Chiffon Cake Problem I would like to know what are the causes for a chiffon cake to have a caved in centre.

After baking, the cake turns out with a nice dome. The cake is overturned immediately to cool. But after about 15 minutes of cooling, the centre of the cake which was previously a dome became caved in (not too deeply caved in, maybe just about 2cm from the level of the cake height.)

So the result after the cake is cooled, the top of the cake (which was previously the bottom) is light and fluffy and the bottom of the cake (which was previously the top) is dense. 

The sides and bottom of the cake are evenly browned. The sides do not shrink as much as the centre. 

The cake is baked at 160C-Fan for 45min for a 5-egg recipe. I use cake flour and a very small amount of baking powder.
Welcome to the site @Jules. Your temperature is right and you did get rise, it sounds to me like your cake was underbaked, which is why it collapsed. When you bake a cake (or any batter) you get rise from the creation of steam, expansion of air due to heat, and leavening agents creating gas from the reaction between an acid and a base. This expansion stretches the batter, which needs to crystallize in order to retain its shape. If you don't bake it long enough the structure of the cake won't hold, and when it cools it snaps back and becomes very dense. 

The times given on a recipe are guidelines, you get different baking times because of differences in ovens, equipment, even the temperature and humidity of your kitchen. Because of this you have to bake to the result you want. The method I use for determining when a chiffon cake is done is to listen rather than touch. A chiffon cake pops a lot, it's done when the popping slows down to less than one pop per second. Firstly, don't open the oven until at least 10 minutes after the cake stops rising, you don't want to disturb it. At that point you'll probably hear lots of popping. I give 5 minutes between pop checks until I hear the popping noticeably lessen, then I check every 2 minutes. You can also use the touch test, you want the cake to spring back, but for me the pop test works every time.