Body Gear Steering and IRUs

[John H Watson]

That IRU was not off; it was in NAV or ATT and sending groundspeed data.

Is groundspeed data active in ATT? If not, did our captain have time to switch to ATT?

And because the control takes the higher value of the two IRUs, 95 kt in this case, or because it takes the average of both, 50 kt in that case, the exceedance above 40 kt disabled the control.

The hysteresis in the wiring schematic is confusing. >50kts disables, <40 kts enables. Does enabling have to occur before disabling? What happens at 45kts?

Maybe it's company-specific.

Jon, how do you read that note relating to Satcom? Does that registration range refer only to item 1? We have a similar note in our QRH, but I can't make any sense of the registration range in ours. I can't tell if it excludes any aircraft. The wiring diagrams seem to suggest all our fleet only has Left IRU input to Satcom (with no switching)

[Hardy Heinlin]

ATT mode doesn't provide groundspeed. You can disregard this part in Miss Marple's theory :-)

Also: >50 kt, not >40 kt.

Every hysterisis needs a start. When a new device is installed, it probably starts at 0 kt. From then on it's easy: At x disable the enabled. At y enable the disabled.

[emerydc8]

Jon, how do you read that note relating to Satcom? Does that registration range refer only to item 1? We have a similar note in our QRH, but I can't make any sense of the registration range in ours. I can't tell if it excludes any aircraft. The wiring diagrams seem to suggest all our fleet only has Left IRU input to Satcom (with no switching)

That's a UPS checklist. I'll have to ask a friend.

[Follow-up] I asked a friend over at UPS which aircraft lose satcom. He doesn't know by looking at the checklist whether it's only N570UP and N579UP or if it's the whole range (10 aircraft). He said UPS only has 13 744s and 4 748s, with 24 more 748s coming. Rumor is that there is an option to buy 14 more in addition to that.

[John H Watson]

Thanks, Jon. Seems like I'm not the only one confused. Who writes these manuals? 

[emerydc8]

I'm convinced that line pilots are usually left out of the loop on checklists.