Another question's answers refer to certain Boeing jets having the capability to annunciate "Land 2" and "Land 3" when flying a coupled ILS approach. What does each of these modes require, and allow, beyond the basic coupled ILS? How are they different from one another?
From "Smart Cockpit"
B777 Automatic Flight (PDF!)
AFDS Status Annunciation
The following AFDS status annunciations are displayed just above the PFD attitude display:
• FLT DIR (the flight director is ON and the autopilots are not engaged)
• A/P (the autopilots are engaged)
• LAND 3 (three autopilots are engaged and operating normally for an automatic landing)
• LAND 2 (AFDS redundancy is reduced; in some cases, only two autopilots are available)
• NO AUTOLAND (the AFDS is unable to make an automatic landing).
With a LAND 3 indication, the autopilot system level of redundancy is such that a single fault cannot prevent the autopilot system from making an automatic landing (fail operational).
With a LAND 2 indication, the level of redundancy is such that a single fault cannot cause a significant deviation from the flight path (fail passive).
An EICAS message is displayed for any fault which limits the capability of the automatic landing system. Aural alerts for EICAS messages not affecting safety of flight are inhibited until after touchdown. Changes in autoland status below 200 feet, other than a transition to NO AUTOLAND status, are inhibited.
From Chapter 9, pages 22-23