VNAV des issue

[jeffhunter 0]

Perhaps this question is borne of my ignorance of the proper operation of the ng in vnav descent, but I repeatedly have problems when I get to 10000 ft. At tod I reset the mcp altitude to (say) 3000 and undertake a normal descent in vnav mode. At 10000 ft vnav disconnects and switches to speed/alt. This would be ok, except I end up in a 6000 fpm descent. I have to repeatedly hit the vnav button (it illuminates and then goes back to speed) until after a while it switches back on.I've read that vnav may not be the norm for descent ops, but rather vs and/or lvlchg.Am I doing something wrong??Also had a weird problem last night. In cruise at FL350 (vnav & lnav), I went upstairs for a while. Came down to find myself in a steep descent at about 2000 ft with an overspeed disconnect warning on the fmc. Managed to salvage it and climbed back to cruise, but discovered that the zoom page had weird graphics with no view buttons/flap postion indicator needle. I'm assuming that I stalled out as a result of a d/l weather sudden windshift(fsuipc notwithstanding), but that wouldn't explain the graphics. Any ideas/regards,Jeff

[Guest Zapper]

Hi Jeff..On your first prob:You should have a hard alt/speed set in the legs page to enable VNAV PATH ..it is shown as an e/d (end of descent) waypoint. By hard I mean that in LEGS the numbers on the right are in UPPER CASE. Just line select the alt/speed for the last WP before your runway into teh scratchpad and reinsert it to teh same place. It willnow appear in upper case and that is where your VNAV PATH will be aiming for.The second problem?... sounds like your PC had a mild coronary.. sorry but I have no idea.Brad

[jeffhunter 0]

OK many thanks,Jeff

[Guest rcarlson123]

Just a small correction, it's speed/alt that you line select into the legs page for your end of descent waypoint, not alt/speed. I can imagine some confusion that could be caused by trying to do the reverse!

[jeffhunter 0]

No worries, I can imagine the problems trying to descend thru 240 feet at 10000 knots!! I guess some people have had the misfortune to do it, tho'.Jeff

[Guest rcarlson123]

Heh ... well, I think the FMC would give an error if you tried to enter 10000 knots, but if you tried to enter your altitude restriction as a flight level, (say, FL190 at 240 knots) which the FMC accepts, but you reversed it an accidentally entered 190/240, then it would end up with a valid restriction of 190 knots at FL240, which you might not realize right away. :)

[Guest aaron_huber]

Aside from the good advice listed above, I suspect that you are getting a VNAV disconnect due to overspeed. The default VNAV PATH descent will keep your plane on the projected path without regard for speed. You have to keep an eye on the speed and possibly add more drag if you are getting to fast. This might occur if the tailwind is higher than expected and the plane has to dive to keep on the path.If you get over a certain speed limit, VNAV will disconnect. The speed limit also gets much narrower under 10,000 feet which explains why you are OK until you get to that point: your speed is within the limit above 10,000, but you are too fast for the new lower limit. I don't recall exactly, but the limit above 10k is some number of kts under the barber pole limit, while under 10k it is only 15kts over the MCP speed (or something like that). Either way, you should see a message on the FMC telling you why VNAV disconnected.The other option is to use the VNAV SPD descent, which will manage the speed for you but not pay any atention to the projected descent path. Or just use LVL CHG, which will also manage the speed and not the path.Hope this helps, and someone please correct me if I'm way off.Edit: More info in this post with the actual limits from Randy:http://forums.avsim.net/dcboard.php?az=sho...33808&mode=fullAaron

[zubart 5]

As far as Vnav is concerned,Below 10,000 feet, the FMC will command a default speed of 240kts. Don't let the airspeed exceed this speed too much (about 10kts at the most ) or Vnav will disconnect. The FMC will not violate the 250kts below 10,000 feet FAR rule, and therefore will diconnect Vnav.If there is a tailwind that is greater than planned during descent, the FMC will command a steeper angle of descent to stay on the Vnav Path and that is why the airspeed will increase. If airspeed begins to exceed the FMC 240kts target speed below 10,000 feet,use the speed brake to keep the airspeed in check. Floyd

[Guest Moxica]

Heeeelp!!Not able to line select speed/alt. Can with waypoints, but none of the speed/alt on the right side can be copied to scratchpad. Can anyone point me in any direction here?

[Guest rcarlson123]

I'm not sure why you are trying to DOWNselect speed/alt entries from a waypoint to the scratchpad.If you want to set a speed/alt restriction for a waypoint, you first type it into the scratchpad then UPselect it to the waypoint by pressing the right side LSK associated with that waypoint. The format to enter into the scratch pad is speed/alt. For example, if you were given a crossing restriction of 11 thousand feet at 250 knots, you would enter:250/11000into the scratchpad then upselect it to the waypoint. You will then need to press the EXEC key to activate the change.

[Guest Moxica]

Thank u for your answer.It is because it would seem to be an efficient way, and the procedure is described this way by Brad in this very topic.Quote: "Just line select the alt/speed for the last WP before your runway into teh scratchpad and reinsert it to teh same place."

[Guest rcarlson123]

Hrm ... I'm confused. Why would you need to downselect then upselect the data if it's already there?

[Guest Zapper]

Hi Guys,Been away on a trip..My apologies, I did indeed transpose teh speed/alt on teh descent page.... oops..but teh reason VNAV disconnect occurs is because the AFDS cannot maintain the VNAV PATH for any of a couple of reasons. As has been rightly pointed out, in VNAV PATH the FMC will disregard speed in an attempt to maintain PATH and if the speed exceeds certain paramters it will give up. I think it will open the MCP SPEED window and leave you in LVL CHG but to be honest I don't let this happen on teh line so am not sure. Will check it out in sim (a real one) on Sat if I get a chance.In teh matter of downselecting a speed/alt for the legs page we do this to create and end of descent point for teh FMC to aim at in its PATH calculations. It is an operation done quite regularly IRL. If our sim won't allow it then key the spd/alt you want into the scratchpad and line select it against your waypoint. I will look at teh sim and see what happens.Remember folks this is not meant to be perfect.. but by gee it comes closer than anything I have seen :) !Brad

[crvm 0]

Thanks Brad. Somehow reading real pilot's remarks and comparing to some folks who are attached to their chairs makes all the difference to me ;). I'll say it ain't finshed yet! The next phase will see the FMC fully functioning and other NG specific items that will once again raise this aircraft another level.. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest Moxica]

High again.It is to lock the speed/alt restriction. When these are placed by user, they appear in upper case, and are locked. Not continously calculated. It is nice to know what altidude to put in the MCP upon T/C and know it will not change in MCP afterwards.