Body Gear Steering and IRUs

[John H Watson]

According to one check pilot, his QRH states that if the "IRU LEFT" Advsy message is visible, body gear steering is not available. However, my Engineering (and KLM) training notes say that either L or R IRU data (<40kts) will enable body gear steering. Also, the Brake System Control Unit (where a lot of the activation logic is found) has two IRU inputs (Captain's selected IRU data and F/O's slected IRU data). This may mean that the Body Gear Steering System has access to all 3 IRUs)

My old airline's AOM does not have this note in it's QRH. Does anyone else's QRH (or other manual) indicate that the steering system will be inop with "IRU LEFT" in view?

Thanks.
Cheers
JHW.


P.S. Note that with the IRU's switched off, the body gear steering system does work. This may mean (amongst other things), that a failure is not the same as the system being off (I think the IRU Left message disappears if you switch off the Left IRU). This may open up a bag of worms. Big sim test: If all the IRUs are in ATT mode, will the body gear steering operate? Is this the same as an IRU fault?

[Jeroen Hoppenbrouwers]

Brain storm... why would the body gear steering require IRS in the first place? Possibly it depends on antiskid? Is there a relationship to be found between availability of antiskid and the IRS requirements?

Hoppie



typo fix

[Avi]

Body gear steering is also a function of speed. It goes center above 20 kts and it enables again when speed goes down below 15 kts so the system needs IRU data to know the speed.

[mark744]

There are 3 IRUs
Dispatch with one u/s is allowable according to one MEL version I have, with no mention of it affecting body gear steering.

[simonijs]

Good Morning,

Neither KLM's (obsolete) AOM nor the FCOM mention any relation between IRU and Body Gear steering.

KLM's QRH contains EICAS advisory messages for "IRS CENTER, LEFT, RIGHT". The condition here is that an IRU fault has occurred; as a result of a LEFT IRU fault, the SATCOM system is inoperative... 

Regards,
Simon

[Hardy Heinlin]

I guess the core question is whether the system considers a deactivated IRU a failed IRU.


Deactivated: EICAS message removed.

Failed: EICAS message displayed.


Perhaps the system only relies on IRU groundspeed data when groundspeed data is valid or intentionally deactivated. If it's not intentionally deactivated and a failure is detected, the data is unreliable and the system can't decide  whether it should ignore the data or not. The system can use wheel speed as a backup.

On the other hand, there's also the question why it doesn't use the other IRU if one IRU fails. Does it it need two IRUs for safety? If so, what if both pilots select the same IRS source? And what if only IRU R fails?


Regards,

|-|ardy

[United744]

.

[RAC]

Hi guys,

Looking at the AMM, the system needs two types of speed...
- GroundSpeed +/- 40kts ... if + disarm the system, if - arm the system.
- Wheel speed +/- 15 kts ... -f - steer, if + centre the wheels.

Now Wheel Speed comes off the Antiskid transducers on the wheels, GroundSpeed has to come off one/more IRU.

But looking in the schematics it is NOT clear which IRU is required for this GroundSpeed info. However, our QRH does specifically say that loss of the Left IRU will disable BGS. So one has to infer that it's the Left only which supplies GroundSpeed to the BGS Ctl Units.

There ARE databuses kicking around, but the aircraft was designe long before the idea of information-sharing by databuses really took off, most of it is still done by point-to-point single wires, aka ARINC 429. So maybe this reliance on a single IRU is just part of the old philosophy - we wouldn't do it like that in a more modern airplane.

HTH,
RAC

[Hardy Heinlin]

Hi RAC,

did you test it in one of the big sims?


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

... as a result of a LEFT IRU fault, the SATCOM system is inoperative... 

Not my SATCOM system     


Hoppie

[John H Watson]

Are there some QRH's more detailed than others? I've looked through 3 different airlines' QRHs (the latest dated Oct 2015) and there is no mention of the BGS being inoperative with the Left IRU inop.

Where exactly in the QRH is it mentioned?

Here's a diagram from one of my manuals:

Body Gear Steering Logic

[RAC]

QRH 11.7 or 11.8... IRS CENTRE, LEFT, RIGHT

Bottom half of step 2...
Note: CAT IIIB autoland not available.
Body Gear Steering not available with IRS LEFT message.

[localiser]

RAC is correct, airlines are able to tweak their manuals including the QRH, which may have additional information which is nice to know. Sometimes this is done in a contrasting colour to highlight the fact that it's a company addition. It's not to say that the info in question isn't available anywhere else, it just makes it easier to reference. Some QRHs are "fuller" in this way than others but all have enough information to contain a failure safely.

[mark744]

Ah yes, same comment in the QRH I have, (missed it at first look)
Note: CAT IIIB autoland not available.
Body Gear Steering not available with IRS LEFT message.

Also MEL only allows dispatch with Centre or Right IRU  u/s  NOT the Left

[John H Watson]

It seems odd that there is no redundancy. If it's only the Left IRU, I wonder if the captain's instrument source switch provides back up.

I know switching off the IRSs enables BGS (no message). Does ATT mode remove the IRS LEFT message?

[Britjet]

I don't suppose they are too bothered about redundancy - a u/s body gear steering (which sometimes happens anyway, typically if there is a hydraulic leak somewhere on the system and it is deliberately locked out).
It just compromises really tight turns a bit..
Peter.

[Hardy Heinlin]

If book A doesn't exactly agree with book B, one book is incomplete or wrong.

Which one agrees with the real 747-400?


|-|ardy

[emerydc8]

Maybe it's company-specific.

[Hardy Heinlin]

If I were Sherlock Holmes or Miss Marple I would suggest the following theory:

Once upon a time on a taxiway a 747-400 captain was attempting a sharp turn after IRU L had failed. That IRU was not off; it was in NAV or ATT and sending groundspeed data. The captain noticed the turn rate was lower than expected. The body gear steering was disabled obviously. The captain shared this experience with other pilots, and the company added a remark to the books that an IRU L fault disables the body gear steering. A mad rocket engineer later found out that the body gear steering was disabled because that faulty IRU L was sending a groundspeed value of 96 kt. The body gear steering control compared this value to that from IRU R which was only 6 kt. And because the control takes the higher value of the two IRUs, 96 kt in this case, or because it takes the average of both, 51 kt in that case, the exceedance above 40 50 kt disabled the control. If the erroneous groundspeed data had been isolated by deactivating the faulty IRU, the body gear steering control would have been enabled. A faulty IRU R could disable it as well. Unfortunately, that mad rocket engineer didn't share his findings with the company.

Perhaps. Something like that :-)


|-|

[cavaricooper]

This is indubitably a three pipe problem... Penzance for me.



C