Pmdg 737 Vnav Descent

[Yrko Philogene]

WellI know that a comparison with the NGX should not be made, but when it comes to VNAV descent, the NGX acts perfectly, the 777 leaves me quite doubtful and it looks to me a bit strange that the real one behaves like that. Whenever I start the descent in VNAV, the thrust goes to HOLD and the system doesn't care about what vertical speed the airplane achieves. Only when it approaches a constraint altitude, it "remembers" of that constraint and both thrust and altitude become controlled again, thus going into level flight. You end up reaching that altitude miles before the waypoint with that constraint altitude. Instead of a continuous descent, it becomes a descent made by steps.

If I start descending before T/D, I knew from the good old 747-400 that the airplane performs a controlled descent at 1250 fpm until intercepting the descent path. Instead this 777 guess what? Goes into HOLD mode and bye bye...

Yup, don't forget that the 777 predates the 737NG series and the VNAV is different. Early descents may not work as you expect them but there was a technical review team consisting of T7 pilots that vetted the simulation, so I trust that there is fidelity. Your CI does have a major impact on descent performance, try playing with that.

But constraint are adhered to in any case (the constraint altitude will just be reached earlier.....unless the Vanv path is still intercepted (thrust reduced). Or the vnav Path will be intercepted sooner resulting in an idle descent from there on to you constraint.

After the first constraint a line/slope is drawn from one constraint to the other resulting in continuous descends that might, or might not be, idle descends (depends on distance and altitude between them obviously).

Thrust mode goes into THR/VNAV SPD and after a while into HOLD/VNAV SPD, ok, as described in the FCOM. But the vertical speed decreases very slowly and the speed tends to decrease to 270 knots. First anomaly: why doesn't VNAV SPD trim nose down in order to maintain 280 knots, also considering that the descent rate is near 0?

Yes, I do, registered version. I tried another scenario with a different descent path and things were better, anyway this VNAV gives me the idea of an almost completely manual descent, 90% of the time the mode is HOLD/VNAV SPD and you have to adjust the thrust to let the airplane fly the descent profile.

Check your hardware. In an early descent VNAV will reduce thrust to descend at approximately 1000 FPM and then go into HOLD and VNAV SPD. My guess is your throttle axis has noise or something that is causing the thrust lever position to change after it's gone into HOLD. It sounds like your hardware is causing the thrust levers to come back all the way to idle.

This could be your problem. Unless you have selected NEVER for the A/T MANUAL OVERRIDE option, you do need to touch the thrust levers. If you've left them forward after takeoff this will limit thrust reduction on descent and in HOLD mode they will follow your hardware input. I find VNAV works just fine unless I forget to retard the throttles before descent.

I did a test starting descent at T/D, therefore I did not touch anything, just dialed a lower altitude on the MCP and let the airplane do the rest, and the descent was PERFECT, with IDLE (then HOLD) and VNAV PTH. It's interesting to also compare the actual vertical speed with the V/S number on the FMC VNAV DES page.

I've used early descent multiple times and only thing I noticed was that the rate was higher than initially expected based on documentation but definitely not excessive as you experienced. I assume your descent is ECON... throw some numbers at us (M/KIAS, ALT, distance to TOD, CI) and maybe we can figure it out.

I believe I figured out what caused it in my particular situation. I saved the flight at TD with time compression enabled @ 4x and then loaded it the next day to finish it. I turned off time compression atleast 40nm before TD, and then used DES NOW about 10nm from TD but I kept getting into these uncontrolled dives. What solved it was hitting the "reset" button in the time compression options after re-loading the saved flight. After that, DES NOW worked just fine. The only thing I can imagine causing this is that certain VNAV calculations were still operating at 4x rate until reset.

I was descending on a STAR with VNAV when p3d atc took me off it early and gave me initial vectors for approach. I followed their altitude instructions and heading (heading select) by inputting them into the MCP with VNAV remaining on. However, Vnav seems to descend very slowly at times doing this and I got the "expedited descent' message often. I wonder why it doesn't get you down faster when I push altitude intervene on the MCP after an instruction by ATC.

So, Is it usual procedure in this case to remain on Vnav even if you are not going to be near your STAR waypoints and their associate altitude restrictions? Is this messing up the VNAV descent profile somehow? Should I be using a different mode when ATC starts vectoring me off my STAR and giving different altitudes than my STAR has?

The first issue here is the default ATC in sims today. The STARs you're using are from the real world. The ATC in the sim doesn't control according to real world procedures - specifically those STARs. Furthermore, it doesn't control with an understanding of the aircraft type. A controller has to control a Cessna differently than he or she would control a 747, or a 737. The sim control function does not do this.

There's only so fast that you're going to be able to descend. As mentioned in the Intro Manual, and many many times here on the forum, the 800 is a slippery plane, so it's a little tougher to get down. Keep in mind, as well, that if you have hardware throttles, you should make sure that they're at the IDLE stop to ensure that they don't interfere when the AT mode drops to HOLD.

The best luck I've had with the default ATC is V/S @ 2300-2600 FPM to 10,000 feet (from cruise) and then FL CH at 220-240 until I'm 30 NM out and then I'll start slowing things down for approach. At least that's with the default FSX ATC. Otherwise they'll be barking at me to expedite descent or I'll hit the FAP waaaaaaay too high or too fast.

When flying with the default ATC I almost always use VNAV to climb, then FLCH to follow descent instructions from ATC. The exception is when I am using an RNAV approach (which the default ATC does ok at, as long as things haven't changed too much since 2006). With an RNAV approach, you can often switch back to VNAV when you are on the approach.

The issue with VNAV once you come off the LNAV path is that it has no idea what your actual track mileage to run is going to be, so its descent profile calculations are necessarily going to become pretty useless pretty quickly. It helps if you "tidy up" the map a little so that the distance to go figure in the FMC is a bit more in line with what you are actually going to fly and therefore gives VNAV some slightly better data to generate a profile with.

Ok thanks all for the advice. I'll definitely start to switch to FLCH or V/S to avoid this issue. I'll check out some ATC add-ons that deal with STARS/SIDS properly too. However, is there any way edit the default ATC to make it start vectoring you later so that that it allows you to fly more of the STAR? I inputted the STAR waypoints into the IFR flight plan but the vectoring started after the second waypoint on the STAR was reached. In real life, how much of a STAR does a typical flight follow before getting vectors?

Depends where you are. In the UK, you'll normally go all the way to the holding stacks and then be either put in the hold our vectored from there (so at Heathrow, LAM, BIG, OCK or BNN), though occasionally you might get a shortcut from further out if it's (very) quiet. Other places it depends a bit but usually you'll go to some sort of IAF (indicated on the charts).

Do not be afraid to use the speedbrake on descent. Do not be afraid to start your descent earlier than the fmc tells you on the navigation display. Do not be afraid to plug in a higher number for descent than the vnav spits out that could be as low as 240 knots at altitude if you fly with a low cost index. Also a helpful addition to the fmc giving you a good planned descent is plugging in the winds on the legs page and winds on the descent forecast page. You have to do this manually in the ngx but the 777 grabs the info from active sky wx engine - David Lee

I watched a video of a Virgin Atlantic 744 from EGLL to KSFO. Just after getting on the STAR which was GOLDEN GATE SIX, they were cleared direct to LOZIT which is about 25 miles from KSFO skipping 4 waypoints of the STAR. Instead of being in VNAV they had to go to V/S and use the green banana. It happens in real life also, not just P3D.

That's why I fly actual flights grabbed from flightaware. Ideally I'm about an hour behind actual time so I can see the ground tracks for departure and arrival. The other day, I saw traffic on the KIAD GIBBZ (tip the hat to NCIS) vectored direct to final at about LDN forming an extended base leg instead of to midfield via STAR and thence downwind. Usually the traffic follows the STAR but I guess traffic was light.

I like to listen in on liveatc feeds in America or Australia almost every big airport has good coverage and Flightaware has all the real routes. I'll pick a flight scheduled within an hour or so and follow it in real time either in the ngx or 777, listening in from liveatc clearance feeds if available all the way to pulling up at the gate. Even setting the same squawk.

3a8082e126