I don't know if this is correct or wrong but when I am at gate with just the external power you can't tune nav radio, i remember to read somewhare that you can setup with just 115 AC if supplied by EXT PWR,APU, ENG.
With this thred I want also to ask another question, after setup the aircarft before to fireup the APU I have a problem with the INS as I always loose electric power going between APU to Engine BUS or Ext PWR with essential power switch, maybe i should understand better the procedure to avoid this problem. The consequence is always the lost of INS Alignment.
Thank's for any hint
Fabrizio