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michal

VNAV with a non precision approach (newbie FMC help!!!)

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Guest dhtmlman

I cannot get the FMC to correctly descent at a constant rate from the final approach fix to the end of the runway at the desired 3 degree angle. What always happens in my non ils approaches is the aircraft descends very low first then gives me a gradual descent to the runway threshold. I am too low too early ... Most approach charts give you the angle from the last fix to descent at. How do you make the FMC perform this.. How do you program descent angles??How do you make the aircraft descend at the 3 degree angle using VNAV so you get a constant rate??

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What aircraft, what FMC? That may affect what and how you can program the FMC.To be honest I find it easier to use the PAPI/VASI approach lights to keep (more or less) on the glide path.IIRC the formula for descent rate is a half of your speed (strictly ground speed). So if on approach at 140kts your vertical speed should be 700ft/min.

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Guest dhtmlman

I have tried using LDS 767 FMC and PMDG 737NG FMC

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Guest Adverse Yawn

A non-precision instrument approach doesn't use a 3 deg descent. Once you are cleared or able to do so you descend to the MDH/MDA (Min Descent Alt/Height) as quickly as is safe and practical to do so, then you maintain MDA/MDA. The point is you need to get the min height quickly to give yourself the most chance of making visual contact with the terrain. Once vis is established you may descend below MDA/MDH. The descent line on the chart merely indicates your minimum allowable alt at set distances from the threshold. Once you are past the FAF you can descend to MDA/MDH as soon and asquickly as you like. The chart will however show a continuation of the gradient, but you will see that it can be as steep as 5-6 degrees.My quess is that in a real life heavy, this will never be flown with VNAV/VS/ALT. It will be hand flown with auto thrust engaged. Any other technique would require too much fiddling about.

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You may get a more realistic answer from the LDS and PMDG forums, however FWIW here are two methods I would use.If you have the approach chart it will give you a profile of the approach. e.g. at 9nm out 3,000ft, at 6nm 2,000ft and so on.If the approach has a VOR or NDB associated with it then you can create waypoints in the LEGS page of the FMC to fly that profile probably down to 500ft and then I would take over for hand flying.Let's say the approach is an NDB one and the NDB ident is XYZ, it is in line with the runway and the magnetic heading 135.In the legs page type in the scratchpad XYZ, you may get a list of XYZ idents, pick the correct one either by its LAT/LON or its frequency.You then need the reciprocal of the heading which is 315 (if less than 180 add 200 and take off 20. If above 180 subtract 200 and add 20).Then add the furthest distance, in the above case 9nm.So your scratchpad would be XYZ315/9. Enter this into the legs page, before the runway entry and in the right hand column enter 160/3,000. This will tell the FMC to be at 3,000ft and 160kts at that waypoint.Do the 6nm waypoint exactly the same, and so on.The above will work with a VOR approach as well.If the approach is a stepped one, as you get with hilly country, just use the stepped profile as given on the chart.With any non-precision approach the final landing phase is up to you, including the flare, as the autopilot cannot do an auto-land.You can of course use the a/t to control your speed and the FMC will calculate your speed and flap settings for you.If you are new to these aircraft, or FS itself, I suggest you fly some precision approaches with full auto-land on and watch how the plane handles it. Then go and practice some manual landings, using the ILS bars in the PFD as a guide.HTH

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Most IAPs have a descent angle associated with them (generally in the ballpark of 3


John Morgan

 

"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

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I pretty much agree with everything Adverse Yawn wrote. Such approach would be hand flown. It is true that NOAA charts show you a semi-glide slope arrangment however Jeppesen charts for the same approach would show you a step-down approach and this is how such approaches are flown routinely. It is only with the latest RNAV type approaches that you get to fly in a ILS-like fashion. To do this in GA world you would need to have something like GNS480 aboard, for a big airliner you need special software for FMS. Alaska Airlines and New Zealand Air are certified to fly such highly unique approaches to some hard to get to airports between the mountains. But the approach you are showing above is an old VOR non-precision aproach and I don't think a typical airliner would attempt to fly it automatically by programming FMC.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.hifisim.com/images/asv_beta_member.jpg


Michael J.

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Guest Darren Howie

These days 3 degree aproaches are pretty much flown everywhere.In the Bus its quite easy as flight path angle is selectable so you fly to the commencement point and descend on the 3 degree path.If not visual at them minima go around is executed.Why?Stabilised approaches are pretty much industry standard these days.Flying to the minima early sounds good but when you get there you will be way low on slope.Autothrust then has to spool way up to maintain level flight.As you approach the 3 deg path over you go autothrust spools down etc etc.As you can see a steady 3 degree approach is far more stable amd far safer.In the chart above you would program the aircraft to arrive over the VOR at Vref +10 at 4500'.Roll her over into a 3 degree descent and monitor your profile on the way down.The Airbus flight path angle is accurate to about 0.2 of a degree.At the minima if not visual go around.There is no point flying level at the minima to the mapt as the minute you level off you are by definition above slope and unstable.Airbus is approved for vnav on no precision approaches in many countries around the world but not where i fly so we use the trk/fpa function.We can use nav for tracking but not vnav so its flight path angle all the way for us.Works nicely.I know the NG has been approved for RNAV approaches out here and can be flown in LNAV/VNAV and it does a great job.Agaian its programmed to fly a 3 degree path as outlined above.DArren

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Darren is correct on this one Michael. The 'dive and drive' technique is not really promoted anymore on regular airlines (in the US that is, not sure about the rest). It has been deemed a bit more safe and allows for more monitoring vs. adjusting during the final approach inside the marker. In the GA world (when I got certified) it was a dive and drive, but with newer GA equipment popping up, this may be changing.I just read a great article on this, I will see if I can dig up the URL (about a month ago or so) :-)

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Guest dhtmlman

Thanks guy for your help it sounds like i have to use the Flight Path Angle then.Is this function supported in the PMDG 737 or LDS 767?? I didn't see it

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No, you can only program the approach using the published waypoints of this approach with their restriction altitudes and then use VNAV. If you go to the PMDG forum there was once a long post there how to use FMC to program such non-precision approach.Michael J.


Michael J.

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It does depend on what FMS you are using, the CRJ-200 uses the Collins FMS-4200 for the FMS.However our airline is not authorized to use the FMS for approaches. We'll still have the ILS/VOR/NDB approach set up in the FMS, but we'll switch to green needles (raw data) in the case of ILS and VOR approaches, or heading mode while staying in white needles (FMS data) for NDB approaches.The autopilot will track the navaid for lateral guidance. For non-precision approaches with step downs, we will set the step down altitude in the altitude preselect and switch to V/S mode and command 1500ft/min down at .2nm before the step down. After we get ALT CAPS then we'll set the next step down and so on until we get to MDA.

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I think the best way to "let the computer do it" is to use a runway extension point, and set a computed hard altitute for it. That way on descent, the FMS will compute to the extension point, not the runway.Here is an example using the LDS767 FMS:Enter your route. In this case, there are no published procedures (in the FMS database anyway) for PHTO. I line select runway 26. This gives the option to set a runway extension point. I set it to 12.0 nm.http://forums.avsim.net/user_files/131739.jpgI switch to the LEGS page, and line select a speed/alt of 180/2500(about 2500ft AFE -- you will have to compute this but this is a good swag) and activate my route. Plan view shows my lateral route.http://forums.avsim.net/user_files/131740.jpgOnce I'm enroute at my cruise alt, I can bring up the VNAV descent page and double check that my extension point is being used for vertical calculations.http://forums.avsim.net/user_files/131741.jpgHere I am about to make the turn to the approach course still in LNAV, and the FMC has switched to the runway alt as the VNAV vertical point. I can use the ILS here as a cross check, and it seems like my 2500ft is OK.http://forums.avsim.net/user_files/131742.jpgI hand fly it in using the FD. Note that this addon airport runway seems a little offset from what's in the FMS database.http://forums.avsim.net/user_files/131743.jpgThis may not be the way it's done "IRL", but it works.scott s..

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