For everyone having LNAV problems - Please Read !

flachi · 01-08-2002, 10:33 PM

Hi guys,

I think I finally found the solution for the LNAV problems that a lot of people encountered. I had exactly the same problems as the ones reported here in the forum, so I guess the chances are good that it will work for you too ...
Feel free to skip to the second section right away if this seems to much to read, but somehow I felt the need to get all pain I went through off my soul ... ;)

Let me first say, when I started on the tutorial flight, the nightmare began. I spent hours and hours trying to figure out what could be the problem, was close to giving up, tried again, only to get more frustrated. I encountered all the known problems (levelling off at 3400 or 10000 ft, LNAV disengages, the bird going into a fatal dive, the RETARD problem, etc...). After reading the post about leftover setting from previous flights, I made it a habit to set the Altitude again on the MCP every time before takeoff, and thus got rid of the unwanted levelling off. Ok, but I still didnt manage to fly the tutorial with the initial altitude restriction of 6000 ft. At some point I got the idea to try adding a SID departure, and found that the VALKO SID is already in the database! Peter McLeland posted this tip and a detailed description here recently too (great job, Peter !). Setting the altitude to 15000 ft and leaving all the work to the FMC really worked !!

But I wasn't really satisfied yet, it still bothered me that the plane was doing unexpected maneuvers, and I wanted to be able to use the FMC/MCP as it is stated in the manual : restricting the climb/descent altitude which is set in the FMC by manual settings in the MCP (like in the tutorial), but being able to resume LNAV navigation with the FMC afterwards. I found that I (sometimes) could restrict the altitude on the climb leg (getting into MCP mode) and then, after re-engaging LNAV, continue with a working LNAV to the cruise altitude. BUT, after reaching cruise altitude, I could NEVER touch the altitude setting on the MCP again, the bird did the most weird things, and I could never return to LNAV mode.

Oh well, better than nothing ... But then I realized that I was cruising at 270 IAS in LNAV mode. Hey, wait, it should be 304 according to the FMC settings. I switched to the FMC, and simply tried something out : on the CRZ page, I pressed the LRC key, the exec button illuminated, then again ECON to return to my previous setting, and hit EXEC. The bird accelerated to 304 IAS. Hmm, I thought, could this be the reason for the other problems ? I dared to change the altitude on the MCP to 13000. MCP mode engaged, climbing down, and after levelling off I switched LNAV on again, and IT WORKED !!! Getting back to 15000, LNAV mode and FMC speed remained on. I tried the same with a climb, and it worked too. After switching back to LNAV, RETARD (this time it really does...) and staying in LNAV after reaching 15000. It seemed that finally _I_ was in control ... ;)

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Ok, so here is my conclusion : Somehow the power settings get screwed up in the FMC when switching to MCP mode, and the solution is to "refresh" them by hitting execute on the CRZ, CLMB or DESC page again. After that, it is no problem getting from MCP mode back to to LNAV.

Here is my advice :

1. Make sure you select the altitude on the MCP before takeoff, even if it already shows your initial climb altitude (switch forth and back to make sure the FMC gets the setting)

2. If you are in LNAV on the climb leg, you can dial in a lower altitude on the MCP to restrict your climb. You will get MCP mode in the FMA, but this is the way it is supposed to be (Pilots, correct me if I am wrong here ...). Dialing in a HIGHER altitude doesn't have any effect, thats why we can set it to 15000ft in the tutorial and have LNAV manage everything on its own. Again, thats the way it's supposed to work (Manual : "During a climb, the aircraft will only climb as high as the lowest altitude constraint.")

3. If you want to get back to LNAV mode, engage the LNAV button. Sometimes it works right away, sometimes you have to "refresh" the FMC settings on the CLMB page. I made it a habit to do it every time I leave MCP mode ...

4. In cruise flight, changing the altitude on the MCP doesn't have any effect (if it does, you should refresh the FMC). To climb or descent to a different altitude, use LVL CHG or V/S which gets you into MCP mode again. If your FMC setting are still correct, you wont have any problem returning to LNAV mode. If not, just refresh your FMC settings and try again.

I only can say this trick works for me like a dream, and I never had any problems again so far (knock on wood ... ;)
Hope it works for the rest of you too ...

But the questions still remains, obviously there are a lot of people out here which have NO problems at all with the FMC, and a lot of people which have. Maybe it is an XP issue (I have XP, and I remember some posts about that too ...) It would be great if the DreamFleet team would investigate a little further into that ...

A final question, this time to the DreamFleet team, which I came across trying the tutorial with the database SID :

1. The VALKO waypoint in the VALKO SID contained in the database has a restricted altitude of 6000 ft. If I enter the SID by hand (like in the tutorial), it is only possible to change the altitude of waypoint EH029 (FMC computed alt 8600), but for EH009 and VALKO (FMC computed alt. FL150) no change is possible. They are obviously treated as "Cruise legs" by the FMC, which cannot be changed by the user (according to the manual). But how would I enter the "official" SID by hand ? If the "database SID" VALKO is still part of the climb leg, it should be the same when entering by hand, right ? Is this a bug ? Or is there a way around it ?

Otherwise : Great job, DreamFleet team, with your 737 I am enjoying FS2002 even more. Keep up the excellent work ... :)

One final comment : I am no pilot, and just starting to learn to fly the "bigger" jets. So if my conclusions or my understanding of FMC operations are completely "off track", please forgive me. It just makes more sense to me now ...

Dirk