747-400 Body Gear Steering and IRUs
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
NSEU,
Methinks the "IRU LEFT" QRH is wrong. My notes state...
BSCU inputs NECESSARY for BGS arming logic ---
1-- Wing gear wheel speed <15kts
2-- Both BSCU air/grd inputs indicate grd mode >10secs
3-- Left OR right IRU grd speed signal <40kts
As far as the BSCU databusses, would they not both be there in case of IRU failure as you have indicated by Capt"s or F/O"s "SELECTED" IRU? Open for correction...
Methinks the "IRU LEFT" QRH is wrong. My notes state...
BSCU inputs NECESSARY for BGS arming logic ---
1-- Wing gear wheel speed <15kts
2-- Both BSCU air/grd inputs indicate grd mode >10secs
3-- Left OR right IRU grd speed signal <40kts
As far as the BSCU databusses, would they not both be there in case of IRU failure as you have indicated by Capt"s or F/O"s "SELECTED" IRU? Open for correction...
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
Methinks the "IRU LEFT" QRH is wrong.
Unfortunately, this is not easy to test in real life or in the sim.
NSEU,
Indeed the QRH and FCOM are always the latest and greatest but that sentence regarding QRH "IRS LEFT" msg seems to contradict the BGS arming logic.
My notes are also old, so I am at a loss as to why the the FO's/CTR IRU's can not assume "command" of the BGS system with that msg displayed.
Another of those aviation anomalies that make you go "Mmmmm...."
Rgds McHale.
Indeed the QRH and FCOM are always the latest and greatest but that sentence regarding QRH "IRS LEFT" msg seems to contradict the BGS arming logic.
My notes are also old, so I am at a loss as to why the the FO's/CTR IRU's can not assume "command" of the BGS system with that msg displayed.
Another of those aviation anomalies that make you go "Mmmmm...."
Rgds McHale.
Join Date: May 2005
Location: Standing at P37
Posts: 187
Likes: 0
Received 0 Likes
on
0 Posts
On our mixed fleet of 744's, Body Gear Steering works just fine, every time during maintenance tows with all 3 IRU's switched off.
Seems to find a way to satisfy whatever arming logic there is.
I realise having them "off" is different than having them "on yet failed" I.E EICAS message present.
Would have to do some digging in the manuals to get to the bottom of it.
Seems to find a way to satisfy whatever arming logic there is.
I realise having them "off" is different than having them "on yet failed" I.E EICAS message present.
Would have to do some digging in the manuals to get to the bottom of it.
Last edited by Spanner Turner; 15th Feb 2018 at 03:05.
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
On our mixed fleet of 744's, Body Gear Steering works just fine, every time during maintenance tows with all 3 IRU's switched off.
Our manuals don't seem to have an answer to this (I've been digging for a while).
So what are some of the possible variations?
1) It's an option to have multiple IRU inputs (This might explain the differences between airline QRHs).
2) Only one IRU is true for all aircraft (the other QRHs haven't been updated yet and the engineering books are wrong).
3) The QRH is correct in saying that that you will have no BGS with the Left IRU message in view... but the guy who wrote it forgot to tell you that you can select another IRU to fix the BGS (unconfirmed) or switch off the IRU to fix the problem (confirmed).
I don't know if the BODY GEAR STRG message appears whenever the IRU LEFT message appears (or if the BGS system needs to have a command (on) signal first). The FIM fault logic has a lot of "and" and "or" statements (with commas and full stops in confusing places).
Join Date: Apr 2008
Location: nowhere
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
That would explain why the logic is the way it is. Covers the piloting side of things at for high speed ops as well as the maintenance side of things at low speed ops when it comes to BGS.