Originally Posted by
Denti
The 737 is perfectly capable of fail operational CAT IIIb operation. It is just that most carriers do not want to spend the extra money on the maintenance for the rudder servo that is fitted additionally. The autoflight system is inherently capable of CAT IIIb out of the box though, it is just not activated for most carriers. And the CAT IIIb fail-operational airplanes still have the same trim up thing going on, both in dual engine and one engine out autolands (yes, those are approved as well if one has the fail-operational version, flaps 30 for landing as well).
That is quite correct. It has a curious 2 APLT 3 IRS reference architecture that provides for the fail-operational capability. With 2 APLT and without the option, it is fail passive with 2 APLTs. There is a note in the FCTM for the plane that says, for single-channel ops, refer to limitations section of the AFM, and in most cases that requires a single channel to be disconnected no lower than 50ft AGL, for FAA and 158ft AGL for EASA....