B737 IRS to FCC pitch/roll coupling

[xetroV]

Hi all,

Consider a B737 classic or NG, equipped with two IRS systems and two FCC's. Why is it that the pitch and roll signals from the IRS's to the FCC's are cross-coupled (i.e. IRS L feeds the pitch channel of FCC A and the roll channel of FCC B, while IRS R feeds the roll channel of FCC A and the pitch channel of FCC B)?

I think I'm probably missing something obvious here, but I really can't think of a good reason for the design to be like this. Isn't it true that this cross-coupling effectively negates the advantages of IRS redundancy, as a single failed IRS will now affect both autopilot/flight director systems, instead of only one?

Thanks!

[Port Strobe]

Just a guess here but if IRS L fed both pitch and roll channels of FCC A and the equivalent with IRS R / FCC B, then a single IRS failure would mean all your FCC eggs are in one basket as such, whereas under the setup you described above means that whilst a single IRS failure does affect both AFDS, a subsequent failure of an FCC still leaves either a pitch or roll channel operative. Working backwards, if the IRS was paired with a FCC in both pitch and roll, then a FCC failure would leave it's respective IRS with nothing to talk to despite being fully operative, and so once again under this "crossed" configuration it affects both IRSs but there's still redundancy built in. That's my 2p anyway, perhaps someone more knowledgeable than I can straighten it out? I think I've confused myself now!

[xetroV]

Thanks for your replies!

Originally Posted by sets

It is failure mode related. The same reason why you shouldn't (can't ?) engage an autopilot with the IRS switch in BOTH-ON-x (on the 737 classic).

Tried it on a simulator: with IRS in BOTH-ON-L/R it seems to be possible to engage an autopilot, even though the procedure states you shouldn't. If this is for the same reason as the IRS cross-coupling, could someone please elaborate on these failure modes?

[xetroV]

Thanks, sets, that makes sense.