FMC: No descent path after ....

[Nick Dobda 24]

Resurrect this topic, I got this message flying into KIND from the west on the SMUKE1 onto 5R ILS approach.

 

I don't recall exactly (I have this problem when I am busy in the cockpit),  but I think the VNAV disconnected automatically as I came into the problem area, making it impossible not to intervene with alternate A/P settings. I guess I had planned on disconnecting anyway, but what I did was control the pitch with V/S and holding the speed to hit my restrictions as low as I could.... clicking the EFIS into that mode that shows the plane's profile showed exactly where I needed to be vertically, adjusting V/S to hit the minimums.

 

Nothing seemed unusual.. the descent path wasn't ridiculous or anything. But I stil have two questions:

 

1) did I recall correctly that VNAV disconnected automatically when it encountered the "impossible descent path"?

2) would this message appear in the real world, and how would a real pilot have handled it?

3) is there no way to program the FMC at the gate prior to takeoff such that the message goes away (short of deleting the restrictions - which I didn't want to do).

 

Side question: speed restrictions over a waypoint, do they expire as soon as you cross the waypoint? After the 210 restriction I wanted to drop the speed to lower the flaps in order to help with descent, so the second I passes I dropped the speed. You're not supposed to maintain the speed for some distance after the waypoint are you?

 

Thanks!

[ALF1 15]

Sometimes you might have to descend at a pretty good clip to meet the altitude restrictions. You might have to do 300 knots from the Flight Levels and set the lower altitude in the waypoint whet it is an between such and such.

 

Also, if you are using real weather the FMC will calculate the wrong path unless you input the correct forecast winds at the different altitudes as per the weather engine. A tailwind will zip you by waypoints faster.

[pcubine 143]

1) did I recall correctly that VNAV disconnected automatically when it encountered the "impossible descent path"?

In 4 years of flying the NGX I have seen the message many times but I have yet to see it cause VNAV to disconnect.

 

3) is there no way to program the FMC at the gate prior to takeoff such that the message goes away (short of deleting the restrictions - which I didn't want to do).

There is no way to program the FMC to prevent the message. To me it is a message that at some point during the descent (probably 11000 ft.) I am going to need to get out of VNAV and use V/S or LVL CHG and the speedbrakes almost continuously

.

Side question: speed restrictions over a waypoint, do they expire as soon as you cross the waypoint?

If I am on the Approach I consider them expired once crossed. If I am on a STAR I consider the restriction in effect until the next speed restriction, 10000 ft., or the Approach.

[ALF1 15]

Inbound ATC will do their best to keep you clean by assingning between 210 and 250 for traffic separation below 10,000. 210 and 230 are favorites. If distances are short between altitudes flaps 1 or 5 and maybe even speed brakes may be needed. Below 4,000 speeds are usualy decreased further to 180-200 kts.

[scandinavian13 5,192]

 

 

1) did I recall correctly that VNAV disconnected automatically when it encountered the "impossible descent path"?

 

I believe this is correct once you get past the point where there is NO DES PATH AFTER.

 

 

 

2) would this message appear in the real world, and how would a real pilot have handled it?

 

Yes. Happens all the time on the LENDY into JFK. See here, which will also include the cause of the message:

 

 

 

3) is there no way to program the FMC at the gate prior to takeoff such that the message goes away (short of deleting the restrictions - which I didn't want to do).

 

Use a STAR that allows for this type of operation. There aren't many STARs here in the States, though, that dump you right from the STAR onto an approach (note that the nearly fully VNAV tutorials were set in Europe :wink: - this is slowly changing here in the States with STARs like the HYPER6 and ATL arrivals). The SMUKE is a mess if you've looked at the chart. Regardless of the transition you chose, the last thing the FMC is told on that STAR is "be at 6000 feet right next to the airport." As I describe in the video, above, the FMC doesn't know what it doesn't know. That sounds like a stupid statement, but think about it: the last info it's been told (by loading the STAR) is "be at 6000 feet right next to the airport." If you, as a person, opened a box of IKEA furniture that contained instructions with missing pages, you'd probably say "no chance until I get complete instructions." The FMC has a similar picture in this instance. It's been given enough information to get to the end of the STAR, but the STAR ends so close to the airport that it cannot calculate a good descent path directly from the last STAR fix to the first approach fix.

 

The message the FMC is giving you is essentially:

"This STAR is putting me in a spot that I can't calculate a reasonable descent path from where it's telling me I need to be by the end of it. After the end of the STAR, it's all you." Since that segment will be vectored, you'll be in HDG SEL + FL CH, or HDG SEL + V/S + SPD, and VNAV doesn't even matter by that point anyway. The MCP is a toolbox. VNAV is great, but it's only one tool. There are other situations that will require the rest of them.

[pcubine 143]

I believe this is correct once you get past the point where there is NO DES PATH AFTER.

I guess I have never seen it disconnect because by the time I get past the point in the NO DES PATH AFTER I went to V/S or LVL CHG 10-15 miles before that.

[downscc 1,327]

This is an example of why we always try to stay ahead of the airplane and anticipate not just the next move but many moves ahead. I use a point 100 nm before TOD do make sure I have an executable descent and approach plan. Just last night I completed a YSSY-KLAX trip arriving on the BUFIE that affords you the opportunity to get from the IAF (SLI) at 7000 to the glideslope at 3600 in about 10 nm. This is not as challenging as the JKF LENDY but 350 ft/nm is something you need to plan ahead for by first recognizing it (there are no FMS warnings) then planning it (speed control is the key).

 

Like Kyle said, the FMS is only a tool. You need to be like a good chess player anticipating many moves in advance and be ready for the unexpected.

 

OBTW: The new YSSY from Fly Tampa is beautiful and I had no problems with VAS and my PMDG B77L. Flight is DAL 16.

[Jetstream96 20]

I guess I have never seen it disconnect because by the time I get past the point in the NO DES PATH AFTER I went to V/S or LVL CHG 10-15 miles before that.

 

From my memory the VNAV goes to VNAV SPD mode in that situation. I could be wrong though.

[Nick Dobda 24]

I took the 5R approach so the star ended at IUUU, leaving me at FL5000 and 210K.. in the FMC it didn't give me a vector though, it gave me a discontinuity, so I pulled IUUU up to the ILS approach, removing the discontinuity... it seemed like they were meant for each other, but alas I got that message and it did disconnect the VNAV.

 

But from what your saying, this is not uncommon, and the fix is to do what I assumed... get off VNAV and fly the plane using other means.

 

Thanks for your answers.

[scandinavian13 5,192]

 

 

leaving me at FL5000

 

In the States, FLs start at 18,000 (FL180). Moreover, FL5000 would be 500,000 feet  :wink:

 

 

 

in the FMC it didn't give me a vector though, it gave me a discontinuity, so I pulled IUUU up to the ILS approach, removing the discontinuity... it seemed like they were meant for each other, but alas I got that message and it did disconnect the VNAV.

 

Don't remove discontinuities unless you're sure they should be removed. In this case, selecting the approach that also begins at that point should have been just fine, but it depends on what you did, and when.

 

 

 

But from what your saying, this is not uncommon, and the fix is to do what I assumed... get off VNAV and fly the plane using other means.

 

In this case the STAR can actually dump onto the approach, but its effectiveness is dependent on your technique. You are correct in that, when things get weird, just take over yourself.

[Nick Dobda 24]

Thanks for bearing with me, quite obviously I'm fresh off the farm with respect to flight simming.

I've been reading everywhere to remove discontinuities - for example a checklist I came across created by CarstenRau... which at least looks like it were created by PMDG has "###Check for no discontinuities, delete any of them###"

You're the first to say leave them in. At least as far as I can recall (I've been doing a lot of reading and could have been told somewhere to leave them in there, and just forgot).

 

Forgive me again, I am guessing that the answers to the questions I am asking are somewhere in the documentation. I do not have hard copies of the thousands of pages, and its so much easier to come here and ask a question then to try and dig through those manuals.

[downscc 1,327]

 

 

Forgive me again

 

No need for that... other than Kyle we are all just like you at various stages in the learning process. Kyle is kinda like us too but gets paid to do this....not sure what form the payment is in; perhaps he collects his compensation in rides on the PMDG company plane.

[scandinavian13 5,192]

I've been reading everywhere to remove discontinuities - for example a checklist I came across created by CarstenRau... which at least looks like it were created by PMDG has "###Check for no discontinuities, delete any of them###"

 

Wasn't made by PMDG, and I'd suggest anything written like that be looked at with a very hesitant eye. Many times, when people use absolutes like that they either aren't telling the whole story or don't know what they're talking about. Many of the issues in aviation, when properly conveyed, should include some form of why when being told, unless it's obvious. Pilots are up front in order to make educated decisions. Educated decisions require education. Simply saying "delete discontinuities without coming to an understanding of why" is a very poor instruction. The disco could mean that you entered something improperly and should review your entry instead of deleting a disco.

 

If your route were planned well, you wouldn't be subject to any of the types of discontinuities that should be deleted. In other words, a well-planned route wouldn't usually have a disco between the SID and route, or between the route and STAR. It might, but a well-planned route is usually planned to fit nicely between these "pitch" and "catch" points. The only other discos are then usually between the airport and the SID, or the STAR and the airport. Unless you know for absolute certain that the discontinuity should be removed, it should be left there. Occasionally, the vectored portion will simply be a disco. Having that gap will ensure the AP doesn't follow an improper command (turning toward an approach fix before being cleared, as an example).

 

Remember that a lot of the information in the sim community comes from other simmers who may or may not have actually flown something before. Additionally, consider that some of the information in the community can also come from pilots who either don't know how to properly convey the information, or simply don't know it that well. I'm not saying "we're surrounded by idiots," but I've fallen victim to trying to sound more educated than I was at certain points. For some reason, people who have just gotten into training tend to fall victim to it, as new those who have recently obtained a private pilot certificate (this latter one had me written all over it, unfortunately).

 

 

Forgive me again, I am guessing that the answers to the questions I am asking are somewhere in the documentation. I do not have hard copies of the thousands of pages, and its so much easier to come here and ask a question then to try and dig through those manuals.

 

As Dan mentioned, there's no need for forgiveness. We're all learning. Heck, I still am.

 

I don't think these answers are in the manuals, but I will say that most of the common ones are addressed in the Intro Manual. Additionally, the PDF versions are a lot easier than the paper. Paper copies, though easier to read like a book, don't come with a CTRL+F function.

[Nick Dobda 24]

Thanks again for everyone's help.

 

BTW I just bought a computer with an 8 core 4GHZ processor, and a bunch of other stuff light years beyond what I was using. So crossing my fingers hoping the uninstall / reinstall goes smooth between computers.

[ALF1 15]

The problem with leaving a discontinuity is that LNAV will disengage upon reaching the last valid waypoint. I think, though not sure, that if that discontinuity is left before a flight you will not be able to engage LNAV. Someone correct me if I am wrong I am not an authority.