A Bit of Confusion on the FMC and VNAV/LNAV

[gpbarth 12]

I don't think TOPCAT's profile matches the new PMDG 737s (the profile in there is from their FS9 version, if I remember correctly), so I wouldn't tell it to export weights at all.

I wasn't worried too much about the weights so much as I was about exporting ANYTHING from TOPCAT to the 737. TOPCAT does look at the aircraft.cfg file (as far as that may go).

[scandinavian13 5,192]

I wasn't worried too much about the weights so much as I was about exporting ANYTHING from TOPCAT to the 737. TOPCAT does look at the aircraft.cfg file (as far as that may go).

 

Sorry, that's what I was referring to.  Fuel, passengers and cargo are all weights.  I don't recall any benefits from the linked config other than exporting the weights.  With the MD-11, because the profile in TOPCAT was built from the current PMDG version, you could avoid the PMDG load manager entirely and just use TOPCAT for that.

[LRW 42]

One of the big lessons I took away from the first tutorial was that the navdata in the FMC doesn't necessarily match the charts.  Whenever I enter plan and the PERF INIT info, only to see a "NO DES PATH AFTER xxx" I always look at the altitude restrictions in the FMC and compare to the current FAA chart.  For example, yesterday I shot the 25L ILS to KLAS.  The FMC (using Navigraph 1305) gave me "NO DES PATH AFTER LARRE".  It showed

PRINO   8000A

LARRE  6500A

SHAND  3800

RELIN    3800

RW25L  2098

 

The current chart from the aeronav.faa.gov site has

PRINO   8000A

LARRE  6500A

SHAND  4900A

RELIN    3800

RW25L  2098

 

When I changed SHAND to 4900A in the FMC then the message went away.

 

Speed restrictions can cause similar messages.  Often there's a descent restriction like 280/12000, which will throw a message because the default descent speed is 261 on the DES page or, occasionally,  a 250/10000 restriction which trips up the default 240/10000.  

 

So many really cool details.  I've been using this aircraft since Sept 2011 and it has become my hobby.  There is so much to explore and learn, and I haven't even tried the Service-based Failures yet.

[pcubine 143]

Whenever I enter plan and the PERF INIT info, only to see a "NO DES PATH AFTER xxx" I always look at the altitude restrictions in the FMC and compare to the current FAA chart. For example, yesterday I shot the 25L ILS to KLAS.

 

 

Larry

Once the STAR and ILS at KLAX (24L, 24R, 25L or 25R) are entered into the FMC and executed I always receive  "NO DES PATH AFTER xxx". This particular one was SEAVU and ILS 25L. Most of the time it is RIVIR and ILS 24L or ILS 24R. I have confirmed that all altitudes agree with the FAA charts. I even added the last fix LADLE because it was on the chart and not in the approach per Navigraph  FMS Data. The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

This is not a problem since I go to V/S mode around 13000 feet but I get tired of looking at two or three times a week for hours on end.

 

Got any ideas? Just left click the images so you can see the CDU.

 

Thank you

Michael Cubine

[scandinavian13 5,192]

 

 

Got any ideas?

 

The system is getting huffy because it knows that you're going to have to decelerate that whole time, while also descending the whole time as well.  It's probably something the real world pilots have their own tricks and work arounds for as well (setting a particular speed on a waypoint, ignoring it and just using speedbrakes to keep the path, modifying the flap schedule, etc - I don't fly/work much over there, so I couldn't tell you what's "normal").

 

 

 

...but I get tired of looking at two or three times a week for hours on end.

 

The "for hours on end" can be avoided simply by leaving off the arrival runway until you start your descent (where you'd get the runway assignment in the real world anyway).  The message may still pop up, but having the runway in there the whole flight is neither realistic, nor is it gaining you any advantage.  Keeping your sanity for the hours leading up to the descent/approach is ideal.

 

 

 

I even added the last fix LADLE because it was on the chart and not in the approach per Navigraph  FMS Data. The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

For what it's worth, LADLE is a fix after your glideslope intercept.  As such, while you need to be at or above that fix on the approach, the glideslope will help you meet that.  While adding it could add to your situational awareness, it doesn't add much.  The altitude indicated at 727 only confirms that the calculated path meets the charted restriction (700A).

[ichundu 10]

 

 

The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

try with a zero in front (0700). if you enter 3 digit numbers i think the fmc interprets it as flight levels, in this case FL700. that's why you get the "INVALID ENTRY" message.

[Spin737 102]

try with a zero in front (0700). if you enter 3 digit numbers i think the fmc interprets it as flight levels, in this case FL700. that's why you get the "INVALID ENTRY" message.

Or how about just /7.

[ichundu 10]

Or how about just /7.

hmm, haven't tried that but i like the short entries. i always enter the altitude restrictions that are below transition level in FL format, e.g. 30 for 3000ft. now that i'm thinking about it what if you just enter 7 or 07? is the slash obligatory for altitudes below 1000ft?

[Spin737 102]

 

 

now that i'm thinking about it what if you just enter 7 or 07? is the slash obligatory for altitudes below 1000ft?

Yep, just "7" should work, too.

[hopskip 253]

You really don't wanna be bumbling around in VNAV at 700ft unless it's an actual RNAV approach, in which case you don't want to be modifying entries.

 

also 727ft is above 700ft (by 27ft no less). So "Above 700ft" and "At 727 ft" is ... the same thing. No need to change it.

 

Besides you're going to be on APP (Glideslope) mode there anyway, so the VNAV is pretty irrelevant (except for altitude check on slope).

 

If the FMS said "400ft" at that point, then there might be cause for concern (VNAV wants to bust minimums... why? And how do we make it stop?)

 

But in this case, I'd do ... nothing.

Actually no, I'd go into the INIT REF page and enter the expected Gross Weight for my arrival and select a flap/vref speed, which you havn't done (The touchdown speed in the legs page is "---" unselected).

[pcubine 143]

 

 

Actually no, I'd go into the INIT REF page and enter the expected Gross Weight for my arrival and select a flap/vref speed, which you havn't done (The touchdown speed in the legs page is "---" unselected).

Trent

The LADLE fix and 727 feet has nothing to do with VNAV. It has merely been inserted into the CDU to see if I can get rid of the NO DES PATH message. It was there at the time I took the screenshot but it was later deleted from the CDU.

 

I select flap/vref from the INIT REF page at the start of the descent. According the 737NGX Tutorial 1 Page 71 the weight on that page is the current weight so I don’t believe any estimated weight needs to be entered prior to selecting flap/vref. Plus I check the INIT REF page at 7000 feet to make sure flap/vref agrees with what is show on the PFD. The screenshots were taken about two hours from T/D so that is why there is no touchdown speed. And once flap/vref is selected the NO DES PATH message still appears.

 

I have done two more flights to KLAX and tried several different things but I still receive the message. The only way I have to get rid of it, is two enter a character in the scratch pad. It blocks the message from appearing but the message is still there. It also probably blocks other messages which I might want to see.

Michael Cubine

[scandinavian13 5,192]

 

 

The screenshots were taken about two hours from T/D so that is why there is no touchdown speed.

 

There's another problem.  You'd never get a runway assignment that far out.  I know LAX runs west ops as much as possible, so you're almost guaranteed a 24 or 25, but selecting it that far is probably only worsening the problem as I've already mentioned.  Here at Potomac, you generally get your runway assignment on handoff to the first area controller (meaning, occasionally not the first approach controller you speak to).  When arriving from the west into DCA, you'll get it from the first controller you speak with (LURAY), as you're descending to 15,000 (somewhere between FL200 and 15,000ish if you're on the new profile arrivals).

 

As I mentioned before, part of the issue is that many STARs leave a lot of variation for controllers.  There are a few posts out there by me regarding the LINDY into JFK.  On its face, it looks like a crazy chop and drop after the LGA VOR, but in reality, you get vectored off before LGA and vectored for about 50nm.  The trainee vectoring charts for LAX don't show anything like NY's vectoring, and actually show that this is one of the facilities that will magenta-line you to the airport.  Here's the VATSIM version:

 

Since this isn't a situation like the NY Scenario (where vectors unknown to the automation are increasing the distance to the destination, giving you more room for a descent), it seems that it's just a matter of speeds.  The message is coming up because the automation is trying to work out the best solution to slow down and go down given the required altitudes, in the distance between those fixes, and it's not getting a valid solution.  *DING* "NO DES PATH AFTER [FIX]."  It can't anticipate if you're going to use speed brakes (or any other drag to help it out), so it just says "given my normal profile assumptions - altitude and speed - I can't get the plane down the way I normally do.  *DING* You figure it out. (NO DES PATH AFTER...)"

 

As I mentioned in the last post, pilots flying into the SoCal area are used to the odd profiles into the airports, and likely have their own ways of handling them.  Max gear extension on the 737 is 270 knots, and I think I've heard of pilots in that area hanging the gear early to get the drag for a more aggressive descent.  Perhaps @MattC would have a few technique pointers.

 

At the very least, since the message comes up likely when you enter the approach, just leave the approach off until a realistic distance (in the descent, on the arrival, below FL200ish).  That way, it will at least have less time to bother you.  As the automation is only there to assist you in doing you job, sometimes when you know something it doesn't, you just have to put up with its complaints temporarily (especially on the LENDY).

[pcubine 143]

As the automation is only there to assist you in doing you job, sometimes when you know something it doesn't, you just have to put up with its complaints temporarily (especially on the LENDY).

 

 

Kyle

I see and understand what you are saying.The main thing I take away from this is that there in no problems with the speed/altitudes at the various fixes on the ILS approaches as entered in the CDU . Even if the Approach Plates info was in total agreement with the CDU/FMC, the message would still appear because a descent to the runway can’t be calculated and done in the VNAV mode. Like I wrote earlier at around 13000 feet I switch from VNAV to V/S mode so if VNAV can’t do it it is no problem.Thanks for your help, suggestions and information.

 

Michael Cubine

[scandinavian13 5,192]

 

 

Even if the Approach Plates info was in total agreement with the CDU/FMC, the message would still appear because a descent to the runway can’t be calculated and done in the VNAV mode.

 

It's not that it can't be calculated and can't be flown in VNAV, it's that the VNAV assumptions are assuming one thing, but you're going to be flying another.

 

VNAV assumes a certain speed until about 12,000 to meet the 250/10 restriction set.

After that, it assumes you'll fly at about 240 until getting closer in.

After that, you'll decel to a lower speed.

At some point after that decel, you'll put the gear and flaps down.

 

You can still remain in VNAV, but you just need to tell the automation what you're doing (set in restrictions, etc).  The issue is that the automation is assuming one set of actions, but you're going to be flying another, so it gets upset about it.

 

...or you could just make it easier and kick it over to V/S as you're doing.