Good for a 14 year old. Sounds like you read your share of Stephen Coonts, Michael DiMercurio, etc... Your story has potential, but not a lot in terms of kinetic energy.
I will let others take care of your grammar & semantics. First of all, kill Mark. CIA assassin types are typically cliche & boring (I had a difficult time sitting through Bourne Identitity for example). I hope he isn't your main charactor, because already we know too much about him. So, maybe you can work your story into something where his ride is hijacked or crashes, bombed, etc... This will also allow you start more explosively. There's a reason why everyone likes a Corvette --it launches really fast! Also, even you didn't want to kill him, we know to much already. A lowly CIA operative has no need for a name in a larger story. Also, in the interest of realism, they would never helicopter him out of DCA. Too expensive. The CIA, like any gov't agency, is a beaurocracy...cheap & boring. The most they would have done is meet him there instead. Otherwise, they'll talk to him tommorow, etc...
Also, don't title every scene. No one cares how many feet above the eastern USA they are, nor do they care if it is UAL or even a 744. That stuff will always be cool to us airplane geeks, for most, it is simply too much detail that will lose most readers. Big rule here... Keep detail like that limited to when it matters to the story! Otherwise you will write a 7,000 page essay about a trip to europe, lol.
Also, unless you are a foreign defector at like senate level or more important, CIA will NEVER spring for First. Besides, a CIA operative will know that you can scan the cabin from the rear more effectivley anyhow,
OFF THE UNITED STATES EAST COAST
Things were quiet aboard the United Airlines flight. The sun had just dipped below the horizon, leaving an eerie twilight fading outside the window. Mark Robertson sat in his first class seat with his earphones on and plugged into the entertainment console, but with no music playing. He was alert as always, but after his latest mission he was taking no chances. He was sitting in the last row, the cabin bulkhead at his back. He was in the right aisle, sitting by the window, the chair next to him vacated. His leather laptop bag lay on the ground at his feet. He was careful not to look too suspicious as he monitored the rest of the cabin, looking and listening for any unusual activity.
Again, it seems like you have good potential here & we certainly would like to see where this all goes! I am guessing that you have gotten more advice than you bargained for, lol, but that can only help right? Take care!