VNAV Problem?

[Guest rellehenk]

I just did a flight from EKCH to EHAM and I had a small problem. (unless it's normal of course :D)Before the T/D-point ATC instructed me to descent to FL220, so I started a LVL-CHANGE descent out of FL340 for FL220.Later on ATC instructed me to cross a waypoint at FL100. So I put the restriction in de FMC, set the MCP to FL100 and hit the VNAV button. But what happened?: The airplane started to climb because the first waypoint on my flightplan had an altitude of FL230.Am I doing something wrong? I tested it twice.Regards

[Guest amerton]

Interesting situation...You started your descent, but the FMC doesn't know about it...Next time : initiate the descent phase by commanding DES NOW, and then get in whatever pitch mode you want.Any one knows how the real FMC would handle that ?anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[B777ER 2,484]

>I just did a flight from EKCH to EHAM and I had a small>problem. (unless it's normal of course :D)>>Before the T/D-point ATC instructed me to descent to FL220, so>I started a LVL-CHANGE descent out of FL340 for FL220.>Later on ATC instructed me to cross a waypoint at FL100. So I>put the restriction in de FMC, set the MCP to FL100 and hit>the VNAV button. But what happened?: The airplane started to>climb because the first waypoint on my flightplan had an>altitude of FL230.>>Am I doing something wrong? I tested it twice.>>Regards>>Well, if I was to hazard a guess, I would say that after you initiated your descent to FL220, you had not crossed the next waypoint before the waypoint that you inputed the FL100 restriction into, therefore your FMC thought you needed to climb to meet the FL230 altitude. Let me give an example:-Your level at FL340. You are 50NM from waypoint 1 when told to decend to FL220. But your T/D point is 15NM from waypoint 1 so you are descending 35NM before your FMC is telling you too.-At 30NM from waypoint 2, ATC tells you to cross waypoint 3 at FL100. Currently your passing through FL160. -You input that restriction into the FMC and select VNAV.-Here is the problem, your FMC did not adjust the altitude for waypoint 2 which according to the FMc was supposed to be FL230. So it starts a climb to meet the altitude for waypoint 2. The reason being I believe is because you selected LVL/CHANGE instead of DES NOW back when ATC told you to initially descend. By selecting LVL/CHANGE, the FMC did not recalculate the altitudes for the waypoints. I may be wrong (usually am) but this may be the cause.

[Guest rellehenk]

Ok, next time I will hit the "DES NOW" button before I start an descent.I thought it was not necessary because I did a LVL CHANGE descent.What I did exactely:1. Cruising @ FL340 20 miles away from T/D2. Instructed to descent to FL2203. Set MCP to FL220 -> LVL CHANGE4. Aircraft leveling off at FL2205. ATC: Cross waypoint at FL100 6: Insert restriction, MCP at FL100, engaged VNAV7: Aircraft starts climbing.I am curious what will happen in real life.Thanks for the answersRegards

[Guest Ray CYYZ]

Level Change is not going to change your mode from Active Climb to Active Cruise to Active Descent. These mode changes in the FMC are done by reaching the TOC (based on FMC Cruise Altitude) then reaching TOD (based on predictions required to make Landing) or by pressing the Descend Now button.Reset the Cruise Altitude in the FMC once you had completed the Level Change to FL220 (step 4a) will clear the 12,000 feet error in VNAV otherwise you will climb back UP instead of down to try and correct the 12,000 feet error in where VNAV thinks you need to descend properly to FL100. If you look at the deviation scale on the ND, you will see it is completely pegged at the end because VNAV is seeing you 12,000 feet off your cruise altitude (step 7).It would be simpler to just reset your cruise Altitude in the FMC then re-engage VNAV, but I don't know real procedures so take it with a grain of salt ;)Ray

[Guest amerton]

Yep...The only thing I can add to your very accurate reply is that *unfortunately* the deviation scale on the ND will stop showing the deviation if it is greater that 9999 ft... As per the real thing...But then, one can refer to the deviation on the DES page...Very interesting topic ;-)anthonyPS : LVL CHG is not an FMC driven pitch mode. You can LVL CHG without even ever initializing the FMC.Anthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest rellehenk]

Thanks Ray, I will try that too.Just did the flight again.Here are the steps again1. Cruising @ FL340 20 miles away from T/D2. Instructed to descent to FL2203. Set MCP to FL220 -> LVL CHANGE4. Aircraft leveling off at FL2205. ATC: Cross waypoint at FL100 6: Insert restriction waypoint3: 250/FL1007: "cruising" at FL2208: LEGS PAGE: first waypoint (1) in the list: 290/FL210 (below my cruise altitude, so this is correct)9: Set MCP to FL100, engaging VNAV10: First waypoint (1) in LEGS-page (same as above) now shows FL 225 (above my present alititude) and increases, aircraft starts to climb.So the FMC knows I am descending, but at the moment I hit the VNAV button, things go wrong.

[Guest amerton]

Reset your FMC cruize alt in the CRZ page (as you have not initiated a descent with DES NOW).LVL CHG is not an FMC mode ! No way the FMC can know that it should consider you have started your descentBut hey... hmmmm there's a small inconsistency there. It seems in cruize mode the FMC in this situation does wrongly not give you the CHECK MCP ALT message and refuse to engage... because your AC is in between target cruize alt and the MCP alt... I have to check that ;-)In principle, in the situation you describe, VNAV should definitely not engage...anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest rellehenk]

So every time I make a descent with LVL CHANGE, I will have to insert my new altitude in the FMC? (if I am planning to make a VNAV descent after it)What I meant with "FMC knows I am descending" is that the values of the altitude-restrictions are changing when I am in a LVL-CHANGE descent.I hope someone can tell what the real procedure is for a combination of a LVL CHANGE descent (before T/D) and a VNAV descent.Anthony, thanks for your help.Regards