As Alien in level 3, the Queen will tell you to get to the bottom of a shaft with a giant fan at the top. As you get near it, another Xenomorph will be swept away and blown into the fan. If you use your Focus, you can see two vents on the opposite wall. select either of them and you can safely jump to it despite the power of the fan. As you go, you will find more vents to hide in. Use them to make your way down.