A detailed explanation of the terms Fail Operational and Fail Passive
Automatic Landing System
Fail-operational Automatic Landing System.
Fail-operational Hybrid Landing System
NOTE: A typical secondary independent guidance system consists
of a monitored head-up display providing guidance which normally takes
the form of command information, but it may alternatively be situation
(or deviation) information.
Pallet has a good book, Automatic flight Control with a section on system
reliability and redundancy - a worthwhile read.
2. Also see http://www.wingsys.com/apalssa.htm (HUD - same as visual)
is annunciated for Fail-Operational mode and Land
2 for Fail-Passive, for three A/P systems. No
Autoland obviously indicates a fault which precludes the use
of A/Ps for automatic landing. The annunciator system is active only when
the aircraft is below 1500' RA and with G/S and LOC capture. Below 200'
RA, the only change permitted in the annunciation is to No Autoland.
Land 3 or 2 annunciations mean that G/S or LOC signals are not being received. If the station fails, the aircraft continues on an inertial track for a short time until the problem is annunciated on the EADI, affected flight director commands are removed and A/P and master caution lights illuminate. The indication on the Autoland Status Annunciators may or may not change.
If a Land 2 condition exists and below 100' RA, an increment of nose-up trim is automatically applied for the flare. If the A/Ps are subsequently disengaged in the approach, a forward control force (20-30lbs) is required to counter this automatic trim condition. It is automatically removed if a multi A/P GA is initiated.
A/P control of the rudder is only active when either Land 3 or 2 is active (hence only below 1500' RA). Additionally, the Runway Align, Flare and Rollout submodes are only available during these times.
The FCCs are powered by separate electrical sources when a multi A/P approach is initiated, so the loss of a bus during a Land 3 approach is not critical, however, during a Land 2 it might mean that it's all over for autoland that day.
This all based on 767-300s
Either mode would work for RoboLander