Added Comments on LAND3 versus LAND2 and the B737
Note that LAND 3 DOES NOT specifically mean 3 APs. Instead it means "Fail-Operational" capability. Similarly, LAND 2 does NOT specifically mean 2 APs. Instead LAND2 means "Fail-Passive" Capability. Both triple Ch APs and Dual-Dual APs were demonstrated to be capable of "Fail-Op" Cat III, and were approved for fail-op Cat III. The modern B737NG with the newer EDFCS AP has been shown to be equivalent to a triple channel AP, for the purposes of "Fail-Op" LAND 3 capability.
For some types of failures (including on the B737NGs with EDFCS), a Fail-Op AP LAND3 mode may revert to Fail-Passive LAND2 mode, ...and when authorized, some operators may continue an approach following those failure conditions, following a LAND3 to LAND2 mode reversion.
This is because the AP has been shown to meet Fail-Op to Fail Passive reversion acceptability. Accordingly, for some operators and aircraft types, Op-Specs may allow the operator to continue an approach following a LAND3 to LAND2 reversion. However, the minima typically are different (need a higher RVR) when that happens. Fail Op Cat III (using LAND3) typically does not use or need any DA(H), since an "Alert Height" is used to assure AP availability even through rollout. Hence RVR limits are typically lower for Fail-Op systems, down to RVR300 ft. Whereas for Fail-Passive APs and FP Cat III operations, a 50' DA(H) is typically used [a required DH], with more conservative RVR minima typically set, now at about RVR 600 ft. (even though both of these minima are actually considered to be within the Cat IIIb range). Decades ago, when the Fail-Passive APs limits were first set for Cat III (e.g., B727) a 50' DH with RVR 700/700/700 was required, leading to Fail-Passive systems once being commonly identified with limits at Cat IIIa. But that constraint is long obsolete, since the 1980s, and now both LAND3 and LAND2 systems can operate to Cat IIIb minima, albeit LAND3 Fail-Op systems generally have considerably lower RVR limits, currently set at RVR300/300/300 ft.
When use of an AIII mode HUD and LAND2 Autoland are combined by an operator, to achieve an equivalent Fail-Op hybrid system, other minima may apply (e.g., minima credit using a Hybrid Fail-Op System).
See FAA AC120-28D for more details.