This is idiomatic, informal and OK. "Go" is being used as an adjective, meaning "all systems go", or ready and fully functioning. If you scan a dictionary you will find "go" has many possible uses and meanings. https://en.oxforddictionaries.com/definition/go https://www.dictionary.com/browse/go

It means we are ready to start touchdown procedure, and like another corrector has stated, it is Radio communication talking procedure or protocol, to help eliminate poor radio communications. However much radio protocol talk has made its way into normal dialogue. and vice versa to sometimes give a hybrid semi official protocol, especially in films.

There are many aspects to this. Yaw is a term used in flying (along with pitch, and roll). Yaw ten degrees in a plane would be to raise the nose up 10 degrees. We are go for touchdown means we are ready for landing, the wheels or landing gear is in place and everything is as it should be. Strictly speaking the grammar is wrong. However it is used partly as a shorter version for speed, but also partly to keep instructions clear because over long distances radios and other devices have not always transmitted quality sound. Simple words such as 'go' are less likely to be misheard or misinterpreted.