Unless its hunting a russian whale why would it be that low? I read it was doing a surface search rather than an ASW mission (maybe duff info) so I don't think it would be at ultra low level for the entire sortie. You'd be amazed what 2,500ft can do to a radar picture! I would expect to hold a Lynx at 5k ft at 70 miles+ and used to track SK AEWs even further than that. The Navigating Officer would normally be responsible for keeping tags on the ships flight at all times and for making sure that the ship was in a postion to recover the helo when required.