This isn't good...

[BlueStar 323]

21 hours ago, skelsey said:

What system provides ground speed information? ;)

I've found time, distance and an E6b to be extremely accurate and reliable in calculating ground speed.  And when it comes to heading nothing is more reliable than a mag. compass.  

blaustern 

[skelsey 765]

 I quite agree!

[Qavion2 39]

23 hours ago, berts said:

John, when you say "my training notes say that IRU signals come from both the Left and Right IRUs"  Don't you mean the Left and Right FMC's?

Just quoting my notes, Bertie, but why would you (or Boeing) want to complicate the steering system by including a groundspeed signal from the FMC? The Body Gear Steering System is armed using a signal from the BSCU (Brake System Control Unit). The BSCU has no inputs from the FMC. Also, nowhere in the Maintenance Manual does it say that you need to have an operable FMC for the steering system to work.
 

 

23 hours ago, berts said:

It is always difficult commenting on a technical issue, because no B744 is exactly the same.  In the case of the RR engined B744, when an IRS fault is detected in flight then automatic selection of an operable IRS should occur for display purposes.

I'm pretty sure that engine type has nothing to do with display switching. What is more likely is that on your RR aircraft/simulator, you just happened to have a particular IRU source selector. e.g. I think most of BA's aircraft had an AUTO selection for the IRU. Our RR's didn't.

My notes do say L or R IRU (in relation to Body Gear Steering), but the Boeing wiring schematics seem to indicate that the instrument source select switching relays are involved and the C IRU may be selectable. Both the schematics and wiring diagrams suggest that two sources of IRU data are available at all times for the BSCU (unless of course both Captain's and F/O's selected IRUs are inoperative)

I

23 hours ago, berts said:

If whilst actioning the QRH the IRS LEFT message is still displayed at this point then the Body Gear Steering will definitely not be available. 

Is this based on what is written in your manuals or is this from real world/sim experience?

 

Further info:

The wiring schematics (32-53-01) show a simplified logic/switching circuit showing two arming relays (A & B). Each arming relay is controlled by BSCU logic which shows (internal) switches in series:

1) Air/Ground (a signal from the BSCU) AND

2) <15Kts wheelspeed AND

3) <40kts IRU groundspeed.

According to the schematics, the logic is contained on BSCU "System BITE card 1". I know the schematic is simplified, because if the IRUs are switched off, the body gear steering still operates. Also not shown is which IRUs are providing the data, so I had to go to my training notes. Unfortunately, the wiring schematics and diagrams don't go into enough detail regarding the sources of IRU data going to the BSCU circuits specifically relating to Body Gear Steering.

Regarding Air/Ground: both primary and alternate air/ground systems must be on the ground for 10 seconds. i.e. One of the left main gears must be on the ground and one of the right gears.

Regarding wheelspeed: 4th highest wheelspeed of the 8 wing gear wheels.

In my notes, the schematic diagram for the BSCU shows a two position "IRU<40kts / IRU>50kts" switch. In the de-energised position (IRU<40), the arming circuit is completed. If the switch is energised to the IRU>50kts position the steering will be disabled. A no-IRU data may leave the switch in the <40kt position (activing the steering).

Hopefully we can get this sorted.

Cheers

JHW

[berts 97]

On 2/10/2018 at 1:36 AM, Qavion2 said:

Just quoting my notes, Bertie, but why would you (or Boeing) want to complicate the steering system by including a groundspeed signal from the FMC?

Is this based on what is written in your manuals or is this from real world/sim experience?

a) I never said any groundspeed signal came from the FMC.  The QRH calls for the crew to select the FMC using the most accurate remaining IRS.  From then on the FMC position will be based on a single serviceable onside or centre IRS and this will give the most accurate FMC positionfor LNAV purposes when radio updating is not available.

b) I always check my facts on technical issues and the non-normal QRH I have clearly states the Body Gear Steering not being available with an IRS Left message displayed.  The issue here seems to be the fact that not all B744's have the same equipment states or mods.  Most of my Boeing technical notes are on a need to know basis and, like yours it would seem, they do not always explain why something happens or the complete picture in the schematics etc.  The only good thing I suppose is that my notes are not as bulky as Encyclopaedia Britannica!

I hope this is now sorted!

Bertie

 

[Qavion2 39]

25 minutes ago, berts said:

b) I always check my facts on technical issues and the non-normal QRH I have clearly states the Body Gear Steering not being available with an IRS Left message displayed.

Thanks, Bertie.

I wouldn't have thought something as fundamental as Body Gear Steering would have options, but I'll look further into this. It's not mentioned in our QRH (in the IRU fault section or elsewhere). Also, I have engineering training notes from two airlines which mention the L & R IRUs.