Right ! it's on page 28 under "Component limitations"
If all vipers are in play and a player wishes to activate the "hangar deck" location, he may choose to move a viper from any space area to the "reserves" so that he may pilot it.
Yes, thank you. I thought I had seen it somewhere. I was expecting it to be under the "Pilot Vipers" section on page 26. Silly me.