B747-400: A/P and IRUs

[Bluescan]

Good morning all!

It has been my understanding that A/P (or FCCs) L, C and R uses IRU L, C and R respectively. Also, the CPT and FO IRS source selectors are irrelevant in regards to the autopilots, in contrast to eg. ADC source selectors.

In my last sim the IRU R failed while flying on A/P R. The NNC finally ended with us putting IRS R to OFF, but there were no mention of any deferred items. We flew on A/P R for the remainder of the sim. Only LAND 2 was available for landing.

I'm no system expert but I would imagine that a attitude source was required for the autopilot to stay engaged. :confused:

Do anyone have insight in how this work?

Best regards,
Bluescan

[NSEU]

The FCC's still talk to each other (sharing IRU data).

There are some Service Bulletins regarding A/P engagement and the availability of "inner loop" data. e.g. pre-SB 22-xxxx, in the air, A/P engagement requres at least two inner loop sensors being valid (i.e. any two IRS's). Post-SB 22-xxxx, in air, single channel engagement is allowed only if the local sensor source is valid (i.e. local IRU data... e.g. Left A/P requires Left IRU)

Some SB's are not mandatory, but, even so, I'm wondering if the term "engagement" is different to "being engaged and remaining engaged".

[CCA]

To add further information.

Each FCC has one, two, or three sources for the necessary data. The FCC must select which source has the best data. To do this, the FCCs use a special source selection procedure. Where there are three sources available, the data from each sensor is available to each FCC. Each sensor has an onside connection (left sensor to left FCC, etc.). This is the local sensor data for that FCC. Each FCC also gets data from the other sensors of the same type through the FCC cross-channel data buses.

[Bluescan]

Thank you NSEU and CAA, that is very informal.

May I ask what sources you have for this information? AMM?
The FCOM doesn't excactly go in to details regarding these systems. :-)

[CCA]

Maintenance technical training manual.