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The Hang Mobile (ハングモービル) is the second boss of Sonic & Knuckles, fought at the end of Flying Battery Zone Act 2.
After the pre-boss destroys itself and the player gives chase after Dr. Eggman/Eggrobo, the wall slams shut behind them and the ship starts to fall. After a short chase sequence, the main ship completely disappears from sight, and Hang Mobile drops in from the sky, swings around a couple of times, then begins to attack.
This boss has two main attacks—he swings around in front and behind of the main platform, causing damage if the player is standing in the centre of his attached limbs; and when he is hanging below the platform, he shoots fire from the top of his head and shimmies left and right, his spiked limbs causing damage if they touch the player. When playing as Knuckles, the machine shoots fire all the time, instead of shooting only when the player (be it Sonic and/or Tails) stands over the machine. The only opportunity to hit him is when he is swinging around—when the boss is underneath the platform, he is invulnerable. After the usual 8 hits, the machine falls to pieces—but beware the spiked limbs, as they stay deadly for a few moments longer than the rest of the boss does. Eggman flies off to the right, past the Capsule for the Act. Once broken, the character runs to the right and spins though an access hatch, sending it flying and the character falling straight towards the desert of Sandopolis Zone.
Curiously, the boss displays as Eggman for Knuckles, and not the Eggrobo who is the main nemesis in his storyline. This is likely due to the fact that the battle's perspective is different--the Egg Mobile and thus its pilot face the screen head on, and so require new sprites, which the artists did not implement for EggRobo. Knuckles was also not originally intended to visit Flying Battery Zone before the level was moved to Sonic & Knuckles. Sonic 3 Complete, a ROM hack by Tiddles, addressed this by implementing sprites edited from one of the ending scenes.