Flipping a Phantom
From Halopedia, the Halo wiki
In Halo 2, start the level Quarantine Zone and continue through the level until you see Wraiths attacking other enemies; destroy them so that they cannot kill you. Once you see a small tunnel with mini pillars, Combat Forms ride Ghosts through the tunnel. Try to hijack one and destroy the other one. If you see Sentinels attacking you or the Flood forms, destroy them and travel to their location. There you will see a hole; go over it and you will see an empty part, maybe with a destroyed or a piloted Wraith. Get out of the Ghost and throw a frag or plasma grenade at the bottom of the Ghost. Immediately get back in and you will be propelled to the higher platform. Once there, go to the mini bridge made of sand and go over it. After that go to the highest part you can get to and you will see the big area just after the steep hill. Go there and in the left corner you will see a Phantom that is in the same situation like the Pelican that flips in Metropolis. Like the Pelican, it says "Press X to flip Phantom ".
In Halo 3, on the level Crows Nest, in the hangar where you must clear it for the Pelican to launch, the Phantoms that drop off the troops can be boarded by jumping into their troop bays. First, jump on to one of the first yellow walls. Then, as the Phantom flies over, jump and hold RB to board the unmanned turret. After successfully on board, press RB while jumping backwards to land inside of the Phantom's cargo area. You must run to the front right corner of the ship (towards the Op Center) and be in the corner of the invisible walls present there. If you do not do this you will be thrown from the Phantom as it turns. Get back on a turret, then wait for a little while and the ship will begin to free fall. You might be ejected upside down while it is tumbling, but the Phantom is flippable (hold RB to flip); however, no message is shown.