Dear CommunityIt's now about 3 Months that I'm flying the 738 and I am happy to announce that I do have achieved quite some basic understandings, allowing me to do a propper flight incl. SID/STAR with FD from A-B incl. smooth landing :-)During this time, I have noted some questions I'd like to ask you - maybe one of you can help me:a) LNAV/VNAV and/or VOR LOC / APPR - ButtonsI have from time to time the situations that those buttons extinguish.Example:I am in final approach on GMME (Algier) to ILS 21. NAV-Frequency 1/2 is set and OK, I engange VOR LOC while I am approaching around 30 degrees from below the GS. Now I engage APPR and the plane catches the GS and everything is OK. I am working my stuff (flaps, speed etc.) and all is OK. Then, while I do an instrument-panel sweep I realise that the APPR button is extinguised (AP main button is on) This is both on virtual cockpit and on 2D-Cockpit. I cannot engage it again. But still - the approach works well - it' s just the button that is off.Question: Is that normal? I don't remember having seen it extinguisedB) FMC Altitude vs. LNAVOK - I am not a pro - all I understand is that there are "white altitude entries" and "pink altitude entries" and if I remember well, the white ones are "fix" and the purple ones somehow "not"However - on that same flight approaching Rabat (GMME) in Morocco I watched the FMC waypoints and their white altitude entires. The plane was on LNAV/VNAV and AP was on. While the plane was following propperly the vertical path, the lateral was not correct. This is what happened:The FMC showed the next waypoint at say 8400 Ft - but my plane was continuing going down (was at 4000 Ft and still going down. It was even lower than the 2nd next waypoint ahead. The altitude settings on the MCP was at 1800 FT (I did this because this is the altitude at which I will catch the GS and I don't want to continuously lower the MCP Alt Settings - I understand that the plane should follow the FMC Alt of the waypoints, right?So, why is the plane going "BELOW" the "WHITE" waypoint altitude?c) 1000 Ft Altitude difference (Plane vs. FSX ATC)OK - this happened very rarely - I can recall it once after take off from LSZH (Zurich Switzerland). I was heading north-east towards Germany. ATC asked me to climb to 7000 Ft. and thus I set 7000 on MCP. Plane is on LNAV/VNAV mode, AP engaged. Baro is set correctly.Now, I climb and cross 6000 Ft continuing towards 7000 Ft. Now ATC (FSX-ATC) says: You are 300 Ft above your assigned altitude. But I'm not! ATC is only happy if I stay at 6000 Ft instead of 7000 Ft.Now, this is something that happened to me very very rarely and thus I don't think that it's a user error - as if so, then I would have that more often :-). Do you have any idea what that is?I have to say that I have not yet installed SP1c - just in case you wonder.Thank you very much for your valuable input!CheersMichel

I've not got the time in NGX yet to be real knowledgeable, but I can assist a little:a: What is your flight mode annuciater telling you? Are you on LOC/GS and if so don't worry about the button on the MCP.b: I'm not really following this one, is the error lateral or verticle? What about the verticle path deviation indicator on the ND, if you are centered then you are on path.c: There is no relationship between the default ATC and the PMDG simulation. Basically, the default ATC is verrry basic, a true game-like feature than barely resembles ATC. Also, nonething in the FMS is available to the ATC module running inside FSX.

Hi Michel,To your point b. Did you see a "B" beside the white altitude entries? If yes, this means that the shown number is the maximum altitude which you are allowed to have there, however the VNAV probably calculated a lower flight path because to save fuel and to meet other crossings restrictions. If there wasn't a "B", which stands for below, beside it than something went wrong, or we are missunderstanding something in the VNAV guidance system.Best regardsJonathan

To the first question, yes that is completely normal. At that point the FDs are committed to APP mode. The only way to get out of that is to go around (hit TOGA) or cycle both FDs off and on.

a. It is normal for the MCP buttons to extinguish in several cases - this is letting you know that the autopilot is "committed" to the current FMA modes or flight phase - TOGA and approach are the two most common examples of this. To get out of either phase when the buttons are extinguished you have to actually cycle the flight directors on and off. You never look at the MCP button lights to tell what modes you're in - always look at the FMA strip at the top of the PFD - whatever's in green there is what you're in, period.b. Magenta on the LEGS page means the number is the FMC's current restricting altitude/speed etc. That's a general rule in Boeing systems - magenta always means the currently active/restricting item. I'd have to see the precise example you're talking about but if the altitude was just in normal size font and white, that's simply a VNAV prediction, not a restriction. Assuming you were perfectly following the path (both vertical and lateral), the FMC thinks you'll hit that waypoint at that altitude and speed based on its prediction algorithms. A restriction that is not the active one will appear in white but it will be bigger/bolder on the LEGS page. You can always see what your current descent restrictions actually are by pressing the DES button. The current restricting altitude is at LSK 1R and the current restricting speed is at LSK 3L. If you see something like "220/NIRSI" that's telling your speed is being restricted by what's entered at the NIRSI waypoint on the LEGS page. You can see things like "204/FLAPS" too, which means that the speed's being restricted from going lower by the current flap setting. "240/10000" is common there too in the initial part of the descent if there's no other speed restriction entered at a waypoint.So, in summary there's three ways speeds and altitudes can be displayed on the LEGS page:Magenta - this is the current restriction/target.Normal size white - this is a path prediction, not a restrictionLarger/bold white - this is a restriction that is not yet active. These can be coded into the SID/STAR/Approach already in the navdata, or they can be entered manually by the pilot.c. FSX ATC is pretty awful with advanced addons like this - if you're using real weather with AS2012 or REX or whatever it doesn't understand the pressures and yells at you for allegedly being too high or too low - does the same with AI traffic too. Get on VATSIM or IVAO - much more realistic.

Hi AllThanks a lot for your kind support! I haven't had the time to drop by lately but I took well notice of your hints/recommendations!CheersMerlinMichel Bühler