Wierd VNAV disconnects

[Guest ph-ndr]

I tried posting this before but seemingly noone had an explanation, and now I'm going at it again for more info, just in case anyone cares.It's fully repeatable and didn't happen pre SU2. The facts:ENZV to ENBR, cruise set to 16000, and using the following SID/STAR:SID LUCKY2R RNW 36 NAVAID RSY NAVAID KRM NAVAID LUCKYSTAR LUCKY1E NAVAID LUCKY 4000 NAVAID BENTI 4000 NAVAID ROBIX 3000 NAVAID GOLAX 3000 NAVAID BR443 3000 NAVAID BR441 3000 NAVAID INTEL 3000 RNW 17In the LEGS page I've modified the altitude constraint on the STAR so that I'm supposed to be passing LUCKY at 16000, BENTI at 10000 and GOLAX at 6000, and I've added RWX-17 at 5 miles with a speed constraint at 180.Flight weights don't matter, even at 153k lbs I manage to be level at 16000 before passing KRM. After takeoff I engage both LNAV and VNAV nicely during climbout, and everything runs smooth, up to the instant I level off at 16000. At that point VNAV disconnects with "VNAV DISCONNECT UNABLE CRZ ALT".The FMC is set up with cruise at 16000 and it will not engang VNAV again untill passing LUCKY. At that point it will happily stay engaged untill I intercept the ILS at ENBR/RWY17.Any taker to either shoot me in the foot or explain it otherwise?

[Guest ilh]

Is the descent path too steep from LUCKY on? You've made some huge altitude constraint changes (4000 up to 16000). But that doesn't seem to explain the UNABLE CRZ ALT.If you DELETE the altitude constraint at LUCKY, what does the FMC show for it? I wonder if there is something going on with having an altitude constraint the same as your cruise altitude.Lee Hetherington (KBED)http://www.precisionmanuals.com/images/betaimg.jpg

[Guest Mark McG]

I have noticed a VNAV Disco when the VNAV ALT CONSTRAINT and the MCP ALT Window have the same value. Deleting the ALT constraint and rengaging VNAV did the trick.

[Guest Ray CYYZ]

You are in effect setting an altitude constraint AT your cruise altitude from the way I read that.Following through the logic, because the Altitude constraint is = to cruise altitude, you will never reach cruise altitude. This is probably tripping VNAV because there actually is no valid cruise altitude available as 16000 is your descent altitude (part of your STAR).Curious if you change your cruise altitude to 16100 feet, does the problem go away?Ray

[Guest ph-ndr]

Just flew KDTW to KORD and had my MCP set at cruise to the same as the altitude constraint on the first leg of the STAR into KORD, worked a charm, and that seems awfully similar.On the route above T/C will be at some point prior to KRM and then T/D will actually be a few miles past LUCKY, so when I reengage VNAV after passing LUCKY it's all happy. Approaching the T/D I'll set the MCP ALT to 3000 and it will happily do a VNAV PATH through the whole STAR and place me picture perfect to capture the ILS just past INTEL. Then it's just enganging APP mode and keep dumping flaps and gears according to schdule and it all becomes a picture perfect landing :)Two more wierd things I have observed on the same route are:* Prior to passing RSY on climbout, the fuel remaining on the progress page ont he FMC will be 0.0 except for the last line at ENBR. Passing RSY will suddenly initialize the values for the other waypoints showed.* Programming the FMC and trying to enter FL200 (tried at heavy weights) I will place T/C and T/D right next to eachother slightly north of LUCKY, and I'm still able to clim nicely to FL200 in time to do a nice and slow 1000fpm descent towards LUCKY to meet a 16000ft altitude constraint there. It seems not to compute something right. VNAV will not engange and give me "UNABLE CRZ", even tho I can fly it.

[crvm 0]

Is there any real reason to place an altitude restriction at your CRZ altitude? If it's just after T/D, how do you expect the FMC to be able to preform such a feat in that little space-time? What I would do is hit ALT HOLD until passing LUCKY then preform a SPD DESCEnt or modify the ALT restricion to lower because DEScent will have initiated before LUCKY, this way you will keep the LUCKY restriction and the bird will temporarly level off at each restriction on the way down. Many ways to preform this if you think about it. One thing we should not expect the FMS to do is fly impossible scenarios ;) [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest ph-ndr]

It's the other way around T/D is a few miles (5-10) past lucky. The reason I ended up doing this in the first place was due to the other oddity with the FMC getting wierds about not letting me use anything higher, like FL200, beause it claimed it couldnt do it (UNABLE CRZ).The plan was to do FL200, and have the 16000ft restriction at LUCKY, and along with the others it would make for a nice climd, and then just coasting along most of the STAR.And then i reported it because it seemed like very odd behaviour :)

[crvm 0]

Sorry I am lost hehe. Why did you say this in the original post" The FMC is set up with cruise at 16000 "? [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest ph-ndr]

Cuisa altitude set to 16000, and as you notes, it's the same as the first altitude restriction in the STAR. I initially tried FL200. but then I got into the wierd thing about T/C and T/D getting stange placements along the path and I went for 16000.Will any screenshots of anything in particiular help?