Drive an AA Wraith

In Halo 3, you can drive an AA Wraith in any level it appears in.

It is to be noted, however, that the Wraith will self-destruct once you leave it. Avoid leaving the Arbiter (if on Single) or any Marine in the turret; they will die, and your allies will blame you for their death.

Walkthrough

 * 1) Find an AA Wraith (recommended on the level, The Covenant ,though can be done on every level it is found on though you can't board or drive the AA wraiths beside the Towers on The Covenant
 * 2) Take out the gunner, if necessary.
 * 3) Stand on the hatch (the door) of the Wraith and melee it off (weapons like Rocket Launchers or Spartan Lasers with a good melee force will smash open the hatch in one hit and not kill the driver). Or, on higher difficulties or with the Tilt skull on, you will probably have to board the Wraith and melee the hatch until it falls off, as a normal melee attack won't even dent the hatch.
 * 4) After the hatch is off, hit the driver with a melee attack so his helmet is knocked off. If you melee the driver to death, it will not work
 * 5) If you boarded the Wraith, get off and jump on top of (not board) the Wraith. Once you see the drivers head, crouch and position yourself so you do not see "Hold RB to board Wraith".
 * 6) melee the drivers power armor off then Holding RB, shoot the driver in the head until he dies (it is better using a Battle Rifle or Covenant Carbine), once he dies you will automatically enter the Wraith.

Multiplayer Walkthrough

 * 1) Find an AA Wraith.
 * 2) Get Player 1 to board the Wraith, with Player 2 holding RB at its front or side.
 * 3) Get Player 1 to melee the driver and gunner normally.
 * 4) As soon as the driver and gunner are killed, Player 2 will automatically get inside.
 * 5) If there is an ally gunner, and the driver gets out, the wraith will still explode.

Explanation
The reason you can't pilot this Wraith variant is because of a rule Bungie included in the development; and that rule is when the driver dies, the vehicle explodes. As you may have noticed, if you attempt to kill the driver in order for it to be unoccupied so you can use it, it will self-destruct when the driver is dead.

If Bungie hadn't included this, you could just get in and out of the vehicle as much as you want. This is where the glitch comes into play. When you kill the driver while holding RB, you are quick enough to bypass this "rule" and get in the Wraith, tricking the system into thinking that the driver never left the vehicle. The rule also explains why the Wraith will self-destruct when you leave it; as you have left the vehicle, the system now realizes that the driver is absent from the vehicle, so it carries out the correct action, thus destroying the Wraith.

Halo 3 ODST

 * The AA Wraiths appearance in Halo 3: ODST is quite limited, but all occupied regular Wraiths are rigged to explode when their driver dies like AA Wraiths are. That way, you will have to use this glitch to get your hands on this powerful vehicle.

Trivia

 * When piloted, the vehicle uses a temporary reticule that seems to be used for vehicles that do not yet have a reticule of their own. This same reticule could be seen in several ViDocs and in the Beta when using locked vehicles like the standard Wraith. It is unknown whether Bungie ever intended for the player to use this vehicle and decided to remove this capability due to its over-powered nature or it is using an edited Wraith, and found it easier not to take away the driving ability but to make it explode. Another thing of note is that there is an animation that is seen only when the player gets out of the vehicle - the turret being contracted into its compartment.
 * If hijacked, the main weapon can be fired at a rate much greater than is seen during normal gameplay, all one needs to do is button-mash on the right trigger. This is most likely because there was no wait between shots programmed, since it was not intended to be driven.
 * On Xbox Live, this glitch will not work except for the host, but on rare occasions the other player attempting to drive will get into the destroyed Wraith as if it were still working, however it is immobile and cannot fire. This is possibly due to lag, as the aforementioned rule cannot be bypassed quickly enough.
 * For some unknown reason, a regular Wraith on the Halo 3 level, "The Ark" is rigged to explode when its driver dies.
 * If the anti-air Wraith takes notice of the player, hijacking it will become nearly impossible. If the player manages to get on top of the wraith without being shot or splattered, the Wraith will continuously drive around and back up in a circular motion, as it is trying to get away from the player. This makes holding a steady aim on the driver's head and positioning one's self so than the board prompt does not appear extremely difficult, due to the fact that the player does not turn with the vehicle.