Understanding 747 FMC & Autopilot Behavior

[NixonRedgrave]

I know at least once response will tell me to go download X or Y mod, but I cannot. I’m flying on Xbox.

That said, here’s what I am trying to understand.

I’ve used the World Map to setup a low altitude flight from KSFO to KPHX. By default, the system used a Direct departure, Direct arrival, and Direct approach. I left them this way.

Looking over the flight plan in the FMC, it shows that all of the LEGS of the flight (beyond the TO and climb out of SFO) are set to 310/ FL370 other than the last one EED at 310/ FL303.

Here’s what I am trying to understand. as the aircraft is in the midst of its current leg approaching ROMOF, the Airspeed Bug suddenly changes to 242 KIAS and the aircraft slows appropriately.

Where is it getting this information? If the FMC indicates 310 for the entire duration of the flight (obviously, I haven’t gotten close enough for ATC to assign an approach), how does the FMS know to lower the airspeed as indicated? I’d also like to understand why it does this in the midst of an actual leg and not at, say, the start of the next leg.

Yes, yes, I know the default sim FMS leaves much to be desired, but this is what I have to work with until it is fixed or someone creates a mod that is available on the Marketplace.

This happens for me with every 747 flight I set up this way, high or low altitude.

[TheSevenflyer]

If you go to the nav log, top right of map screen after you create your plan, you will see in there that
the plan has a speed of 231 kts at KPHX.
Since the plane interprets that as a constraint, it starts to slow down for it after the last waypoint, which is EED.

EDIT
If you select ILS26 as your runway, and HYDRR1 as your approach, you will get a much better speed profile

[NixonRedgrave]

Right, however, the slowdown is occurring as I’m in the midst of the KARNN to ROMOF leg. There isn’t a speed reduction there, is there?

I’m still in the midst of this very flight, so I can’t pop into the World Map, quite yet. I’ll check it out after I land.

[TheSevenflyer]

No, you passed one at Vinco for 250 kts, but you are clear after that according to the nav log.

[NixonRedgrave]

Ok.

I’m looking this over again and I have to put my hand up and say, “I was partially in error.”

I’ve been doing a bit of flying in the 747-800, these last few days, trying to get on top of the assigned approach procedure weirdness. All of my flights have had a speed reduction to 242 KIAS at some stage, I’m assuming this is occurring at the Nav Log’s “Descent”, but that isn’t reflected as a waypoint in the LEGS section of the FMC.

Today, on this flight I mistook the speed change prior to ROMOF as that speed change. It wasn’t. Upon closer inspection the aircraft was maintaining 268 KIAS. I’m thinking my confusion was in the transition from climb to cruise (when the auto throttle backed way off) and possibly the switch from knots to Mach.

Here’s where the confusion continues, however. My basic question still stands. Why does the FMC not reflect what the flight plan in the World Map is stating for speed? Even the speeds on the World Map Nav Log are confusing. Are these listed at Ground Speed? The Nav Log is showing 501 knots at EED.

What is also weird is what is up with all these tiny speed changes? Are these reflections of what the winds are projected to be doing at that point in the flight?

• ROMOF 497 Kts
• Cruise 498 Kts
• Descent 491 Kts
• GFS 496 Kts
• GORSY 496 Kts
• EED 501 Kts

Regarding “Cruise” is that also, like “Descent”, a hidden waypoint in the flight plan that the FMS is actually going to utilize?

Furthermore, why does the FMC show the duration of the flight at 310? At 37,000 ft., 310 KIAS is outside the maximum airspeed for the aircraft. Given the cruise altitude, why aren’t the cruise speeds listed in Mach? Additionally, it didn’t fly at that speed, nor try to even get close, so how did it choose what to cruise at?

Anyhow, all of these inconsistencies are very confusing when trying to correlate what the FMC reflects and what the aircraft is going to do and when it is going to do it.

[TheSevenflyer]

The speed changes are because you are flying in Mach speed control.
When controlling in Mach speed say .84, airspeed reduces while climbing, while Mach number stays constant.
Reverse on descending.

[MikeyAir3272]

The FMC works good in the 747.
This is what I do and how I learned the FMC.

1. Make GPS flight plan. Your fav airports. Set a FL of 35000
2. Get in the sim
3. Once in the cockpit go to the FMC. Note your heading if you are on the runway.
4. Click int/ref > POS Enter your heading and Ref airport. Then enter the gps pos by clicking on it. Then enter it by gps ref below. Then you can move on to the rest of the flight plan by entering your SIDS and STARs. I also recommend Navigraph if you are going to airliners,

[NixonRedgrave]

Thanks for this.

My issue(s) aren’t so much in how to use it or program it, but more about how what information it displays seems to conflict with the behavior of the aircraft.

I can definitely choose a SID & STAR from the World Map, and have done so with previous flights, just not with this particular example.

[MeridianSquid71]

OP-Posted an Indepth Cold and Dark Checklist for 747/787 in plane section on here, at least couple months ago. In that I go into some depth on the FMS/FMC system. MSFS in my opinion, based on watching numerous YouTube videos and trying out what they did, does not setup correctly from World Map route to the unit. That I cannot also exactly figure out how to use it is also issue. One thing MSFS does not do, is set the planes correct position if the battery has been disconnected from plane. For it to be successful, it has to know it’s starting location. Suggest you read my post if you can find it. Tried to get it into the "tutorials section, but system would never let me post there.

Even after that, and doing several test flights, the plane does not fly the route correctly, nor descend per ATC rules/requests, and sometimes completely missis Airport selected in Destination. That I put in SID/STARS runs plane all over map prior to getting to final location. RMAG and several others make it look simple, well not for me.

[NixonRedgrave]

@TheSevenflyer

The speed changes are because you are flying in Mach speed control.

Ah! Of course! Got it! Thanks for that.

How about this then? What is telling the autopilot what Mach speed to set once we get into the mode?

Like I stated, the FMC is displaying 310, but not a Mach value. How is it selecting the Mach value it does? I believe it was at .820.

[NixonRedgrave]

@MeridianSquid71

That I put in SID/STARS runs plane all over map prior to getting to final location.

It certainly does this. Much of my test flying lately is sorting out how to get out of the mess it puts itself in.

Basically, I’m finding once I am assigned an approach I have to:

• Press LEGS
• Press Left Soft Key 2
• Press Left Soft Key 1
• Press EXE

Or, if it has added a USR waypoint:

• Press LEGS
• Press Left Soft Key 3
• Press Left Soft Key 2
• Press EXE

This will bypass the bogus double entry of the assigned Transition waypoint or the erroneously entered USR waypoint that, for whatever reason, causes the aircraft to turn around or fly in circles or what have you.

It will instead proceed to a proper waypoint and continue as it should.

Even with all of this, is still have to override the FMC speed to slow enough to allow for the very abrupt descent profile. I also have to play with the altitude knob as ATC keeps changing its mind about where it wants me as I descend.

All in good fun, though!

[TheSevenflyer]

I suspect it does the math, calibrated for altitude.
In actual fact, it’s probably the other way around.
After FL280 (app) the FMC changes from indicated airspeed to speed measured in Mach, so I suspect the plan does the change.

[NixonRedgrave]

Now my next task is to figure out why Ground at KLAX sees fit to send me and my passengers to the punishment section.

This is a most unbefitting gate for a 747!

[Michail71]

@TheSevenflyer

If you go to the nav log, top right of map screen after you create your plan, you will see in there that
the plan has a speed of 231 kts at KPHX.
Since the plane interprets that as a constraint, it starts to slow down for it after the last waypoint, which is EED.

OMG, I’ve never noticed that there!

MSFS goes to great efforts to hide more complicated stuff. But since I’ve started a month ago the aids and hidden options have been more a hinderance.

Thank you!

[Chaezaa]

I checked the 747 file on PC and the standard cruise mach is set to 0.82

[NixonRedgrave]

I’ve taken the basic premise of what @Microfone9986 has outlined here…

Best usable airliner for xbox?

I also play on Xbox. I plan the flight in advance, according to the weather arrival airports from the Internet and manually type the flight plan in MFC. Very often, when I fly up at the arrival airport, the ATC gives me a different lane. Then I change the approach lane to FMC. After that, I evaluate the new route plan and if the flight point is not on the way, then I use DIR to set the next point so that it is on the way. That way I don’t have to fly in circles. You can practice doing this, for…

…and programmed a basic flight plan directly into the FMC of the 747 while sitting on the runway. I’m not bothering with ATC, because I want to see how the FMS behaves once I program an approach shortly after takeoff.

The flight plan departs KSFO, follows various waypoints in the SF Bay Area and returns to KSFO in a loop just as he has done with KJFK.

What is interesting is (without all the pressure of ATC during a “normal” airliner IFR flight as setup in the World Map and directed via ATC) being able to actually see how the FMS misbehaves after the approach has been assigned. Since the waypoints are all so close together (rather than spread out by massive distances in a “normal” flight plan), you can easily see what the aircraft is up to when it does the strange turn away from the assigned Transition waypoint.

In my case, I can see that even though the top waypoint on the LEGS page is the Transition, the FMS has, instead, directed the aircraft to the final waypoint I’d programmed into the FMC, as seen on the final page of RTE.

I want to experiment a bit more and report if this is the consistent behavior across the board, or just in this particular case.

Anyhow, it’s giving me some insight into why we see such erratic/unexpected/unintended behavior with the autopilot once we assign an approach.

[NixonRedgrave]

I’m really not a video person. My entire world of expressing myself is via reading and writing.

I’m also pretty certain I’m not using the Autopilot incorrectly. Not only are there a number of issues with FMS logic and “flyback” after programming an approach, but there are threads here discussing autopilot issues with both the 747 & 787.

If you’re using an aircraft with the G1000 NXi and have the free Working Title module from the Marketplace installed, you are not going to experience this issue since WT have sorted it out. They are working on all the systems for the future, so it will come good, but for now it’s pretty screwy when using the other systems (G3000, GNS, Airbus/Boeing FMS, etc.) across aircraft.

[TheSevenflyer]

The FBWA320 experimental version is very good as well.

[MeridianSquid71]

All - FWIW - the AP issues have been present to some degree since Release almost 2 years now. They are better, but certainly not perfect by any means. Having a manual to learn FMC/FMS would be nice with some test route inputs to see what pilot/plane does.

One Youtuber said you can copy already completed flight plans into FMS or enter the same data at any rate and plane will do that route, and show the correct waypoints transitions etc., correctly. I have not tried it.

[NixonRedgrave]

Unfortunately, I’m trying to keep myself constrained to the Xbox — both for my own use and to help other Xbox users.

This excludes me from using mods.