In Level 3 as the Alien, the Queen will tell you to get to the bottom of
a shaft with a huge fan at the top. When you get close to it, another 
Xenomorph will be blown into the fan. If you use your Focus, you can see
two vents on the opposite wall. You can use either of them and safely jump
to it despite the power of the fan. As you continue, you will find more 
vents to hide in. Use them to make your way down.