If an airship crashes when defending a city, you could build a specific tower that has crew rush to the fallen ship, and repair the remaining modules, but not replace the obliterated ones. The crew would of course be in danger as they run through open air. Along with this, i think it would be a good idea for the option of the surviving crew to evacuate an airship that has crashed, so that they could instead help one of the defending towers.
Captain, Engineering Corps
Or when the attacking falls, they try and take the defense over and use it to destroy the other ships...
Noted - the major engineering focus of version 5 is really on letting crew run around outside of ships, which should enable things like this. :)