There are a couple of FRUSTRATING steps on how to fly Jun's falcon in the level Winter Contingency in Halo Reach
Step 1Start at rally point Alpha on normal or easy (you will get shot less) so that you could quickly go to the truck. Get in the truck and look up. You will see a falcon go over a mountain. That's Jun's falcon. Take the truck and make sure that Carter and Jun are in. They should be able to cover while you follow these steps. Make your way towards the falcon while you are on the truck.
Find a Skirmisher or Jackal that has a Plasma Pistol. Take it, get back in the truck, and keep on going towards the falcon.
This is where it gets FRUSTRATING (sorry I needed the capital letter, itallic, bold effects). Once you get to the cliff you will see a pathway that will lead you up part of the way. Go up and then you would have to use the truck to kind of jump off of and then move it farther, jump off it, so on, and so on. If you succeed to get up CONGRATS! You passed the FRUSTRATING part.
Next you will have to use your Plama Pistol and extreemly overcharge it and fire. DON'T miss the falcon. It will then fall out of the sky. If you were lucky it should roll down the hill. Jump down (please dont take it too literely and fall to your death) and when you can quickly hop in the falcon. Then you may fly around as much as your heart desires (well, more like how far the area boundries go).