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Guest valterg

FMC speed restriction

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Guest valterg

Hello,I realized that PMDG 747-400 have some problem to follow SID speed restriction introduced in FMC.I set the SID speed and altitude restrictions for two waypoints in FMC LEG page (in this case, the maximum speed in two waypoints just after departure is 190 kts).The bird, instead of follow this restriction rule, it went up directly to 250 kts, the 10000 ft general speed restriction.Interesting, checking the VNAV page (ACT ECON CLB), I noted that the speed restriction was set correctly in FMC (the 1R line inform correctly the speed restriction for the next waypoint, that is, 190 kts).The LNAV, VNAV and A/P was properly set just after the departure.A snapshot of the FMC VNAV page and cockpit it can be seen here: http://forums.avsim.net/user_files/143687.jpgThis picture shows the second waypoint, but the speed restriction for the first waypoint had the same problem: the 747 kept 250 instead of 190 kts.Any comment will be welcomeThanksValter - Brazil

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Guest valterg

any idea?

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Guest D17S

Me too. Tried to get the FMC to target speeds at the first several, consecutive waypoints during a TO / Climb sequence. Just to get some variables out of the way, I set acceleration and thrust reductions below the first speed/waypoint target. This would get Vnav off the initial Vspeed target and roll back thrust to a less aggressive

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Guest valterg

The departure and landing are the most critical phase of the flight. The SID plays a fundamental role for organization and safety of the traffic.FMC is the main tool for pilots to ensure that the airplane will follow these rules, granting the safety of the departure and air organization operations. Pilots and traffic controllers expect that the airplane will strictly follow the planned departure, in particular speeds and altitudes. All traffic operations depend of that. Ultimately, the FMC is responsible for the exactitude of this critical phase of the flight.Of course, pilots can handle that, but at cost of a great work overload, so confidence in FMC and autopilot system is fundamental in heavy airplanes to cockpit management.No doubts, PMDG 747-400 is a wonderful and well care project, and its FMC is one of the best that I have seen in a simulation. Of course, the complexity to develop this system may lead to some idiosyncrasies. Nothing more natural.PMDG 747-400 FMC has problems to handle speeds and altitude restriction in SID operations that, unfortunately, compromise the expectations of the system, at least for users who matter with details.Actually, this and others problems will only be noticed by the people who spend their time to use, test and push the program to the very details, and take care to report it to the developers to maximize it reliability and value.As we are doing now.I really expect that the people at PMDG take a look at this report, check it and, if it will be the case, try to correct.ThanksValter - Brazil

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Guest D17S

They've been leaving us to our own devices lately, but this time we need some help. Bet they're busy working that 744F . . . so they can get on with the important business of that MD11! Go get 'em guys, but maybe just a quick word? If it's a bug, still not too late to get it in the 744 PAX update/patch (hopeful nudge).

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Guest sputnik

The speed that you see in the screen shot is for cruise speed at altitude. The 318 knots is target indicated airspeed at that particular cruise altitude (you're actually going much faster). If you have the fmc properly programmed with proper weights etc and are using vnav the computer calculates the proper speed for to climb for a given gross weight/thrust configuration. The blank lines below are to enter speed restrictions at lower altitudes; 250 below 10000 for example. If the fmc is programmed correctly the aircraft performs by the numbers.Steve G.

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Guest sputnik

I think I may have misread the question...What does the legs page look like? Did you enter and execute each indivdual speed restriction for the SID (Bold speed and Alt.)?sg

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Guest valterg

Yes, all legs was properly configured and executed. One evidence that FMC was adequately adjusted is that in the ACT ECON CLB (see the picture), the 1R line shows the expected speed at CGO, one waypoint that have the SID speed restriction. As you can see, the actual speed is far above the determined, even before the waypoint.Sam, one that is discussing this thread, had the same problem.You can test it yourself, making a simple flight plan, and determining in two or three consecutive and near waypoints, just after the runway, some speed restrictions, below 250 kts. You must arm the LNAV and VNAV just before the takeoff and turn the A/P on just after 500 ft above the ground. You will note that the LNAV will work fine, but the speed will go straight to 250 kts (until the plane will stay below 10000 ft), ignoring all the speeds restriction placed in FMC.ThanksValter - Brazil

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Guest sputnik

What SID are you flying? It would be very unusual to still be at 190 kts at 10000 ft. On a normal departure you want to get the airplane cleaned up as soon as possible. I don't think I've seen speed restrictions below 250 knot in a SID. As a matter of fact most pilots would be asking atc if they can break the 250 kt restriction before 10000.sg

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Out of curiosity what is the SID in question?190 knots is mighty slow for anything more than a moderately loaded 747. You'd still have flaps out and such forth.Not saying you've got it wrong or anything, I'm just surprised to see a SID calling for a 747 to be maintinaing 190 knots climbing through FL100 or more.


Mark Adeane - NZWN
Boeing777_Banner_BetaTeam.jpg

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Guest D17S

These are just test flights. The 190kt setting was just a speed to keep it well above the yellow but below the red at 20 flaps. Again, this is not an operational scenario. We're in our XB-744 with our helmets on. Wouldn't it be fun to have an XB-744 skin of some creative modern, Evil Knievel sort? Maybe one of our artists . . .? That way you guys (in Vatsim)could see these test flights comming plummeting out of the sky trying to recover from a flat spin. Clear! Gotta love that reset button.

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Guest valterg

Let me explain that the 190 kts restriction in the actual SID is only for the first turn (not showing in the picture), just after the departure and before intercepting a determined radial from CGO.Since the airplane did not keep the restricted speed at this point (it went directly to 250 kts) just after the departure, I made some tests, "stretching" the restriction for two more waypoints, just to watch it behavior. I varied the target altitude for these waipoint just to check if keeping the same altitude between the waypoints (below 10000 ft), or allowing free climb (as showed in the picture), it would change something.I took the care to keep the 747 very light, without any payload and a minimum fuel aboard and flaps set accordly to minimize the impact of weight and atitude in this test.In busy airports, the rule is to speed up to the maximum possible the departures, but sometimes occur the opposite, especially when conflict occurs between landings and takes-off in very near airports that share the same airspace, as between SBSP and SBGR, in S

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Hey Valter,I just tried a short flight using three waypoints each set at about 15-20 miles apart.First waypoint restriction was 190/5000.Second waypoint was set at 220/10000.Final waypoint was 250/15000.Restrictions were set using LEGS page.VNAV set properly before takeoff but could not get LNAV to work until airborne.I have this from time to time for some reason.Ran Throttles up to 70% hit TO/GA and plane accelerated to 190 and then pitched up and climbed to 5000 and leveled off until reaching first waypoint.Plane then accelerated to 220 and pitched up and climbed to 10000.Same on the last waypoint.It did stay between 5 and 10 knots faster than what was programmed but I was over 650,000 so maybe that is why.Also I noticed you already have 350 set in MCP.Is this realistic?From the videos I've watched the crew usually sets and resets altitude as given by ATC until reaching desired level.I'll try another flight when I get a chance.Michael

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