Load up new Alexandria from mission start on any difficulty. This requires a Jetpack.
Just do the level up to the point where you have to defend the Oni Tower. Instead, head back to the club place with the 4 hunters, don't go in the club, instead go behind the building next to it and look where the building kinda sinks in.
If you look at the ceilling of this sinked in place, you will notice a switch, try to get your falcon's wings on the tiny ledge you can see not far from the switch, if you park it right, it won't fall.
Jetpack up and press the switch, get back in your falcon and start flying towards a building that looks like a giant O shape only abit dodgier.
Fly through this ring in a falcon and you should instantly switch to a pelican.
Fly through it in a banshee to pilot a phantom (warning, the phantom can go through walls but if you go through them far enough you will die). You cannot use the weapons on these aircrafts, but you can go all the way out the map up to the carriers.
You can even use them to finish the level off by pushing the shade turrets of the building near the Oni tower. While in either aircraft you are invincible, however if you get out of the pelican it will just disappear, you cannot get out of the phantom though.
So finally after all these years, bungie has let us pilot these vehicles, but in all fairness, just limiting them to an easteregg gets aboit boring after a while, still I recommend you do it because it is just plain awesome.
Extra note from XxSTORMHAWK xX:
He is right up until the weapon part, because you can use the turrets and for the pelican nose turret you have to go close to the ground and your friend crouches and walks towards the cockpit while holding X or RB. But only the AI can use the phantom nose turret.
[more cheats for Halo: Reach here]