Sky Deck

Sky Deck (スカイデッキ) is an action stage found only in Sonic Adventure, and is the eighth stage for Sonic the Hedgehog, fourth stage for Miles "Tails" Prower, and fifth and final stage for Knuckles the Echidna. As its name implies, it is a section of the Egg Carrier that acts as the opposite of Amy Rose, Big the Cat, and E-102 Gamma's internal action stage, Hot Shelter; and is mostly set on the exterior of the ship in the open sky instead of inside of it.

Lower Assembly
Set primarily on the left wing of the Egg Carrier, the first part of this stage is available to both Sonic and Tails, and both share a similar goal: Reaching the end of the underside of the large wing where a giant active cannon stands in their way. After being launched from a large cannon mounted to the side of the main body of the ship underneath the wing, the duo must cling to circular canisters and cross steel rigging hanging above empty air below to gradually make their way towards the large cannon at the wing tip, passing through two midway points that separate the three sections of the wing on the way in the form of a small hanger and a group of mini cockpits, presumably controlling the weapons systems on board. The route here mainly consists of flying chopper bots, as well as occasional segments where drop down mini cannons will attempt to destroy the walkways out from underneath the duo. Afterwards reaching the underside of the tip of the wing, which is completely dominated by a large hanging three story square shaped scaffolding of sorts around the massive central cannon; to run along it avoiding the cannons walkway destroying fire, until they are face to face with it, in which case a single rocket must be fired into the barrel to finish the first part. If playing as Tails, he must beat Sonic to this point in order to win. This marks the end of Tails portion of the stage.

Upper Airfoil
Afterwards, Sonic apparently jumps through the wreckage of the cannon, ending up on the top of the wing where the airships periodic altitude changes become far more deadly, each of Doctor Eggman's speed changes whipping up powerful winds that threaten to throw the two overboard if they don't grab on to something. And at this point, the ships considerable arsenal also becomes more of a threat. From here, the player starts on the very tip of the wing, and must move to the first large wing section where the remains of the destroyed under-cannon now sit towards the main body of the ship, avoiding fire from the many other automatic and single shot energy cannons scattered about the airfoil as they make their way towards a raised section where they can climb up to move to the middle section of the wing. Grapple plates on the floor, as well as a large tower assembly of sorts with a scrolling light bar around the base; both functioning perfectly as hold points if the player is caught by strong winds. Once they've reached the monkey bars leading up to the second part, the tip and everything on it is then apparently destroyed, falling away as the player must race collapsing segments of the large hanger here while constantly released mini airships plague him with missile payloads. Once at the end, leaping into the third and final section of the wing (which is also the very beginning of the stage on the underside) to battle yet another giant cannon, though this time with the cannon randomly firing at different areas, forcing the player to have to pick the rocket the cannon is currently aiming at in order to not miss. After taking this second cannon out, the player moves into the ship through an unknown path, and may end up inside the ship between the two wings on the main body.

Storage Bay
Here, Knuckles also joins, and the stages for both characters are identical beyond the altitude manipulation. For Knuckles, the stage starts leading the player right next to a joystick of sorts sticking up from the ground which he can then push to adjust the heading of the aircraft, either turning it into a downwards pitch and causing gravity to pull the player towards the large opening at the rear (though they won't fall out of it, no matter the pitch), or into an upwards pitch that causes gravity to pull towards the entrance of this stage. With left and right tilting that way accordingly. Left causing the doors in the storage pit nearest to the rear opening at the rear of this area to spill barrels, as well as the rail mounted boxes further ahead of that to shift to the left accordingly, while right does the opposite. Ceasing the barrels and shifting the boxes to the right. Tilting the plane down, or pushing the joystick forward; causes the six doors beneath the joystick platform on the wall here to spill random personal items and ship equipment like mops, buckets, sandals, and even robomaids; while pulling it back stops this. Knuckles having free reign over the area, while Sonic must deal with scripted changes in gravity the closer he gets to the joystick platform from the rear opening... generally far more severe than what Knuckles can do, such as the whole stage turning on it's side and forcing Sonic to use handrails as monkey bars. This area ends when Sonic reaches the roof assembly above the six door pit with the handrails where he can drop to the Animal Capsule to release them, with Knuckles part ending once he finds the three Master Emerald shards.

Trivia

 * This level and Hot Shelter prove that the Egg Carrier is much larger than it physically appears to be, because the characters only explore these extremely large sections of the ship with more areas still unexplored.
 * This is the only stage where one can see most of the Action Stage from the Adventure Field it begins at, and in turn can also make out recognizable aspects of the Adventure Field while in it. Looking to the right of the entrance to Sky Deck, one can make out the parts of the wing they will pass through as well as the large Eggman-face emblazoned cannon cover that houses the cannon they fight in the first part. And while in the Action Stages first two parts, Sonic can make out the front of the Egg Carrier and it's rear lower engines easily.
 * In the second part when Sonic is running across the top of the left wing, the tip is visibly destroyed along with parts of the hanger portion. However, upon returning to the Adventure Stage, if the player changes the ship back to its transformed mode (DX version only), the left wing will still be intact. The same if the player looks at it while it is tucked against the side of the ship in normal mode.