Hello there,I was cruising in VNAV mode to Teheran and I was looking into my approach procedure. I played with some fixes and waypoints, switched from ILS to a VOR/DME and back and then I noticed that my FMC went into descend mode. The descend path indicator with the diamond appeared and the CRZ page wasn't accessible. I still had a big part of my flightplan to go! I remember reading about a similar issue and if I recall correctly there was a way to to get back in CRZ mode. My whole VNAV path was wrong also, so I had to descend mostly with LVL CHG. I eventually landed safely at my destination, but I would like to know how I can prevent this from happening again. What causes this?Thanks in advance!

Hello there,I was cruising in VNAV mode to Teheran and I was looking into my approach procedure. I played with some fixes and waypoints, switched from ILS to a VOR/DME and back and then I noticed that my FMC went into descend mode. The descend path indicator with the diamond appeared and the CRZ page wasn't accessible. I still had a big part of my flightplan to go! I remember reading about a similar issue and if I recall correctly there was a way to to get back in CRZ mode. My whole VNAV path was wrong also, so I had to descend mostly with LVL CHG. I eventually landed safely at my destination, but I would like to know how I can prevent this from happening again. What causes this?Thanks in advance!

You may have accidentally deleted a waypoint whilst "playing" as you put it and that WP could have been associated with TOD, and down she goes....be careful out there. Sounds like the FMC recalculated to give you VNAV errors !!Paul

Hi all,I also have a similar problem with 737-800.Initial climb was manual (no AP), and after clearing flaps, at around 6000ft, I angaged AP in LNAV and V/S mode.After passing 10000ft I set MCP Alt to 25000 and engaged VNAV mode (CRZ was set to FL290).Aircraft started to pitch UP a lot and commanded speed was 240; also, seem that FMC switched to DES - strange thing.I was not able to reach again CLB and CRZ FMC pages and not able to use VNAV mode.Though I continued my flight with LVLCHG and V/S modes and was able to land successfully in the end.My question is: what can cause this flight phase switch and how can be avoided?What is the right procedure to climb with VNAV?Other posts mentioned that as soon as aircraft is in clean configuration VNAV can be used for CLIMB.I'm currently not able to do this.FMC was programmed OK, all pre-flight initializations were done.LEGS page show speed/altitude constraints which appeared reasonable to me.I can mention one more thing: at that moment (VNAV engagement) an approach was not set in FMC, but a destination airport existed.Thank you for any suggestion/opinion,Vladimir

The LEGS page should have clues. Also, check the CLB/CRUISE/DES pages to verify info there is as expected.

The LEGS page should have clues. Also, check the CLB/CRUISE/DES pages to verify info there is as expected.

Thanks for the answer Dan,But I was unable to reach CLB/CRZ pages; only DES page was accessible at that moment.I don't remember what was the target for the descent (DES page), sorry.I'll try again and give more details if will happen again.Thanks,Vladimir

Thanks for the answer Dan,But I was unable to reach CLB/CRZ pages; only DES page was accessible at that moment.I don't remember what was the target for the descent (DES page), sorry.I'll try again and give more details if will happen again.Thanks,Vladimir

You really shouldn't be climbing past about 3000' AGL in anything other than VNAV. I don't think there's much "style" in choosing that one. I think every operator I've flown for or jumpseated with does the same.In your preflight, did you check the CLB, CRZ and DES pages? You should have your cruise alt in your pressurization panel, FMC and your MCP (when you're cleared to it).

You really shouldn't be climbing past about 3000' AGL in anything other than VNAV. I don't think there's much "style" in choosing that one. I think every operator I've flown for or jumpseated with does the same.In your preflight, did you check the CLB, CRZ and DES pages? You should have your cruise alt in your pressurization panel, FMC and your MCP (when you're cleared to it).

Matt,Since I'm not a pilot in reality (just love to know as much as I can about this) I've tried various AP modes for climbing/descending.General opinion seems to agree that VNAV is used for climbing and LVLCHG for descending - at least from what I've read till now.Yes, CLB/CRZ/DES pages were consulted before takeoff and everything was normal.I tried yesterday another flight and this time everything was OK, no strange things occurred.Probably something was messed up by me and I don't remember exactly what right now or a small software glitch.Anyway, thanks for the answers!Vladimir

Always check the route in the LEGS page during preflight, make sure you don't have discontinuities and waypoints are where you expect them to be. This is easly done by stepping in the PLN view on the ND.

I think I isolated my takeoff problem.In the end I was able to reproduce the issue in the following situation: after takeoff, AP being engaged in VNAV mode, I've played a little with REPLAY.When I returned to climbing (finished with REPLAYS), aircraft began to pitch up/down chaotically and VNAV got disconnected at some time.Further, flight phase switched to DES and VNAV became useless.So, it seems to be a problem if AP was engaged when replays were played back.If aircraft is hand-flying and sim interrupted for REPLAYS no issues occur.Vladimir

That is interesting. Thanks for reporting back.