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.

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...

Not sure, McHale. QRHs and FCOMs are usually the latest and greatest. My engineering notes are probably as old as the aircraft themselves. The QRH seems very specific in the wording (i.e. it actually reads "Body Gear Steering not available with IRS LEFT message". This would allow operation with the IRUs off. I'm not sure if the message is displayed with the IRUs in ATTitude. If it's only the Left, then maybe Captain's IRU Source would allow Right or even Centre.

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.

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.

Agreed, but with IRSs off, the IRS LEFT Advisory Message is removed (so the QRH statement is partially true). I even asked Boeing why BGS was still available with the IRUs off. They said it's just the way the logic works.

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).

This makes sense. Perhaps when they were designing the system and came up with the requirements for Body Gear Steering activation(which is listed in an earlier post), the person with input from the maintenance point of view said that there would likely be situations where a maintenance crew wants to taxi the aircraft(in which case BGS would be desirable) yet all three IRU's will be selected off. Therefore Boeing designed the system to accomodate this situation after considering that there would likely never be a situation where pilots would be doing higher speed ground operations with three IRU's selected off, so it was therefore a reasonable design.

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.