It can be used as soon as any team has captured its flag location. Throughout the match, any player from either faction can activate the air raid. Once inside the bunker, the player is completely invulnerable and remains so until the bombing run is over. It will "recharge" for another use within a few minutes.
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The Air Raid Bunker appears in the Battlefield 4: China Rising expansion pack. Similar to Battlefield 1943, the control point containing the bunker must be held by either team in order to call in a bomber, but the bunker can be used by any player from either team. It allows a player to control the Bomber and designate areas on the map for JDAM strikes.
In the expansion pack Battlefield 4: Final Stand, the Air Raid Bunker appears only on the map Operation Whiteout. Unlike other maps, it takes the form of a surfaced submarine frozen in the ice. It can be entered by a hatch on the top of the submarine.
For about five seconds after an enemy first enters the bunker, the player may kill him (and prevent the air raid) by firing the Defibrillator at the door. This is the only occasion on which a player in the bunker may be killed.