The Helicopter event is one of metagame events. It occures only once (unless it's altered in the sandbox settings prior to starting a playthrough).
The helicopter event will occur between days 6-9, and between 9 A.M-8 P.M. On the day that the helicopter event occurs, the Automated Emergency Broadcast System station (AEBR for short) will announce 'Air Activity detected.' at the end of every hourly automated broadcast (beginning at 9 A.M the day of the event) warning the player in advance of the helicopter. It is recommended that players stay inside for the entire day, as the helicopter will draw hordes towards the player if they're spotted by following them.
Overview[]
This event (in which a helicopter flies overhead) although unseen by the player will be heard by the player and all nearby zombies. When a helicopter spots a player it follows and attracts zombies to any building the player may hide in. Forests as well as buildings may provide visual cover from the helicopter. The event might be disastrous so you should prepare quickly if you hear either the radio activity or the helicopter itself. For braver players it presents a rare opportunity to draw out effectively all zombies in a wide area. Deaf characters face the additional challenge of not being able to hear warnings of the helicopter event. By default, this event will only occur once per playthrough.