So Jim,
So if I create an airport in ADE(in which I can't write an approach code, yet,) and I don't give it an ILS code, than AI will still land there? I may be running on half a memory stick here, but I thought in FS9,all AFCAD wanted was a runway ILS for AI to land. Is FSX different? I don't have approach codes for any of my own airports and AI land.
Bob
I just read this.
Lets address FS9 AFCAD and FSX ADE. Both FS9 and FSX use the AFCAD or ADE for ground movement only. That is to say all the lines that we see are for the plane once it lands and do not have any effect on a airplane approaching or touching down on a runway. The approach ILS code if present is used (not present then a fallback) to align the AI Plane flying to the center line and a touchdown point of the runway texture. The runway texture is the go between so the arrival AI plane transitions from flyable to on ground moving behavior.
I know many will argue with the runway texture because some set a runway to zero width thinking no texture exist. However, ACES has confirmed that there is no such thing has a zero width runway texture but zero is always .001 width when processed by the .dll. If there was no runway texture listed in the XML and zero meant zero then the AI plane cannot transition between flying and wheels on ground where AFCAD/ADE take command.
There is a pecking order used for the AI Plane (see below) because it fly's on coded data unlike a User Plane. The AI is contantly getting desired instructions in code vs what is actually happening and sent back to the desired for changes which is a closed loop system. A set of FD's should be ideal based on what FS says but many designers use a lesser empty weight vs actual to manage the flyability characteristics. This in turn causes other problems because the FD's are fighting the desired vs actual code and some AI planes fall out of the sky and taxi on the ground to a runway (that is a 2 fold problem).
The perfect world says all airports have at least one set of ILS instructions written in the database which then FS try's to use that runway as the default based on a scoring system.
A ILS runway and approach code is the only clear weather approach that FS9 and FSX knows. That means regardless of weather the Approach Code ILS XML instructions always take precedence over the hard code .dll instructions. That also means a poorly written set of FD's that the AI plane has is getting some help in order to fly the ILS written XML approach code in both VMC and IMC airport conditions.
Now that same AI Plane may just fall out of the sky on approach if for some reason (fair weather) ATC assigns a runway with a lower score (no ILS approach code) and the AI fallback on the hardcode .dll approach. That code is not very forgiving and the AI Planes FD's better be able to handle the no ILS XML approach code.
So FS rules for IFR Flightplan
1. If a ILS exist in the XML approach code then that takes precedence (over the default .dll) giving the AI a written set of instructions for vectors to final headings, descent altitude for the FAF, the Glidepath degree slope, trigger points for contacting Tower, missed approach turn direction, etc. It does not matter what weather is (IMC or VMC) the ILS approach code always wins.
2. If weather is clear (VMC rules apply) and a lower score runway becomes the default (no-ILS approach code) based on winds then FS has to use a defualt .dll set of instructions for the Transition from flyable to wheel on ground (reads the contact point structure).
Because there is no ILS approach code in XML the AI Plane must rely solely on a hardcode that vectors to final headings (based on true runway heading), a default 2000 ft AGL for a no FAF T_waypoint, a glidepath calculated from the runway texture touchdown point, no triggers except a hardcode rho value from touchdown, a missed approach turn to either side, etc.
Many AI Planes are never tested by the designer for a clear weather no ILS approach code in the database. If they did they would see sink rates (geardown in a turn to final) can cause a AI Plane to start descending (no written hardfloor altitude)and not reach a runway or even overfly a runway and go missed all the time.
3. Only when weather is IMC (below minimums) and I will say it again. Only when weather is below minimums does ATC go looking for additional approach code in the database. All the various type approaches (LDA, Localizer only, Backcourse, GPS, RNAV, VOR and NDB's (with or without DME) will be used in a pecking system by ATC and the AI Plane to seek a set of written instructions for the desired vs the actual.
That means if no ILS exist and weather is below minimums then ATC reverts to a pecking order of approach codes. A LDA is second in priority and a NDB is last in priority. ATC looks at the weather then looks for an approach code if no ILS exist for that runway and uses the pecking order to capture the written set of instructions for the AI Plane. If both a LDA and a NDB exist in the code ATC will instruct the AI to fly the LDA approach over the NDB. If a NDB approach is the only one that exist then ATC will tell the AI Plane and the User to fly the NDB approach as per the written XML.
Any written XML approach is better then the default hardcode approach for the AI Plane because in the XML there are needed hardfloors to always seek back to if the AI sink rate cannot recover back to the actual altitude vs the desired altitude.
If you see a AI plane that cannot land properly in clear weather (no ILS) then set your airport to IMC if other approaches by default exist. ATC will then revert out of the hardcoded .dll and use the XML written instructions. We see bad AI plane behavior at airports like TNCM because RWY 09 is not a ILS to work off of. Many complain that the AI land short in the ocean or cannot get down at all to land. Set the weather to IMC at TNCM and the AI plane will now fly a coded XML for the VORDME approach. This gives it a fighting chance to fly a better code. This helps to illustrate that many AI Planes are designed with poor FD's and the weather related VORDME approach code is much better then a visual hardcode approach.
Probably more the you wanted to know but it might also benefit others that read you ILS altitude concerns.
Thinking about it, a 8mm lower glideslope won`t kill me. 
