747-400 Body Gear Steering and IRUs
One of the inputs into the Body Gear Steering System on the 747-400 is IRU groundspeed. However, there is ambiguous information on which IRUs are being used.
The groundspeed, wheelspeed and air/ground information is collated in the Brake System Control Unit for the purposes of Body Gear Steering control (nosewheel angle is external to the BSCU). My Engineering training notes say that either the Left or Right IRU can be used. However, the BSCU has two groundspeed databusses: Captain's selected IRU and F/O's selected IRU. This might suggest 3 sources of IRU data is available for BGS logic. Having said that, some airlines' QRHs have a note saying that if (only) IRU LEFT is inoperative, BGS will be inoperative. Other QRHs don't.
The BGS system seems like a fairly fundamental system to me. I'm not sure why there would be optional IRU inputs. Would anyone have any further detailed information on this?
Thanks.