VNAV drops out. question

[Guest]

Greetings Captains, There is something about VNAV which I am not understanding. My flight plan is from KBUF-CYYZ. Here it is, with the FMC alt/speed restrictions: KBUF. TRAVA. LINNG. UDMIK. XIGAM. DULPA. MOBEM. 250/7000 250/5000 180/2500 175/2000 172/2000 172/1900 RWY05 (CYYZ) 172/0569In any case, initially, my MCP ALT is set to cruise alt, in thiscase 7000. I depart and arm VNAV and LNAV. That part is fine, after the 1st waypoint TRAVA, I set my MCP ALT lower so that the FMC willinitiate a descent in VNAV mode. That does happen, and the FMCwill reach LINNG at the speed/alt restrictions. Sometime afterthat, all of a sudden, VNAV mode drops out, and ALT HOLD mode isarmed on the MCP. So I figure, I re-set the MCP ALT mode to 100'lower than the MOBEM waypoint, my last waypoint which is to establishmyself on the localizer. So I set it to 1800, figuring VNAV willcontinue to hold the aircraft at 2500, then 2000(XIGAM), 2000 (DULPA)and finally at 1900 (MOBEM) so I can get established. No dice.VNAV will re-engage, and then suddenly drop out. I can get it toengage again by readjusting MCP ALT, but I don't believe I would haveto keep doing this. How can I get the FMC to keep the ALT/speedrestrictions I want all the way to the last waypoint? Why doesit drop out, and why do I have keep re-intervening by re-adjustingMCP ALT. If I set it to the lowest altitude I will be flying atto be established on the LOC, (1900), why doesn't it stay engaged?Any advice to a newbie from the experts would be appreciated.Cheers!-b763

[Guest bisinchi]

The altitude restrictions can be programmed on the Legs page of your FMC, then the plane will try to comply to the speed/altitude restriction.When you first selected a lower altitude than your cruise altitude for descent, which altitude did you select? I suspect that if you have selected an altitude higher than an altitude restriction height, the plane will hold that altitude without trying to reach the lower altitude.Francois

[Guest fwsov]

Found this to be an interesting scenario so I did a quick simulation to check what the aircraft actually did. On an approach into melbourne I put an altitude restriction of 3000' on BUNKY and set the MCP to 2000'. At 3.5nm before bunky the aircraft reached 3000'. The result was the aircraft levelled off to meet the altitude restriction and VNAV did NOT drop out.Here's a pic to clarify this:

[Guest]

nope. still not working.The first picture is the fmc speed/alt restrictions:http://ftp.avsim.com/dcforum/User_files/3cd59be4466c48d1.jpgNext snapshot is cruise, before LINNG, with VNAV PTH engaged and LNAV,all is okay at this point:http://ftp.avsim.com/dcforum/User_files/3cd59bed467fc89c.jpgand final snap is, before i reach the next waypoint (UDMIK), VNAV disengages, and ALT HOLD engages, and i had touched nothing.http://ftp.avsim.com/dcforum/User_files/3cd59bf4469040f2.jpgAny ideas?cheers!b763

[Guest]

Francois,I selected 100' lower than the lowest altitude restriction in theFMC, which was 1900 for the MOBEM intersection, just before LOC capture. (So the MCP was set to 1800'). This should have alloweda descent to 1900' without any intervention on my part, but itdoesn't. See the bitmaps attached in the next post to see what Imean.I must be missing something.Cheers-b763

[Guest]

Francois,I selected 100' lower than the lowest altitude restriction in theFMC, which was 1900 for the MOBEM intersection, just before LOC capture. (So the MCP was set to 1800'). This should have alloweda descent to 1900' without any intervention on my part, but itdoesn't. See the bitmaps attached in the next post to see what Imean.I must be missing something.Cheers-b763

[Guest fwsov]

Try putting /3000A to specify the aircraft to stay at or above 3000 at whatever waypoint. Not sure what the actual system logic is here with the FMC but i'm sure some real 767 can shed light on the subject.regards, Andrew

[Guest]

Andrew, Tried this, but no dice. Still the same behavior as per the bitmaps above. Perhaps HSPOV or Ian can comment on this. I have tried this several times, to no avail Cheers, -b763

[Guest]

What was your VNAV mode during each of the phases of flight?Did you have your CRZ Altitude entered into the FMC? For such a short flight, your CLB/CRZ/DES modes will switch very quickly. Also, if you did not reach your CRZ altitude, the FMC will still be in CLB mode and will do strange VNAV things when you are trying to descend. Did you reach CRZ altitude on such a short hop for a 767?I would suggest you try a bullet proof route that I fly all the time, and make sure you properly set up the latest STARS/APPS from Ian and Lee (search this forum). CYOW-CYYZ - routeing is CYOW9(sid) V300 J546 YSO.SIMCO1....your FMC should be programmed with YOW as the first waypoint, then J546 to YSO, then select the YSO.SIMCO1 arrival (FMS) and select your runway. Your cruise altitude will be FL310, although you will only be at FL310 for a few minutes. Cost index recommend 70.Assuming you operate the FMC and MCP controls properly, you will have NO problems with this flight, I can gurantee it! (I have flow it about a hundred times in the PIC767 ;-)Rob

[Guest]

Rob,You might have something here :-)CRZ ALT = FL330Cost Index 80CYYZ STAR was YOUTH1Does this take precedence over the first waypoints alt restriction?If so, I feel :)Will try the route you prescribe - I have the latest STARs from Ian.Cheers,-B763

[Guest]

KBUF to CYYZ is about 40 miles distance, FL330 would be impossible ;-)Rob.Try a longer route.

[Guest HPSOV]

In a bit of a hurry now, so I will just post that what you are seeing is perfectly normal operation of the MCP.There is a reason it keeps dropping out of VNAV when it does, try selecting different altitudes on the MCP and see what happens.Will post the answer when I get a chance!

[Guest]

Where did you get the ADI guage from. Thats not the stock PIC767 one. Looks good.

[Guest Ian_Riddell]

>Where did you get the ADI guage from. Thats not the stock >PIC767 one. Looks good. Which version of PIC do you have, Talon1? This EADI has been available for quite some time.If you are running FS2002 and have the PIC FS2002 update, select "PIC" from the Menu Bar at the top of the screen, then "Settings", then "Realism and Carrier Options", then "Pegasus EADI".Hope this helps?Cheers.Ian.