Glidepath Angle on multiple segments from database but only one to E/D

[rayngwt 0]

Hi, I was flying a multiple segment final approach to VTSP Vordme Rwy 27, and I can see in the database in access that there are 2 segments prior to the MAP that have the GPA vertical angle. However, when the procedure is loaded, only one GPA to the E/D at the MAP is shown. When on VNAV PATH, the acft behaved as it should with only one GPA in that it only observes the last segment to the E/D. Naturally I was flying a constant descent procedure and I think the FMS should accept multiple GPAs. I did not meddle with the procedure after loading it. Any clues? ... thanksRaymond

[Spin737 102]

The path is going to be only one descent from the FAF (or TOD, in theory). That's the point of the VNAV approach - one smooth controlled descent. If you want to do stepdowns, do it in V/S.

[rayngwt 0]

Hi Matt, Thanks, but i think that is possible, as in Arinc 424, the guidance for the FMS providers is to code the database in such a way. There are procedures where there are multiple GPAs prior to the FAF. And even after that, it should allow that too, of course they should be the same angle though.

[Maximilian_Groeber 0]

Hmm, I don't know if this is exactly the point you are aiming at, but, the VNAV approach profiles usually contain altitude constraints "at or above" until you reach the IAF or the FAF. In this case, the aircraft will continue on a constant descent, that's true. You can alter the altitude restraints to "at" at certain points, at least at the start and the end of your desired intermediate descent segment. Then the aircraft will descend according the entered altitudes (it even will level off in VNAV if programmed so). Just have a look at the FMC altitude constraints and make sure there are no unnecessary "A" or "B" letters. In the VSD you will notice a hour glass symbol (two arrows above each other) as well. Hope that is what you'd like to know... Maximilian Gröber

[Spin737 102]

Hi Matt, Thanks, but i think that is possible, as in Arinc 424, the guidance for the FMS providers is to code the database in such a way. There are procedures where there are multiple GPAs prior to the FAF. And even after that, it should allow that too, of course they should be the same angle though.

What is your goal or application?

[rayngwt 0]

Thanks for the inputs.There are approaches in mountainous areas where there is a gradient leg to the FAF then another gradient down to the E/D (TCH or MAP). The database is coded as such but am not sure if the real airplane can perform it in reality and also shown on the CDu as such. I don't have a pic here but if you have access to VLLB (LAos) new procedure in Nov for reference. In that app, its a 3deg down to the FAF then 3.46 down to the E/D. Not having a GPA will result in the airplane possibly going down below the glidepath although Vnav path is FMAed, however the airplane will still observe the Alt at the FAF but it already has violated the required gradient. Tks.

[Paul_Smith 92]

Thanks for the inputs.There are approaches in mountainous areas where there is a gradient leg to the FAF then another gradient down to the E/D (TCH or MAP). The database is coded as such but am not sure if the real airplane can perform it in reality and also shown on the CDu as such. I don't have a pic here but if you have access to VLLB (LAos) new procedure in Nov for reference. In that app, its a 3deg down to the FAF then 3.46 down to the E/D. Not having a GPA will result in the airplane possibly going down below the glidepath although Vnav path is FMAed, however the airplane will still observe the Alt at the FAF but it already has violated the required gradient. Tks.

If you want multiple gradients, then set constraints on each leg. But this is not a constant descent approach so I don't see what the problem is?

[rayngwt 0]

I guess when on Vnav pth without an angle, the flight path may actually descend below the stipulated 3deg prior to the FAF. I know that is not a CDFA as the FAF hasn't been reached, however that does not comply to the approach requirements as there may be obstacles below that an OIS (obstacle identification surface) was intended in the design. Basically, the main question was ... can the CDU have multiple angles after the FAF or before, as coded in the databse as shown below.When the procedure was loaded, only the last angle to the MAP was shown, that means the acft may Vnav pth (idle thrust descent) and not keep to the angle, which means it did not clear the obstacle protection surface. TrackCode, Waypoints, Country, Navaids.Ident, TurnDir, Course, Distance, Alt, VnavIF DME12 VT PUT 002500 TF DME07 VT PUT 001900 TF 43DME VT PUT 01047A 2.98 TF 24DME VT PUT MAP 2.98

[Paul_Smith 92]

You can descend at greater then 3degrees when you are above the ideal slope. This is normal if you step or level out to control speed etc. It does not breach requirements in any way.

[rayngwt 0]

Yeah, I know, I hope I don't go below .. say I hit the altitude at all wpts prior and am not 'on top'.Looks like I keep confusing myself with Feelthere wilco data. Guess PMDG data does not contain GPA as per 'database' yet. The last wpt either Rwy or MAP always has a system-calculated angle to it, it does not come from the 'database' and as per mentioned in the FCOM manual, it keeps to the angle till the E/D. Thanks and sorry for confusing you guys. Great learning for me.

[Maximilian_Groeber 0]

Ahh, now I guess what you're aiming at... You have certain altitude restrictions at certain waypoints which you would manually "connect by drawing a straight line" and you don't want the aircraft descend below that imaginary line, is that correct?Well, the aircraft exactly does this as long as you don't enter "above" or "below" constraints. You would enter the final GPA on the approach reference page. This is what you can use for an IAN approach, and the aircraft will try to descend down this path as long as you do not specify something different. As soon as you enter altitude constraints, they have priority over the final GPA. You can't specify multiple angles, as far as I know but you have to use altitude constraints. However, charts always outline the altitude restrictions as well so that should make things not too complicate. A little example I just flew 30 minutes ago :-) The VOR/DME approach RWY 05 into Christchurch. If you'd like to have a chart, look here. At the last waypoint, you end up about 2.250ft above a standard 3° GP. I entered all the constraints into my CDU and the aircraft just flew like drawed on the approach plate profile, completely ignoring any GPA. Of course, you then have to make sure that you reach the runway without a parachute... For example you may fly a circle at NZQN just in front of the runway. The same thing works for approaches with different descent angles, for example VNKT. I tried the VOR/DME approach RWY 02. There you have a nice mix between various descend angles. The aircraft may fly a bit too high, especially if you enter the constraints "at or above", but it never descends below that imaginary profile line that connects the minimum waypoint altitudes. Just remember: The GPA is important for an IAN approach and if you want the FMC to auto-calculate the altitude restrictions for the IAF/FAF. But if you have constraints on that, they have priority.And if you'd like to hit the specified altitude prior to any waypoint, you have to select FLCH or V/S and use the autopilot "manually". Hope that helps... Maximilian Gröber