the-sonic-rainboom

http://www.fimfiction.net/story/51740/1/the-sonic-rainboom/the-lone-chapter

Well, I'm surprised nobody's commented on this yet at all. Well besides the one spambot, but you can't do anything about those.
The concept is a tried and true one; there's been a few fics that I've personally seen that cover the inner monologue of Rainbow during the Sonic Rainboom. This one doesn't really break the mold. It's a very straightforward depiction of what Rainbow probably was thinking, and there's not a lot of real further development of Rainbow's character. And that's also fine, because there's not a real need to develop her character more, nor is there a lot of room to develop her.
Therefore, what we are left with is a very descriptive account from Rainbow's point of view with a lot of excellent imagery. The description of the sound returning after the Rainboom, and the many different things described in the descent were all very evocative and I could imagine everything very clearly even beyond what I saw from the show; I could see them through Rainbow's eyes. All of this makes a simple yet very strong piece that was very enjoyable to read.
However, I had a few issues throughout. First, the Mechanical Issues. Take for instance this:
"By now, Rainbow's wings were aching, but she ignored it, pushed higher and closed her eyes."
There are so many clauses in this single statement that it comes out very broken up and stop and start. There's a few instances of this where the clauses are so short that they and combined where it would make more sense to simply create multiple full sentences and flesh them out. Going back to the above example, it might flow better and be more understandable as "By now, Rainbow's wings were aching, but she ignored it. She pushed higher and closed her eyes." Even though the sentences are still very telly vs showy, just dividing them like that creates the individual ideas rather than a list of things happening in succession.
The oxford comma tends to appear and disappear in your lists throughout, and since you utilize quite a few lists its quickly becomes apparent that its not consistent. The above sentence does not use an oxford comma, however it appears in other places such as "It was a horrible, high pitched, and very, very loud scream". I'm not typically one to be grammar nazi, but the inconsistency stood out to me enough that I felt the need to mention it.
Going back to the previous statement, there's a few issues with telling vs. showing. The up front wording you've put into many of Rainbow's paragraphs make it feel as though you, the author, are simply talking at us, the audience. You're overall descriptors for the environment, for the world and events are full and excellent but it seems when you're describing the character's thoughts and feelings it comes up short. Things like "By now, Rainbow's wings were aching, but she ignored it" and "Pride was what Rainbow Dash felt", these are missed opportunities for the embellishment that is elsewhere in the piece. Simply saying that her wings are aching doesn't have the same impact as Rainbow's wings throbbed in protest as she pushed herself higher and higher. Just as an example. But there are a lot of instances of show vs. tell and I would encourage you to go through and find those places where you are just stating what is happening instead of describing what is happening.
However, there's also the opposite issue involved in the story where there's simply unnecessary descriptors, or what I like to call groaner lines. They are the lines that feel like they are being delivered to a five year old because they might not pick up on the subtlety of it. The show itself is pretty good about avoiding these, but there are a few scattered about this piece. Places like "It was like it was in her dreams, only now this was real!" It just makes the statement feel wooden and unnecessary.
As for the italics, its a fairly universal symbol for an internal thought or statement. I don't really mind the italics, however you do tend to run multiple thoughts in the same paragraph when a line break would help the clarity. Places like
"“In History, maybe!” Rainbow Dash retorted, her voice breaking slightly “See you boys at the finish line.” She grinned, radiating confidence and cheekiness. Rainbow Dash sighed inwardly. I could do with some of that confidence right about now".
This could have certainly used a line break before Rainbow Dash sighed inwardly as its a separate thought. The same goes for a few lines later "I've got to do it for them, for my friends, for Rarity. It's the only way. Rainbow Dash gulped. Somehow." There are a few of these. Also, "Satisfaction. She thought" does not need the She thought at all.
Of course these are my opinions about mechanics and flow, and you are more than welcome to take them or leave them at your discretion. I think you have a great grasp on making descriptions interesting and engaging and if you work on your character development and branching out from the canon and what is set in stone, I think you'd be able to do a lot. I hope this helps you and even if you disagree with specific examples I hope you'll take the overall critiques to heart.
Good luck to you,
-Duskrider