Navigation controller from within a UITabBarController

An iOS App I am developing begins with a UINavigationController, and a navigation link from that displays a UITabBarController

The UITabBarController has three UIViewController within it, and for one I wish to be able to start another Navigation to another ViewController.

I had imagined the best way to do this would be to derive the View Controller for the tab in question from UINavigationController instead of from UIViewController.

However, the root Navigation Controller is initialised in AppDelegate, so presumably something similar is needed for the one in the TabBarController?

What initialisation is necessary for the second Navigation Controller, and, where should it be done ?

Thanks