VNAV DES

[FOD 23]

Hi Chris,This is interesting. Could you confirm that in a 757 VNav descent, as you pass a waypoint, you do NOT get the VNav path bug jumping to indicate well below the required path? If this is the case, grateful if you could let us know what tweaks managed to clear it.John Rooum

[Guest Knikolaes]

THAT I get, but she always levels off to meet the restrictions. I guess I should have been more specific . . . . . Beforwe when I was trying to reproduce this issue, my 757 would just dive and NOT stay on the path at all. Now it does. At waypoints it does seem to have an odd bit where it "pops" under at a waypoint. The god news here is that it doesn't climb to try to catch it. She just levels off and gradually re-meets the line.I think my diving issue before had to do with computer resources free, etc. On the waypint issue, I am sure that has to do with gauge coding. In this sense, I am still happy that she comes down and seems to meet the restrictions as dictated in the FMC.Note -- I didn't notice what we meant here until John replied above to my post. I just reenacted the KAVQ to KCOS cargo flight and noticed the waypoint "pops". I think I misunderstood the thread when i first replied, getting it confused with the "diving" that my 757 was doing during VNAV approaches before I cleaned my system. Apologies for the miscommunication.

[FOD 23]

Chris,Thanks - exactly what I am seeing: the aircraft is suddenly below the path as it passes a waypoint so it reduces the rod - or levels off depending on the size of the error - until it is back on path.I tried delaying the descent point for 20 miles and this improved things when I reselected VNav descent.Tonight I will try delaying by 40 miles [from FL370 and will equate to the descent point used by the 777] and see what that gives me.John Rooum

[FOD 23]

A couple of follow ups, which might help the fix, on what exactly appears to be happening to the 757 VNav. On 2 identical flights, I was cruising at FL370 as I hit TD:Flight 1. I delayed descent by 45 nm [to the usual PSS 777 TD point] then went back into VNav. At this point I was some 13000 ft above the programmed VNav path. The aircraft started descent at the usual 3 - 3500 fpm and gradually "caught up" at each waypoint until we were back on path at about 8000 ft.Flight 2. I initiated a VS descent at about 2300 fpm - we met every waypoint at around the correct height until 10,000 ft when I adjusted the rod to meet the lower VNav schedule.It seems that the 757 VNav is based on the 777 programming but with a revised, lower, planned rod above 10,000 ft. However, it appears that this revised rod is not passed to the AP which still uses the 777 rod when "on path".John Rooum

[Fritz Bayerlein 0]

I'm still a bit surprised and concerned that we've not received any formal acknowledgment of this big problem from the PSS staff. Anybody in there?

[opherben 0]

Give them a break, they will be back to us on the issues not just this.>I'm still a bit surprised and concerned that we've not>received any formal acknowledgment of this big problem from>the PSS staff. >>Anybody in there?

[Guest NormanB]

>I'm still a bit surprised and concerned that we've not>received any formal acknowledgment of this big problem from>the PSS staff. >>Anybody in there?We are yes. The beauty of a support forum is that there are some folk here with more knowledge than me. John, one of the tester was already answering elements.A lack of a PSS banner doesnt mean its ignored.

[Guest rich90]

Thank you Norman for the answer, now I hope you are working on the fixes...because of the problem in VNAV DES and THR HOLD (see the other topic) I have to disconnect auto throttle every time before descent...

[Guest NormanB]

Just on an aside, why would you be disconnecting the autothrottle? One of our 757 skippers mentioned numerous times that in real life he never ever ever lets the real plane do a VNAV descent.

[Guest rich90]

I never let the plane do a VNAV descent because of the problem I explained before, I am unable to do a descent with FL CH because Throttles move themselves to IDLE and I can't move them so the plane goes down with very high descent rates (-5000 -6000ftm). So I use FL CH with Auto Throttles off. In this way I can move throttles...

[Guest Steve_Park]

>Just on an aside, why would you be disconnecting the>autothrottle? >>One of our 757 skippers mentioned numerous times that in real>life he never ever ever lets the real plane do a VNAV>descent.He must not fly much in the US...with the CDA experiment that UPS is using at KSDF, makes a lot more sense to use VNAV. And how would you fly a descent like "Descend via the CIVET5" or "KORRY3" with five or more altitude crossing restrictions without using VNAV? I can always just plug in a pretty close number in the V/S window, but you know what they say about "Close".Steve Park

[Guest N555TZ]

>>Just on an aside, why would you be disconnecting the>>autothrottle? >>>>One of our 757 skippers mentioned numerous times that in>real>>life he never ever ever lets the real plane do a VNAV>>descent.>>He must not fly much in the US...with the CDA experiment that>UPS is using at KSDF, makes a lot more sense to use VNAV. And>how would you fly a descent like "Descend via the CIVET5" or>"KORRY3" with five or more altitude crossing restrictions>without using VNAV? I can always just plug in a pretty close>number in the V/S window, but you know what they say about>"Close".>>Steve Park>Steve,From my personal experience, I do not use VNAV often(90% no VNAV) nor do I require it to fly into LAX using the MITTS/CIVET arrivals on my flight simulator. ALSO, I do happen to know a few 757 rated guys who used to fly into LAX all the time for ATA, and they did not use VNAV, and did not have very much to say about VNAV.Common knowledge is that Southwest Airlines flies into LAX.......they do not have VNAV on their airplanes, and they fly the MITTS & CIVET arrivals.Here's a Southwest 73NG inbound to LAX on the CIVET5...http://flightaware.com/live/flight/SWA971....note, NO VNAV on ANY Southwest aircract.I am not rated in the 757, and can only forward here information that I know of and about VNAV.P.Meyer

[Guest Steve_Park]

>Steve,>From my personal experience, I do not use VNAV often(90% no>VNAV) nor do I require it to fly into LAX using the>MITTS/CIVET arrivals on my flight simulator. ALSO, I do happen>to know a few 757 rated guys who used to fly into LAX all the>time for ATA, and they did not use VNAV, and did not have very>much to say about VNAV.>>Common knowledge is that Southwest Airlines flies into>LAX.......they do not have VNAV on their airplanes, and they>fly the MITTS & CIVET arrivals.>>Here's a Southwest 73NG inbound to LAX on the CIVET5...>http://flightaware.com/live/flight/SWA971>....note, NO VNAV on ANY Southwest aircract.>>I am not rated in the 757, and can only forward here>information that I know of and about VNAV.>>P.MeyerWe know why Southwest does not have VNAV or Autothrottles...they are removed by mod - lets not have libelous statements here please.. Why do you supposed Boeing and Airbus put VNAV(Managed Mode) in the flight computer in the first place, so it would not be used at all, so pilots could show off how skillful they are without using the autopilot??? Vertical Navigation mean's exactly what it say's. I assume you do use the Lateral Navagation mode and are happy with it not turning back and forth at random rates? That is all I and everyone else are asking PSS to look into for the next patch. Feel free not to use VNAV if you wish, however it was put into the aircraft for the professional ATP pilot to use and I have used it very succesfully for over 10 year. If ATC tell's me to descend and maintain a certain altitude at their convenience, then yes, I use the MCP to reset the altitude and V/S mode to get there comfortably. But with the published STAR's, and controllers tell you to descend VIA the arrivial, there is no reason not to put the altitude restrictions into the FMS and let VNAV hit every one, first time, every time.Steve Park

[Guest N555TZ]

It's my experience control wants you at your assigned or published altitude about five miles before the fix, and at a decent rate less than 500fpm within a 1000 of the altitude.There's really no fixed altitude for most of the fixes on the CIVET/MITTS arrivals.A controller will not say descend VIA the arrival.P.Meyer

[opherben 0]

Mr. Meyer,the controller should. That clearance (see FAA ATC manual) encompasses the whole STAR procedure including all fixes, altitude and speed restrictions, in one short clearance. There is no reason whatsovever to not issue it, as the arriving aircraft are this way nicely separated in 3 dimensions for arrival, and from departures. Just look eastbound at LAX towards CIVET and you will see two dozen aircraft turning on their landing lights at 18,000, nicely staggered in two straight lines for north and south runways. Departure procedures are designed to not intersect arrivals, in any airport except those resrticetd by terrain, such as deep and narrow valleys as in LOWI. If your ATC is working improperly or inefficiently, and you are a rated aviator, it is your obligation as a professional to talk to the FAA, because this is a meaningful deviation from proper air traffic flow management.>A controller will not say descend VIA the arrival.>>>P.Meyer