Troubles Making Accurate Descents With 737-800 On Autopilot Using LNAV/ VNAV

[jeromewillisch]

Hi all, I have been experiencing intermittent troubles with making accurate descents on autopilot using LNAV and VNAV with the 737-800 for ILS landings. Before I explain the issue, it might be worth mentioning that I use Navigraph to obtain arrival/ approach info (final approach altitude, ILS frequency, touchdown zone, etc.) and Simbrief for my flight plans.

After reaching cruising altitude, I configure all of the necessary descent/ approach info, including the final approach altitude on the autopilot panel. The plane always begins descending at the planned TOD juncture, but in the midst of descending, it will sometimes almost level off at an arbitrary altitude and continue the descent extremely slowly. As an example, the plane could basically maintain 10,168 feet when it's supposed to be at 6,000 feet at a waypoint that has been reached. This issue does not occur on every descent and I have not been able to pinpoint any specific flight characteristics that could be causing it. I configure the same autopilot mechanisms for every descent and there are plenty that go as planned. Does anyone know what could be causing this?

Thanks,
Jerome Willisch

[Captain Kevin]

Jerome, is there by chance a speed restriction that you're coming up on that the plane is trying to slow down for.

[jeromewillisch]

I get no indications or FMC messages to suggest a speed restriction, including the "unable xxx knots at x" message. Is there, perhaps, some other way to know that this would be the cause?

Jerome Willisch

[gnt586]

This used to happen in the old FSX days with a wind change and the FMC recalculating the decent path, I've not experienced this within msfs though yet,

[toby23]

I'm not sure if it's the same issue but I find it hard to get the 737 to match my designated speed or preset speed restrictions. For example, this afternoon on a flight, I set a SPD INTRV of 250 KIAS on a descent and the aircraft was unable to achieve this, hovering around 268 KIAS. I have no idea why. Engines were idle and VNAV SPD was displayed. I feel that I can't trust the VNAV to hit specific speeds at specific waypoints in the planned route, which is what I thought that whole point of using VNAV is for, but this could very well be user error as I have just started in the 737.

[Steve-Kane]

I had a flight similar to this a few weeks ago into LAX, where I was much higher than the altitudes I would expect as I descended. I think my issue was that I forgot to set the proper amount of fuel at startup, and the default fuel setting made me much heavier than I should have been (it was a short-ish flight). My guess is that the plane was having a hard time slowing down and descending at the same time due to the higher-than-normal inertia. Could this be a possible cause of your experience?

[jeromewillisch]

Hi Steve, I do tend to do shorter flights ranging from 30-60 minutes of air time which of course require lower amounts of fuel. As I mentioned, I use Simbrief for my flight plans where I obtain total fuel amounts, which include 45 minutes of reserve. These total fuel amounts typically range between 14,000-16,000 pounds and never require the center fuel pumps to even be turned on. While I'm positive that I input these total fuel amounts provided by Simbrief into my FMC correctly, could these amounts be too much fuel for shorter flights between 30-60 minutes?

Jerome Willisch

[Captain Kevin]

I get no indications or FMC messages to suggest a speed restriction, including the "unable xxx knots at x" message. Is there, perhaps, some other way to know that this would be the cause?

Jerome, without looking at what you're looking at, I'm really just guessing at this point. In any event, have a look at the PFD, and if it's showing a lower commanded speed compared to what you're actually flying, for instance, showing a commanded speed of 220 knots when you're doing 260 knots, there's a possibility that it's trying to slow down for a speed restriction. The FMC would also tell you what the speed restriction is at a given waypoint.

[jeromewillisch]

Hi Kevin, thank you for this tip. When you mention commanded speed, I'm assuming you mean the speed in which the purple pentagon is pointed at on the speed strip located in the PFD? As for the FMC, is there another way to interpret what it's telling me to be a speed restriction for a given waypoint aside from the "unable xxx knots at x" message? If these solutions turn out to not work, I will post a picture of my PFD and FMC next time I run into this issue.

Jerome Willisch

[Steve-Kane]

I'm not sure if it's the same issue but I find it hard to get the 737 to match my designated speed or preset speed restrictions. For example, this afternoon on a flight, I set a SPD INTRV of 250 KIAS on a descent and the aircraft was unable to achieve this, hovering around 268 KIAS. I have no idea why. Engines were idle and VNAV SPD was displayed. I feel that I can't trust the VNAV to hit specific speeds at specific waypoints in the planned route, which is what I thought that whole point of using VNAV is for, but this could very well be user error as I have just started in the 737.

If your engines are at idle and it's not slowing down, then the only two ways to slow it is to either add drag (i.e. speedbrakes) or lower your descent rate. Did the FMC say "DRAG REQUIRED" at the bottom?

Energy management of this plane is definitely a skill I'm still learning. Business jets with A/T (Longitude, HondaJet) are much easier to slow down than this plane, to say the least.

[toby23]

No, there were no indications but I have been liberally using the Speedbrake to slow to meet restrictions. It just happened again on another flight. The restriction was 250 but the aircraft would not go below 260/265. I'm not sure what I'm doing wrong here, maybe it's just the way of the 737.. Thank you for your help Steve

[Victor Augusto]

No, there were no indications but I have been liberally using the Speedbrake to slow to meet restrictions. It just happened again on another flight. The restriction was 250 but the aircraft would not go below 260/265. I'm not sure what I'm doing wrong here, maybe it's just the way of the 737.. Thank you for your help Steve

There is a tendency to pick up some extra speed when temperature is hotter than ISA, and to lose speed when it is colder, winds also play a huge role, accurately filling out the descent forecast should help with more accurate descent paths, additionally some STARs may require steep descents. If for whatever reason the aircraft is struggling to maintain profile, the speed brake is your friend use and abuse it.
When not speed restricted you can select a higher descent speed in the FMC, which may help you restablish your path.

Also note that during a geometric descent the aircraft will not reduce vertical speed to comply with speed restriction, so when in this condition always plan ahead.
you know the aircraft is in a geometric segment when FMA goes from ARM to FMC SPEED.

as a side note:
Pmdg VNAV path is very accurate with ISA temps and 0 wind, but when you add all the extra variables, including the Msfs crazy thermals, maintaining the profile can be a struggle.

[Steve-Kane]

Good point, Victor Augusto . I do typically "Request" the Descent winds on the Descent page (part of the Simbrief export files to PMDG), and would assume that having accurate winds will greatly help the FMC to properly calculate a vertical path.

I was watching 737NG Driver on Youtube yesterday, and I now also realize that I haven't been entering the QNH setting of the destination airport into the Descent page. I think this is also important for calcs, since if the baro is considerably different than 29.92, your path be way off when you hit the Baro knob at the transition altitude.

[toby23]

Pmdg VNAV path is very accurate with ISA temps and 0 wind, but when you add all the extra variables, including the Msfs crazy thermals, maintaining the profile can be a struggle.

Thank you Victor. I've used V/S as well, which I've read is a no-no, but works great for me to get me down in time. I've also started adding my own restrictions into the LEGS section to try to anticipate slowing down in time for certain sections. It might just be the approaches that I'm flying are complicated for non-human ATC to vector me around. I'm excited to try BeyondATC and see if that's more true to life, if it comes out.

[Captain Kevin]

Hi Kevin, thank you for this tip. When you mention commanded speed, I'm assuming you mean the speed in which the purple pentagon is pointed at on the speed strip located in the PFD? As for the FMC, is there another way to interpret what it's telling me to be a speed restriction for a given waypoint aside from the "unable xxx knots at x" message? If these solutions turn out to not work, I will post a picture of my PFD and FMC next time I run into this issue.

If you look at the VNAV descent page, it should tell you. This is a screenshot of a previous stream I did, and it is a different airplane in a different phase of flight, but the idea is the same. You can see in the upper right corner of the FMC, it lists the waypoint and the altitude restriction associated with it. If there was a speed restriction, it would also be listed there.
100.png

Thank you Victor. I've used V/S as well, which I've read is a no-no, but works great for me to get me down in time. I've also started adding my own restrictions into the LEGS section to try to anticipate slowing down in time for certain sections. It might just be the approaches that I'm flying are complicated for non-human ATC to vector me around.

To be clear, it's not that you can't use V/S. After all, it wouldn't be there if it wasn't supposed to be used. You just have to be very careful about using it since it doesn't give you speed protections like VNAV would, so you could easily stall the airplane or overspeed the airplane if you aren't paying attention. As for adding your own restrictions to the legs page, I wouldn't recommend doing that. If you're flying with ATC, say on VATSIM or something, and they issue you a speed restriction that's different from what you entered in, you could easily get screwed up if you aren't careful with what you're doing. What you could do is use the speed intervene instead. The speed window will appear on the MCP when you do that, and you can adjust your speed there.

[toby23]

Thanks Capt. Kevin. I was only adding restrictions that exist in the real world and are not, for whatever reason, in the Navigraph db. Thanks for your help.

[DDowns]

Thanks Capt. Kevin. I was only adding restrictions that exist in the real world and are not, for whatever reason, in the Navigraph db. Thanks for your help.

You got an example of one of those? Perhaps the restriction was something like EXPECT XXXXXX? None of the expect constraints are in the procedures because they only apply if they are in the ATC clearance. Of course in simulation not using ATC it makes sense to manually add them but I don't want to assume this is what happened in your case.

I urge you to learn to use LNAV/VNAV. Otherwise you miss out on learning how the Boeing operates and this knowledge is pretty much the same for all airliners (with variations of course).

[mshev]

Leveling off at an altitude close to 10,000ft sounds like your plane is trying to slow down to meet the 250kt restriction below 10,000ft (it's set to 240kt in PMDG by default).VNAV is not magic, you will need to help it slow down and catch the profile when the plane is having a hard time doing so by itself. I suggest reading and watching videos (NG Driver has good videos on it) about LNAV and VNAV before treating it as this some sort of wizardry that just magically keeps your plane on profile during descent.

[klokous]

To make an accurate descend, you shoud:
- Fill the DES FORECAST on FMC with WIND informations, QNH, ISA DEV and TL (DES page on FMC, then FORECAST on the down left)
- Make sure that your speed programmed by your FMC over the IAF is your flaps up speed (VREF40 using your ELAW + 70 kts). Insert it on the LEGS page, typing 210B/, for example. If there's a speed restriction below your flaps up speed, of course you will follow the spd restriction and ignore this item for that case.
- Make sure that the fix just before the IAF has a programmed suitable speed comparing it's distance to the IAF. For example: if your speed at the IAF is 205kts, but you have a fix 5 miles before the IAF in which the programmed airspeed there is 240kts, you probably won't be able to reduce. So change that speed to a lower one.
- VNAV has 2 modes: VNAV SPEED and VNAV PATH. Make sure to always be on VNAV PATH, because on VNAV SPEED the VNAV will try to get and maintain the TGT speed and it will ignore your descent path, so you will most likely become high on the path. If you are on VNAV SPEED mode, just make a speed intervention and increase your speed by like 10-20kts. Your aircraft will increase it's V/S and when you reach the descent path again, it will switch again to VNAV PATH and will continue the descent on the correct path. Then you can return to your previous speed.

Common flaps schedule:
- IAF: flaps 1
- 2 to 3 miles before IF: flaps 5
- 3 miles before FAF: gear down, flaps 15
- 1 mile before FAF: flaps 25
- FAF: landing flaps (30 or 40)

This can obviously change due to the size of the approach procedure or due to your favorite airline SOP

Hope this can help you a little bit.
Regards!