NGXu VNAV disconnect when setting up approach

[CBT_Phoenix 0]

Hi guys!

 

I've been flying with the PMDG 737 NGXu for about a month now, and I recently had a problem with the CDU on the 737-900ER.

I was performing a flight from KORD (FSDT) to KLAX (FSDT), with the latest AIRAC FMS data and charts. About 100NM before my last route waypoint I start setting up the arrival procedures in the CDU. The approach I used is ILS RWY06R, and the STAR is MDNYT2 (MDNYT2.06B) with HAKMN transition.

When I go pressing EXEC on the CDU, it seems the aircraft cannot compute the SPD/ALT for the arrival waypoints and I get a "VNAV DISCONNECT" message.

 

The only way I was able to perform the approach was to change to ILS RWY06R, FIN transition, and flying direct from HAKMN (last enroute waypoint) to FIN.

 

Is there something I'm doing wrong or missing here? I'm relatively new to the 737, as I come from years of 777 so maybe there might be something different to do I'm missing out.

Thanks in advance for any help!

 

Giovanni

[somiller 694]

Haven't flown the NGXu in a while, but I'll try to get time today to replicate your flight and see what I get. I've also got the latest Navigraph AIRAC, so It should be a good comparison. Were you flying real world weather, and if so, what date and time (UTC) did you reach 100NM before last enroute waypoint? What was your cruise altitude?

Kind Regards,

[Jim Young 2,356]

When I'm on flights with the 737NGXu (or the PMDG 777/747), I'll lower the altitude to 1000 feet as soon as I reach the cruising altitude because, if I wait until I near TOD and change the altitude then, sometimes it does not "see" I have lowered the altitude, especially if I'm within seconds of the TOD.  If I forget to lower the altitude before the TOD and continue on with the cruising altitude, I will have to enable VS to begin the descent and, once the aircraft begins to descend, I'll enable the VNAV and watch to make sure the aircraft continues descending (have to manually change the speed too as required by the FMC Legs Page.  I suspect I need to go through my "Before TOD" Checklist or get my Co-Pilot to watch more closely as we near the TOD.  Sometimes the co-polit is asleep on the couch...

 

[spesimen 135]

check your descent speed.... often it can be too high with a high cost index. the behavior you describe can occur if it is set too high/fast for the the aircraft to be able to meet the numerous speed and altitude restrictions that are in that STAR... you can get into a state where it is ambiguous because you are telling it to go one speed but it can't hit the restriction and do the commanded speed at the same time so it blanks out the calculations. typically for a STAR like that where there is a hard speed limit it can be helpful to set that speed in the descent page instead of using the econ value (set it to 280/.78 for example).  alternatively setting a lower cost index that results in an econ value that is closer to the restriction is also possible..

there is another thread about it here from some time ago. 

 

 

happy flying!

cheers,-andy crosby

 

 

 

 

[threexgreen 3,138]

If you wait too long to set up the approach routing in the FMC you're in danger of running too late for VNAV to follow the arrival with its altitude and speed restrictions because you're already past the TOD point that would have been required to follow the STAR restrictions.

Don't leave the STAR and approach empty. IRL, you select the STAR which is usually known beforehand and is mostly unlikely to change as well as the expected approach during preflight. That way you will get a reliable route and descend calculation with all the restrictions of the STAR factored in.

[threexgreen 3,138]

1 hour ago, spesimen said:

check your descent speed.... often it can be too high with a high cost index. the behavior you describe can occur if it is set too high/fast for the the aircraft to be able to meet the numerous speed and altitude restrictions that are in that STAR... you can get into a state where it is ambiguous because you are telling it to go one speed but it can't hit the restriction and do the commanded speed at the same time so it blanks out the calculations. typically for a STAR like that where there is a hard speed limit it can be helpful to set that speed in the descent page instead of using the econ value (set it to 280/.78 for example).  alternatively setting a lower cost index that results in an econ value that is closer to the restriction is also possible..

In that case you would normally get an "Unable XXX kts at YYYYY" or "Des path unachievable" message. I've never seen VNAV completely disconnect because it can't meet restrictions.

[spesimen 135]

25 minutes ago, threegreen said:

In that case you would normally get an "Unable XXX kts at YYYYY" or "Des path unachievable" message. I've never seen VNAV completely disconnect because it can't meet restrictions.

well, it's easy enough to reproduce if you are curious to see it. check out the other thread i linked there is a much more specific example with high CI and weights. i don't know precisely why the situation where it blanks out all of the calculations in the STAR is different than the "Unable/unacheiaveable" messages but i've seen both behaviors. i think the latter errors show up when you can correctly calculate the descent but then deviate too much from it, whereas the blanked entries occur if you have not yet started/calculated the descent and it is simply unable to calculate it due to the ambiguity i mentioned earlier. i believe in practice it is somewhat rare as it requires a STAR with a speed restriction and also an unusually aggressive CI, in the other example thread it was because the last waypoint before the star was commanding a 330 knot descent followed by a 280 restriction only a few nm later. with a more conservative CI the descent is already going to be in the 280 neighborhood. 

happy flying

cheers,-andy crosby

[CBT_Phoenix 0]

On 12/23/2019 at 6:03 PM, somiller said:

Were you flying real world weather, and if so, what date and time (UTC) did you reach 100NM before last enroute waypoint? What was your cruise altitude?

I was flying with ASP4 historical weather set at 23/12/2019, 22:00ish Zulu, and I was cruising at FL340.

 

On 12/23/2019 at 7:58 PM, Jim Young said:

I'll lower the altitude to 1000 feet as soon as I reach the cruising altitude because, if I wait until I near TOD and change the altitude then, sometimes it does not "see" I have lowered the altitude, especially if I'm within seconds of the TOD.

Instead I usually prepare for the arrival 100-150NM from tod (or last waypoint) and then I closely look to the instruments, so that when I'm about 50Nm from TOD I start lowering the altitude.

 

22 hours ago, spesimen said:

check your descent speed.... often it can be too high with a high cost index.

This might be the cause, I'll try to reproduce the scenario with a lower cost index, as I usually fly with very high cost idexes (in this case was around 200).

 

Thanks everyone!