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ahsmatt7

VNAV DISCONNECT flying the COORZ3 out of KDEN

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I saw the post regarding this issue out of KSNA. I however and flying out of denver with on a departure with altitude constraints and cant engage vnav. It wont calculate altitudes.

 

Any ideas?

 

the route is as follows.

 

COORZ3 VOAXA Q136 OAL INYOE.DYAMD2

 

i got the issue when i loaded the imported roite from PFPX as well as a manual input of the route.

 

Thanks ahead of time for the help.

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Are you on the ground doing preflight?  Did you select departure runway? Which one?  PERF page set with CI? What initial altitude?

 

I looked at COORZ and don't see any obvious problem, look at your preflight again. You provided the barest information to make it a guess game.... a little more to go on is helpful.

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Are you on the ground doing preflight?  Did you select departure runway? Which one?  PERF page set with CI? What initial altitude?

 

I looked at COORZ and don't see any obvious problem, look at your preflight again. You provided the barest information to make it a guess game.... a little more to go on is helpful.

yeah you have a point lol

 

i set up the box like i normally do. PERF page is all filled out with a CI of 85 and final altitude of FL360. I planned for a departure off of 34L for weight and balance reasons.

I planned for a full thrust takeoff. The TAKEOFF page was filled out with the winds and the slope....of 0.0 as well as flap setting and V-speeds from TOPCAT.

 

Initial altitude was set to 10000 ft like we normally get cleared for in the real world out of Denver.

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Yeah we need more info.  ZFW, FOB, destination (or NM).  I assume DEN to SFO?  No very far, so you shouldn't be heavy.  Are you filling in every box?  Max altitude is below or at what the FMC suggests?

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Yeah we need more info.  ZFW, FOB, destination (or NM).  I assume DEN to SFO?  No very far, so you shouldn't be heavy.  Are you filling in every box?  Max altitude is below or at what the FMC suggests?

Here is the first page of the release.

 

The cruising altitude was the same as what the box calculated for an optimum cruising altitude

 

The first picture is how i set up the PERF page.

 

 

wxyKeg.png

 

 

o48Iug.jpg

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I really doubt the route has anything to do with the problem. As long as the PERF INIT is good, you should be good.

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I really doubt the route has anything to do with the problem. As long as the PERF INIT is good, you should be good.

I just added a picture of how the box was set up. See if you can find what I may have missed.

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I can't see anything wrong... I just preflight a B738 for your trip and as soon as INS came up I had the VNAV solution.  This is probably one of those head slap moments.  Should work as expected.

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I just added a picture of how the box was set up. See if you can find what I may have missed.

What are you seeing when you can't engage VNAV?

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I totally found the problem! SkyWest doesn't even fly 737-800s in Alaska colors. (At least, not yet...)

;-)

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I totally found the problem! SkyWest doesn't even fly 737-800s in Alaska colors. (At least, not yet...)

;-)

hahahaha i only jse that format because its what im used to. as for the alaska reg....im too lazy to make another profile!!! hahahahahhaa

What are you seeing when you can't engage VNAV?

when i try to engage VNAV either on the ground or in the air i get, VNAV DISCONNECT in the scratchpad. there also aren't any calculated altitudes for any of the fixes in the legs page unless they are charted altitude constraints.

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Maybe you did not press Execute after completing the ACT PERF INIT page.

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wow, i just tried it and i'm seeing the same thing.

it only stopped showing the calculated altitudes after i added the dyamd arrival.

i actually did DYAMD3 because that's what's in my airac.

anyway i was really surprised. i noticed that if i removed the /FL270A restriction from the DYAMD waypoint that it would calculate everything again. and if i reload the STAR from the DEP/ARR page it un-calculates everything again.

it also works if you add a speed restriction to DYAMD such as 280/FL270A.

pretty interesting. i don't know why that restriction would fake it out like that, seems like maybe it is getting confused about the speed at that waypoint because of the 280 restriction on the next waypoint at LAANE.

edit: i think it might also be related to a CI of 85 giving you an econ descent of 330 knots somehow figuring into those restrictions not working quite right..

when i set a cost index of 17 it also calculates the altitudes even without modifying the restriction that comes from the STAR. it also works with CI of 85 if you set a 280/.78 in the DES page.

 

edit 2: i flew the rest of the flight, one thing also i noticed is the release fuel of 22770 seems high, that probably contributes to the econ mode wanting the fast descent. i landed with 8700lbs of fuel. when i planned it in pfpx the release was 17528. not sure exactly about why that would be so different.

 

 

cheers
-andy crosby

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A CI of 85 is pretty high, I use 80 in my BBJ because a billionaire flies it:p

 

I have flown that exact route several times in a B738 without issues and many times flown the arrival in a multitude of airframes, I usually use CI40 in a B738/9 regardless I set the descent speed to 280/.79  if there is a constraint in the arrival to avoid the need for deceleration.  Still, interesting that you found a problem at the FL270A constraint, especially since my path is usually well above FL270 crossing that fix.

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A CI of 85 is pretty high, I use 80 in my BBJ because a billionaire flies it:p

 

I have flown that exact route several times in a B738 without issues and many times flown the arrival in a multitude of airframes, I usually use CI40 in a B738/9 regardless I set the descent speed to 280/.79  if there is a constraint in the arrival to avoid the need for deceleration.  Still, interesting that you found a problem at the FL270A constraint, especially since my path is usually well above FL270 crossing that fix.

haha I have always used that when I fly SWA stuff. Anyways, Thanks for the help on everything. I toned down the cost index to 25 or so.

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