FMS ?DES NOW? working? Options

[kolman]

Hi,

Is the ?DES NOW? function of the FMS working properly? I tried it on several flights now, after resetting the FGCP altitude and pushing LSK6R (DES NOW), initially nothing happens. Then, after a while, the aircraft sometimes starts a descent, with a varying rate of descent. In addition, the VNAV path indicator shows strange information?s.

Best regards
G. Kollmann

[wothan]

Hi,

Is the ?DES NOW? function of the FMS working properly? I tried it on several flights now, after resetting the FGCP altitude and pushing LSK6R (DES NOW), initially nothing happens. Then, after a while, the aircraft sometimes starts a descent, with a varying rate of descent. In addition, the VNAV path indicator shows strange information?s.

Best regards
G. Kollmann

I?m using the "DES NOW" function almost every time, when starting my descend. It gives me a more smooth transition from level flight to VNAV Descend. I don?t have any problems with it.

Wothan

This post has been edited by wothan: 18/09/2007, 15:26

[VulcanB2]

Hi,

The further out you start the DES NOW, the shallower the descent. It will obey the first hard altitude in the FMS that is below your cruise altitude, so if you are at FL360 and you told it your cruise altitude is FL360, and the first hard altitude constraint is say FL130, and you press DES NOW 300 nm from that point, it will descend at about 150 ft/min until it reaches FL130.

Best regards,
Vulcan.

This post has been edited by VulcanB2: 18/09/2007, 18:30

[kolman]

Thanks for your responds!

The further out you start the DES NOW, the shallower the descent. It will obey the first hard altitude in the FMS that is below your cruise altitude,

Where did you find this information? I could not read a detailed description of the "DES NOW? behavior of a MD-82 in the Maddog 2006 manual. My Airbus or Boeing aircraft will start a 1000-1200 ft/min descent to intercept the calculated VNAV path, but this could be different with this aircraft. However, if I push DES NOW about 15 NM prior T/D actually nothing happens. Maybe I should add, that I use the Maddog 2006 with FSX and an AMD Athlon.

Best regards
G. Kollmann

This post has been edited by kolman: 18/09/2007, 19:24

[Egbert Drenth]

Thanks for your responds!

The further out you start the DES NOW, the shallower the descent. It will obey the first hard altitude in the FMS that is below your cruise altitude,

Where did you find this information? I could not read a detailed description of the "DES NOW? behavior of a MD-82 in the Maddog 2006 manual. My Airbus or Boeing aircraft will start a 1000-1200 ft/min descent to intercept the calculated VNAV path, but this could be different with this aircraft. However, if I push DES NOW about 15 NM prior T/D actually nothing happens. Maybe I should add, that I use the Maddog 2006 with FSX and an AMD Athlon.

Best regards
G. Kollmann

Hi,

DES NOW should work the same as in the Boeings:

"The DES NOW prompt is displayed when the descent page is not currently active. Pressing the adjacent
key followed by the EXEC key causes the system to provide guidance at an approximately 1250 fpm descent
rate using the active cruise speed schedule.
Throttles goes dormant to allow for adjustment of vertical speed. A new T/D is displayed at the intersection
of the planned descent path, where the aircraft captures the path."

"Descend prior to the T/D point
4. New altitude on the FGCP.............. SET.
5. DES key...................................... SELECT.
6. DES NOW> ................................. SELECT.
7. EXEC key ................................... SELECT.

- Check that approximately 1250 fpm descent is established until original profile is captured. Throttles are
clamped to allow for manual adjustment of descend rate through thrust."

This post has been edited by Egbert Drenth: 18/09/2007, 19:44

[wothan]

And be aware of the different VNAV logic wether the altitude set in the FGCP is armed or not ( disarm by right clicking the altitude set knob).

If the set altitude is armed (amber altitude /FL readout in the FMA) the FMC will disregard any altitude constrains.

If the set altitude is not armed (No amber altitude / FL readout in the FMA) the FMC will try to comply to any altitude constrains.

This is also true for the climb. If Your SID has any altitude constrains, then make sure that the altitude set in the FGCP is disarmed. Also the FMA will show "FMC EPR" if altitude is armed ans "FMC SPEED" if unarmed.

Wothan

[kolman]

Thanks for your help. Today I was able to execute two correct VNAV descents started with ?DES NOW?. Nice descent with about 1200 ft/min until intercepting the VNAV path. But one a third flight I got the same problem as many other times before. I m still investigating the problem, but I think it may be related to the procedure files. Some STAR?s seem to mess up the complete VNAV descent!

Best regards
G. Kollmann

[dante]

And be aware of the different VNAV logic wether the altitude set in the FGCP is armed or not ( disarm by right clicking the altitude set knob).

If the set altitude is armed (amber altitude /FL readout in the FMA) the FMC will disregard any altitude constrains.

If the set altitude is not armed (No amber altitude / FL readout in the FMA) the FMC will try to comply to any altitude constrains.

This is also true for the climb. If Your SID has any altitude constrains, then make sure that the altitude set in the FGCP is disarmed. Also the FMA will show "FMC EPR" if altitude is armed ans "FMC SPEED" if unarmed.

Wothan

Hi,
where have you read that different fms behaviour depending on ALTITUDE armed or not?
I checked the MD80 alitalia a.o.m. and there's nothing written there about what you said.
Directly from AOM:

To commence descent before t/d:
-LOWER ALTITUDE ON FGCP ALTITUDE WINDOW: SET/ARMED
-DES NOW: PUSH
-EXEC KEY: PUSH

then the manual specifies this:

to bypass waypoint's vertical constraints:
-DES DIR (LSK6R): PUSH
-EXEC KEY: PUSH

I think you are wrong, but I'm not sure.
Anybody knowing the truth??

Bye

This post has been edited by dante: 20/09/2007, 19:15

[wothan]

Hi

I really don?t remember where I have read it, but I think it was somewhere in this forum.

Make a try, it really works as I have written.

Make a flight with the FMC programed with a SID. The SID should contain a number of waypoints. Now put in some hard altitude constrains for these waypoints.

Start Your takeoff with an altitude armed in the FGCP ( amber altitude readout in the FMA). With an altitude armed and after Your takeoff run selecting Climb mode on the TRP look at the FMA it says "FMS EPR".

Now look how it handles Your altitude constrains, cause it doesn?t. It will simply disregard the constrains.

Now execute the same procedure, but this tim right click the altutide set knob on the FGCP and verify that theres no amber altitude in the FMA, it should be blank.

During the climb the FMA now ays "FMS Speed" at perfectly follows Your set altitude constrains.

The same logic is present for the descend.

But be aware the autopilot will still level off at the altitude set on the FGCP altitude set knob but only when in VNAV mode.

Wothan

[dante]

Hi

I really don?t remember where I have read it, but I think it was somewhere in this forum.

Make a try, it really works as I have written.

Make a flight with the FMC programed with a SID. The SID should contain a number of waypoints. Now put in some hard altitude constrains for these waypoints.

Start Your takeoff with an altitude armed in the FGCP ( amber altitude readout in the FMA). With an altitude armed and after Your takeoff run selecting Climb mode on the TRP look at the FMA it says "FMS EPR".

Now look how it handles Your altitude constrains, cause it doesn?t. It will simply disregard the constrains.

Now execute the same procedure, but this tim right click the altutide set knob on the FGCP and verify that theres no amber altitude in the FMA, it should be blank.

During the climb the FMA now ays "FMS Speed" at perfectly follows Your set altitude constrains.

The same logic is present for the descend.

But be aware the autopilot will still level off at the altitude set on the FGCP altitude set knob but only when in VNAV mode.

Wothan

I'll investigate.. I rarely use vnav, i prefer ias mode.
Anyway there's anyone of the maddog devel team that can confirm if the maddog was programmed the way Wothen says?
Hi all!

[kolman]

I don?t know, who is right with this altitude constrains problem, but both versions do not work for me on the following short test trip.
GCRR (Lanzarote) RWY 03; TFN1N departure; ILS RWY 12 TFN transition at GCXO (Tenerife Norte).
My picture shows the FMS prior takeoff. The FGCP altitude is set to 8000. It does not matter if I arm the FGCP altitude or not, the FMS (VNAV) will always ignore the 4000 at D262L and climbs direct to 8000.
But this was not the only problem on this flight. I added a holding at TFN prior to the approach (179/L 230/8000). I reset the MGCP altitude to 8000 at about 15 NM prior T/D and push ?DES NOW? ? nothing happens, except the VNAV path is showing up an is dropping below, but the aircraft stays at FL200. I use ?VERT SPD? to get down to 8000. Finally, when reaching TFN the aircraft ignores the holding and continues on the last track.
I tried exactly the same flight with two other major FS addons without any problem. This aircraft level of a 4000 during the departure, they start a proper descent with the use of ?DES NOW? and they enter the holding at TFN.
One last thing. While comparing this aircraft, I noticed, that the ?DES NOW? option is showing up all the time on the DES page of the FMS, even during the climb phase. I don?t know if this is correct for the MD82. I think it should not show up prior reaching the cruising level or within a specific range to the T/D.

Best regards
G. Kollmann

 MD82_TFN1M_1.jpg ( 348.2K ) Number of downloads: 32

[wothan]

Hi Kolman

Did You change the TRP from TO to CL before reaching 4000 ft ?
Did You ensure the FMA says "FMS SPEED" ?

I know from Boeing aircraft:
"VNAV SPEED" means, it will try to keep the speed disregarding any altitude constrains.
"VNAV PATH" wil try to obey the altitude constrains and (within a margin off appr. ?15 kt) disregard the speed.

I learned this the hard way when I bought the Level-D 767-300 for FSX, coming from the Level-D 767-300 FS9 version. Their new VNAV logic in the FSX version really was a challenge for me to understand. I started a really long thread on the Level-D support forums, trying to convince the developers that there where something wrong, only to find out that it was myself not understanding the logic.

I?m not saying that You are wrong. Maybe theres a problem with Your SID or something else. The way I have described the VNAV logic here, is what works for me.

But the MD-82 is NOT a Boeing, and I know that some of the autopilot logics behave different between Boeing and Mc Donald Douglas.
In fact RW pilots coming from an MD-82 to a Boeing are missing the enhanced functionality of the MD-82?s autopilot, It?should be more flexible.

How close to the T/D are You engaging "DES NOW". I Guess You shouldn?t select "DES NOW" before Your are within 20 Nm from T/D.

The VNAV logic I have described was posted by "Vulcan" somewhere else in these forums, plese search. For me it works flawless.

The newest flight simulator aircraft addons are really complicated and can be very frustrating, but suddently the light bulb goes on and You find the reason something isn?t doing what You sought it should do. Sometimes it?s a bug in the product, a t othertimes it?s only Yourself to blame, but when the light bulb goes on it?s really satisfying and thats whats makes this hobby so fun.

Hope You figuring it out..

Wothan

This post has been edited by wothan: 21/09/2007, 9:03

[kolman]

Hi Wothan

Here are to pictures which show, what I figured out so far. First one, with the SID as it is, the aircraft is climbing through 4000 ft and ?FMS SPD? is not showing up on the FMA.

 MD82_TFN1M_2.jpg ( 391.22K ) Number of downloads: 31


Thereafter I deleted the 1600A and the 3200A form the SID and inserted a ?hard? 4000 at D049M and at LZR. The second picture shows the aircraft now leveling off at 4000 and ?FMS SPD? is showing up at the FMA.

 MD82_TFN1M_3.jpg ( 305.01K ) Number of downloads: 25


For me it seems, that the aircraft ignores the 4000 at D262L because it reaches a higher altitude prior this waypoint becomes the active waypoint. And this behavior looks very unrealistic to me! The SID is from the latest Navigraph cycle and only adapted to work with the Maddog 2006. This means, that this is real world data. I think you will often find a SID, where you initially have some soft altitude constrains and on a later waypoint a hard constraint.

Best regards
G. Kollmann

[wothan]

Hi Kolman

I just tried a flight with some soft constraints between present position ( on the route) and the waypoint with a hard altitude constraint.

You are right, the autopilot disregards the hard altitude constraint, but also the soft constraints e.g if you have a soft constraint saying 4000B (4000 below) it simply climbs above disregarding the constraint and eventually busts through the hard altitude constraint.

I doubt that this can be right. The FMC should read ahead and recognize the next hard altitude constraint waypoint and level of according to these constraints.

Wothan

[wothan]

Well I investigated it a bit further.

If the waypoints are part of a SID (and I surpose it also applies to STARS) then You can?t put in any constraints for them. Well they do show up on the FMC, but are disregarded.

If You select a runway for take off and manually insert a number of waypoints ( it could be the same waypoints as in a SID) and assign them with costraints. Then the autopilot / FMC perfectly follow them.

Sorry for my short memory, but I think that I have seen this behaviour mentioned for the FMC of another addon aircraft.

My last test is to make a "fake" SID including constraints, and see how the FMC handles that, wich means that constraints that are included in the SID, and by that I mean the procedure file, will be obeyed.

Will come back later with my results..

Wothan

[wothan]

...and here I am again, with some fresh knowledge..

I found:

Waypoints who are part if a SID entered via the DEP/ARR page cannot be altered. You might enter a Constraint and it will show up on the FMC, but the constraint will be disregarded.

Waypoints who are part of a SID and where the constraints are "build" into the SID will be handeld correctly.

And I must take my words back regarding armed or NOT armed altitude in the FMA because it doesn?t matter, the constraints will be followed regardless if the altitude is armed or not... sorry if I was confusing You.

It sounds correct, since in real life, ATC are giving You the SID and STARS. These SID and STARS are procedures that make sure that the controller knows what You will do during the SID and STAR portion of the flight. T
he SID / STAR descibes both the lateral as well as the vertical path You have to follow. Any alterations to that will make the SID / STAR invalid.
If the controller want You to fly another vertical profile, then he will tell You, and You have to revert to V/S mode.

Wothan

[kolman]

Waypoints who are part of a SID and where the constraints are "build" into the SID will be handeld correctly.

Hi,

I really appreciate your help to figure that out, but as I already mentioned, the SID is real world data, now altitude constraint was entered manually, and it does not work!!!
A few minutes ago, I tried one last test flight without any SID, STAR and approaches. I entered every single waypoint separately and then I added the necessary altitude constrains manually. The result was frustrating ? no improvement at all. The last MD82 flight for a longer time ? I continue with the big Boeing and its excellent new VNAV behavior!

Best regards
G. Kollmann

This post has been edited by kolman: 28/09/2007, 13:33

[wothan]

Former reply deleted.....

There seems to be some kind of problem with the constaints logic.

I tried the following type of flightplan:
(3000=Constraints / 140= predictive altitude

SID:

FIX01 600A
FIX02 2100A
FIX03 140
FIX04 6000
FIX05 140
FIX06 140
...
...
FMA says FMS EPR and climbs above the first two soft constraints, fine.
But from here, flying towards FIX03, it disregards the Constraint at FIX04, rocketing through 6000 ft, and reaches FIX04 at 9000 ft, leveling of at FL140 (my cruise flightlevel).
FMA still says FMS EPR.

This cannot be right.

I also tried to reinsert the /6000 ft constraint, but to no avail.

It seems that the FMC doesn?t calculate the predicted altitude at FIX 03 correct.

I hope the developers will look into it.

Wothan

This post has been edited by wothan: 21/09/2007, 20:17

[dante]

Former reply deleted.....

There seems to be some kind of problem with the constaints logic.

I tried the following type of flightplan:
(3000=Constraints / 140= predictive altitude

SID:

FIX01 600A
FIX02 2100A
FIX03 140
FIX04 6000
FIX05 140
FIX06 140
...
...
FMA says FMS EPR and climbs above the first two soft constraints, fine.
But from here, flying towards FIX03, it disregards the Constraint at FIX04, rocketing through 6000 ft, and reaches FIX04 at 9000 ft, leveling of at FL140 (my cruise flightlevel).
FMA still says FMS EPR.

This cannot be right.

I also tried to reinsert the /6000 ft constraint, but to no avail.

It seems that the FMC doesn?t calculate the predicted altitude at FIX 03 correct.

I hope the developers will look into it.

Wothan

I hope this weekend I'll find time to try something similar.
I'll write my results.
Bye!

[wothan]

Hi Dante

Sounds great :-)

It looks like it?s only for the climb, that the FMC doesn?t calculate the predicted altitude correct.

Wothan

This post has been edited by wothan: 21/09/2007, 21:38

[wothan]

for almost a week ago ,I PM'ed both Davide and Stefano from the devolpers team about the issue, but none of them has responded nor read my message.

I wonder where they are ?

Wothan

[VulcanB2]

Hi,

EIDT: Hmmm..... it seems if the hard altitude is not the current waypoint, it will ignore it, but if the hard altitude is at the current waypoint it will abide by it if it hasn't got there yet.

Best regards,
Vulcan.

This post has been edited by VulcanB2: 02/10/2007, 20:09

[wothan]

Hi,

EIDT: Hmmm..... it seems if the hard altitude is not the current waypoint, it will ignore it, but if the hard altitude is at the current waypoint it will abide by it if it hasn't got there yet.

Best regards,
Vulcan.

Exactly.

You can get around it by adding altitude constraints for all waypoints between Your current position and the waypoint with the disired altitude constraint.
Try adding the same altitude constraint for the in between waypoints as at the waypoint where You want a hard altitude constraint just preceded with a B for "AT OR BELOW".

Like this:

Waypoint 1 ---/---
<---Your present position
Waypoint 2 ---/---

Waypoint 3 ---/---

Waypoint 4 ---/---

Waypoint 5 ---/6000 (Hard altitude constraint)


Changes it to this:

Waypoint 1 ---/---
<---Your present position
Waypoint 2 ---/6000B

Waypoint 3 ---/6000B

Waypoint 4 ---/6000B

Waypoint 5 ---/6000 (Hard altitude constraint)


VNAV Descend seems to work ok.

Wothan