Greetings!Using Tim Metz's v2 tutorial, 737-800, and the flight plan filed is:KTNT CURVE V35 DHP J79 VRB LBV RSW CYY MARCI KTNT (at FL270).I have programmed the FMC fine - and one the LEGS page, i wanta constraint for DHP to be 270/15000, which I enter.After departure, before reaching CURVE, at around 4000', and in LNAV and LVL CHG mode, I set MCP altitude to 27000, and then push VNAV.then I get "UNABLE NEXT ALT" This is before CURVE, which gives me more than 20nm to DHP.I have attached a picture below indicating the issue. You can see the check FMC light is lit, and also you can see on the ND that I am before CURVE and DHP is the next waypoint, DTG=16nm. The constraint is 270/15000.I have tried multiple times, and always get this error. I tried pressing VNAV on/off to disconnect & Reconnect it, and it always says check FMC and I see "UNABLE NEXT ALT".cheersJoshhttp://forums.avsim.net/user_files/92096.jpg

Hi Josh,Usually these are the following reasons why you get that message:1)Distance to short (which it might be your case only 22 NM to WPT and you are only 4800FT, so you have left to climb 10200 FT, that will translate in about 28 to 30 NM) 2)A/C too heavy3)Wrong cost index in PERF INT4)Wrong setting in N1 LIMIT PAGE (maybe you have CLB2 or CLB1 versus CLB)5)I would set MAX ANGLE speed in CLB PAGE.Try to delete DHP speed/altitude constrain and see what is the FMC predicting for that wpt passage.Also give some more information regarding WT and what kind of settings you have on FMC. Very difficult to find a solution or explanation without all the data.I would like to point out that I am not a 737 driver, but I am a 744 driver and these scenarios happen during daily operations and the reasons are just the ones above mentioned.Let us know how it works and what your findings are.

Alex,I think I figured out my mistake..If i push VNAV *before* i reach the MCP altitude (4000'), PFD goes into N1 and VNAV SPD.If I wait to first reach MCP alt, ALT-ACQ..ALT-HOLD.. and then push VNAV, i get "UNABLE NEXT ALT"What am I doing wrong? Is the FMC not able to VNAV Climb after MCP ALT is reached?Here are my settings:======================act perf init=============gw/crz: 120.8fuel: 14.7zfw: 106reserve: 5.0cost: 150crz alt: FL270n1 limit========clb-1 profile, 22K Deratetakeoff=======flaps 5v1=119Vr=122v2=129legs=====curve 250/7768dhp 315/15000pbi .773/FL270vrb .773/FL270lbv .773/FL270rsw .773/FL270cyy 315/FL257marci 315/FL193ktnt 240/13initial MCP settings=====================alt=4000hdg=94 (rwy 09)ias= V2+15procedure=========at 400' = CMD A (AP 1)push LNAVLVL CHG at 4000' and ALT HOLD1) set MCP ALT=FL2702) push VNAVERROR: UNABLE NEXT ALTThank-you!!cheersJosh

I'm thinking the airplane is leveling off at 4000 feet just a little to long and/or the 15,000 feet contraint may be just a little to close. Go to the LEGs page and DELETE the altitude constraint abeam the waypoint in question and read the predicted waypoint crossing altitude. If it's more than 15,000 feet then you know the problem.Try not leveling off at 4000 feet and see if the airplane can make the altitude constraint at 270/15,000.Note: Don't think you'll see too many speed constraints during a climb in real world flying, I think its unusual, but maybe I'm wrong.*****

Floyd

>Alex,>>I think I figured out my mistake..>>If i push VNAV *before* i reach the MCP altitude (4000'), PFD>goes into N1 and VNAV SPD.I've seen this happen, usually when you change the MCP altitude just after the airplane starts to capture the previously set altitude (i.e. 4000).The way to resume the climb is to deselect and reselect VNAV, OR hit the ALT INTV button.What I THINK is happening is the sim treats the MCP ALT like a waypoint constraint once it captures the MCP ALT, and pressing ALT INTV clears it. But hopefully one of the programmers can explain it better.What I (and others) expected to happen is either:When reaching the MCP altitude (4000) ALT HOLD should go active and VNAV should extinguish, ORResetting the MCP altitude higher will resume the climb.It's odd to be sitting in VNAV in the climb phase and not be climbing.

What I THINK is happening is the sim treats the MCP ALT like a waypoint constraint once it captures the MCP ALT, and pressing ALT INTV clears it. But hopefully one of the programmers can explain it better.What I (and others) expected to happen is either:When reaching the MCP altitude (4000) ALT HOLD should go active and VNAV should extinguish, ORResetting the MCP altitude higher will resume the climb.It's odd to be sitting in VNAV in the climb phase and not be climbing. Anytime the MCP is set to anything other than the FMC criuse altitude, you enter a temporary level off situation and it enters ALT HOLD (VNAV light extinguishers or SHOULD). The FMA should still show FMC SPD/ALT HOLD which is a "special situation" with the FMC controlling SPEED and not like a ACQ - ALT HOLD from a non FMC mode where SPEED is MCP controlled I believe. I wrote this a while back about this on the forum - "QUESTION: What happens if you have a DIFFERENT altitude set on the MCP altitude window than what is set for your cruise altitude on the FMC?ANSWER: This will cause the aircraft to temporary level off at the MCP altitude. The FMA will display FMC SPD and ALT HOLD, the MCP SPD window will stay blank. Reset the MCP altitude and engage VNAV to continue your climb." If on the other hand you have VNAV ALT mode selected from the styles menu and enter a similar temporary mcp altitude (other than FMC cruise altitude), VNAV will enter VNAV ALT mode insted of ALT HOLD mode and VNAV will stay engaged (FMA will show VNAV ALT) not sure about the MCP VNAV light but believe it goes out. The FMC maintains the MCP altitude and FMC target speed for the current phase of flight, in this case most likely ECON climb target. You re-set the MCP and push the VNAV INTV button to resume the climb. Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |

Check the undershoot error at DHP on the climb page, OR delete the constraint on legs page without execing, and checkout what the prediction would be.It's probably just that the FMC thinks it is unable to meet the constraint.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

> If on the other hand you have VNAV ALT mode selected from the>styles menu and enter a similar temporary mcp altitude (other>than FMC cruise altitude), VNAV will enter VNAV ALT mode>insted of ALT HOLD mode and VNAV will stay engaged (FMA will>show VNAV ALT) not sure about the MCP VNAV light but believe>it goes out. The FMC maintains the MCP altitude and FMC target>speed for the current phase of flight, in this case>most likely ECON climb target. You re-set the MCP and push the>VNAV INTV button to resume the climb.That's the ticket, and explains why the MCP VNAV button stays lit... I'll look again at the mode annunciator and check for VNAV ALT.As mentioned before, VNAV ALT and the use of ALT INTV is one of the modes that confuses lots of folks - it struck again!

Anthony,FMC constraint without alt. restriction is 315/15600.CURVE: 250/7700DHP: 315/15600the rest are .773/FL270here is the climb page:http://forums.avsim.net/user_files/92165.jpghere is the legs page (with the restriction):http://forums.avsim.net/user_files/92166.jpgI *do* have VNAV ALT mode selected from the Styles menu.Initial MCP Altitude= 4000'http://forums.avsim.net/user_files/92168.jpgSo, if i push VNAV *before* i reach the MCP altitude, no problem.If I wait until it is ALT HOLD, and push it, then I getUNABLE NEXT ALTFolks - feel free totry out my flight plan posted above from Tim Metz's tutorial v2 to confirm what might be happening.Thanks for all your help.cheersJosh

AnthonySorry, here are the proper #'s for the FMC without the Alt constraints:http://forums.avsim.net/user_files/92181.jpghttp://forums.avsim.net/user_files/92182.jpgMy mistake. cheersJosh

Folks,Never mind - Randy is right,, after leveling off at MCP alt 4000' if you reset MCP alt and push VNAV, ding-dong FMC UNABLE ALT.*however*, if you push ALT-INTV and then VNAV, no ding-dong FMC and life is good.Notice FAM mode in PFD.http://forums.avsim.net/user_files/92184.jpgIf i do the same thing again, that is, allow ALT-HOLD at 4000', and push VNAV, it doesn't do it. question though..FMC says UNABLE NEXT ALT..why does LVL CHG still work.....?thanks everyonecheersJosh

LVL CHG does not care about FMC constraints. It is not an "integrated" mode.It will climb / descend until reaching MCP alt, and then revert to ALT CAP / ALT HOLD.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com