How does Autoland work below decision height (200 ft)? Ie, what guides the plane until the radar altimeter notes the jet is at the right height to begin to flare? I've been searching the internet, and it looks like the plane simply holds its attitude for around 100 feet until the RA kicks in.
An autoland can be performed when a CAT II or III ILS (Instrument Landing System) approach is available (bad things can happen otherwise: Why don't pilots always use autoland?). The decision height for these approaches is lower than the usual 200 ft:
- Cat I: DA 200 ft or higher
- Cat II: DA 100-200 ft
- Cat III A: DA < 100 ft
- Cat III B: DA < 50 ft
The pilots must have the runway (or runway lighting) in sight when reaching the decision height. Otherwise, a go around must be executed. But this does not mean that the ILS signal is no longer available. The autoland system continues to use the ILS signal together with the RA (radio altimeter) to fly the aircraft. That means the autopilot uses the localizer deviation to control the aircraft laterally (during the approach, the flare and even after landing) and the glide slope deviation vertically until the RA indicates it is time to start the flare (at this time glide slope is ignored).
The exact details will of course depend on the aircraft model. Here is how the autoland works on a Boeing 737 NG (source: FCOM v2 4.20.19 Automatic Flight - System Description):
500 Feet Radio Altitude
The pilot is required to check for the presence of LAND 3 or LAND 2 in order to continue the autoland. If the second autopilot in CMD remains armed and does not engage, LAND 2 or LAND 3 does not annunciate. Instead, the amber NO AUTOLAND annunciation alerts the pilot that dual control has not been established and the autoland is to be discontinued.
450 Feet Radio Altitude
The alignment mode is enabled which provides rudder compensation for the purpose of decreasing large crab angles produced by crosswinds, and to control the adverse moments caused by an engine failure. The automatic correction for aircraft crab angle due to crosswinds and engine failure enhances flight crew runway perspective and provides optimal aircraft position for initiation of rollout control. In a strong crosswind, the airplane does not fully align with the runway but lands in a slight crab. Sideslip is limited to 5 degrees. This mode is not annunciated.
400 Feet Radio Altitude
The stabilizer is automatically trimmed an additional amount nose up. If the A/Ps subsequently disengage, forward control column force may be required to hold the desired pitch attitude.
If FLARE is not armed by approximately 350 feet RA, both A/Ps automatically disengage.
The A/P flare maneuver starts at approximately 50 feet RA and is completed at touchdown:
- FLARE engaged is annunciated and F/D command bars retract.
- FLARE engaged is annunciated and with LAND 3 annunciated, F/D command bars center.
- the A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown. A/T FMA annunciates RETARD.
- the A/T automatically disengages approximately 2 seconds after touchdown.
- the A/P must be manually disengaged after touchdown. Landing rollout is executed manually after disengaging the A/P.
ROLLOUT arms when LAND 2 or LAND 3 annunciates. At approximately two feet radio altitude, rollout activates:
- ROLLOUT replaces the VOR/LOC roll flight mode annunciation
- the autopilot controls the rudder and nose wheel steering to keep the airplane on the localizer centerline.
- rollout guidance continues until a full stop or until the autopilots are disengaged.
Autoland can only be performed with CAT II/III as far as I know. The minima of CAT II/III are usually quite a bit lower than 200ft AGL. Also, as the localizer is placed at the far end of the runway, the aircraft will not overfly it and therefore not lose it on landing. The aircraft will just follow the localizer and glideslope until the radar altimeter triggers the flare and leaves the glideslope.
To improve the reliability (or reduce risk of disturbance) there is also a second hold-short line a bit further away from the runway which is being used when CAT II/III is in progress. This assures no other aircraft reflects or blocks localizer or glideslope signals.