (courtesy of NASA/JPL and Fs2004) that the sun was 27 degrees above the horizon and approximately 21 degrees in front of the UA175 aircraft's starboard wing." ) We just need a rough idea of what's going on, to guide the investigation. So it appears that the sun must be a bit North of due East, and fairly low on the horizon.
Since the sun is just barely shining on the North side of the tower, this means that the South side must have a thin shadow wedge across its entire face. This shadow wedge was very thin near the East edge and thicker toward the West edge. Is it thick enough where the plane enters the shadow to create a gap between that edge and where the plane hits the wall?
Fortunately, we can look at the earlier frames from this video, and the one shown on the right gives a very clear indication of the size of this gap. This frame shows the plane immediately after the nose cone impacts with the tower wall, resulting in a bright flash.
Notice that there is a gap between the flash where the nose cone impacts the wall and the visible leading edge of the fuselage, and the dark area in that gap is the fuselage, which is almost invisible in this light. It is still difficult to tell where the surface of the tower wall is, but it must be close to that flash.
The next image shows the same frame with an overlay indicating where the edge of this shadow intersects the fuselage. The sunlight passing just in front of the corner of the tower is just outside of the shadow on the South side, and it can therefore reflect off the plane. Between that shadow edge and the surface of the South wall, there is no direct sunlight, so that is why it is relatively dark, which makes it difficult to see any details of the impact with the wall. The flash makes its own light, which we can see.
The vertical angle of the sunlight is not exact, because I can't tell from this closeup what the angle of the camera is relative to the tower. (I used 27 degrees, the same as the angle of the sun above the horizon at that time.) But the surface or planar edge of the shadow must intersect the fuselage of the plane in the area indicated, and the surface of the wall must be somewhere below that, close to where the flash is.
More frames of this sequence from the Spiegel video are shown at WebFairy where it is suggested that this illusion is due to a hologram. While a hologram visible in all the videos from many angles is not possible, the illusion of the plane disappearing into the wall is really just a confusion about where the plane enters the shadow on the South side of the tower. No video fakery is required to add a plane to this video, and no video fakery is required to explain what we see in this video.
Here is a great visual illustration of the shadow wedge from a series of pages on suspected video fakery (i.e. several more puzzles) on a page titled Michael Hezarkhani Video / CNN Best Angle "The Money Shot Files For Bankruptcy"
The author introduces the image saying: "But the entire south wall was darkened by a 13 degree wedge of shadow cast by the tower over itself and the UA175 aircraft during its impact (the shadow cast by the starboard wing on the fuselage has not been shown in this graphic):"
I can't vouch for the precise angles depicted, and the position of the plane, but this is at least approximately correct, and should make it clear to you that there was a shadow wedge on the surface of the South wall, and that the plane must have flown through it.
Unfortunately, the author uses this shadow wedge to suggest that there should be no illumination at all of the plane as it enters the shadow. But he confuses where the plane has entered the shadow wedge and how much should be visible.
Here is a cropped and scaled photo of WTC2 from Aman Zafar's WTC gallery taken minutes after the second plane hit, looking toward the North-east. From this angle, the sun is apparently still behind the tower. Notice how the sunlight brightens the smoke that has moved out of the shadow wedge.