I'm not sure if FSX models the autoland correctly. I'm still using FS 2002, the B737 - 400 in FS 2002 will fly coupled aproaches (LOC and G/S capture) but won't autoland.
Anyway this is how it should work (B737 NG)
Approach (APP) Mode Dual A/Ps
Approach mode allows both A/Ps to be engaged at the same time. Dual A/P
operation provides fail–passive operation through landing flare and touchdown or
an automatic go–around. During fail passive operation, the flight controls respond
to the A/P commanding the lesser control movement. If a failure occurs in one
A/P, the failed channel is counteracted by the second channel such that both A/Ps
disconnect with minimal airplane maneuvering and with aural and visual
warnings to the pilot.
One VHF NAV receiver must be tuned to an ILS frequency before the approach
mode can be selected. For a dual A/P approach, the second VHF NAV receiver
must be tuned to the ILS frequency and the corresponding A/P engaged in CMD
prior to 800 feet RA.
Localizer and Glideslope Armed
After setting the localizer frequency and course, pushing the APP switch selects
the APP mode. The APP switch illuminates and VOR/LOC and G/S annunciate
armed. The APP mode permits selecting the second A/P to engage in CMD. This
arms the second A/P for automatic engagement after LOC and G/S capture and
when descent below 1500 RA occurs.
The localizer can be intercepted in the HDG SEL, CWS R or LNAV mode.
Glideslope (G/S) capture is inhibited prior to localizer capture.
Localizer Capture
The LOC capture point is variable and depends on intercept angle and rate of
closure. Capture occurs no later than 1/2 dot. Upon LOC capture, VOR/LOC
annunciates captured, SINGLE CH is annunciated for A/P status, the previous roll
mode disengages and the airplane turns to track the LOC.
Glideslope Capture
Glideslope capture is inhibited prior to localizer capture.
The G/S can be captured from above or below. Capture occurs at 2/5 dot and
results in the following:
• G/S annunciates captured
• previous pitch mode disengages
• APP light extinguishes if localizer has also been captured
• airplane pitch tracks the G/S
• GA displayed on thrust mode display (N1 thrust limit).
After VOR/LOC and G/S are both captured, the APP mode can be exited by:
• pushing a TO/GA switch
• disengaging A/P and turning off both F/D switches
• retuning a VHF NAV receiver.
After LOC and G/S Capture
Shortly after capturing LOC or G/S and below 1500 feet RA:
• the second A/P couples with the flight controls
• test of the ILS deviation monitor system is performed and the G/S or LOC
display turns amber and flashes
• FLARE armed is annunciated
• the SINGLE CH annunciation extinguishes
• A/P go–around mode arms but is not annunciated.
Note: After localizer and glideslope capture during a dual autopilot approach,
CWS cannot be engaged by manually overriding pitch and roll. Manual
override of autopilots causes autopilot disengagement.
The A/Ps disengage and the F/D command bars retract to indicate an invalid ILS
signal.
800 Feet Radio Altitude
The second A/P must be engaged in CMD by 800 feet RA to execute a dual
channel A/P approach. Otherwise, CMD engagement of the second A/P is
inhibited.
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.
Flare
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.
• 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.