[MD11] RNAV APP - JFK RW31R

[3-2-1-Now 126]

Hi,I tried an RNAV approach to JFK RW31R last night, and all was going well, until I set minimum PROF descent altitude on the approach.I'd selected the approach (RNAV RW31R), the STAR (CAMRN4), and entered the minimum profile descent altitude as per the current charts. Everything looked good.I descended to 3000 ft in PROF mode. This was fine. Levelled off, and was waiting to turn finals. I selected 400 ft in the altitude window.As the aircraft turned finals, I hit PROF.On the FMA I had PROF TO 400/PITCH 1900.It got to the next waypoint at 1900 ft (correctly) then just levelled off. Shouldn't it have just kept descending all the way to the next waypoint (runway) levelling off at 400 ft simply because this is what was set in the altitude window?I had to go around the first time, and select ILS the second.This is the first time I've ever flown RNAV, so there is possibly something awry in my technique.Can someone confirm if it is required to press PROF after it has levelled off due to MCP altitude constraint or not?Best regards,Robin.

[Guest henry_chinaski]

To what was your MIN PROF altitude set in the FMS?

[downscc 1,327]

Did you set the altitude dial lower than 1900? She should continue the descent in PROF mode unless the altitude setting prevents it, which the pilots will set per ATC clearance. This prevents the plane from deviating from clearance.

[3-2-1-Now 126]

Yes.When selecting the arrival, I set the minimum profile descent altitude as 420 (as per the chart).Altitude was set to 400 ft (below the 420 ft altitude set in the FMS; I was anticipating taking manual control).I was at 3000 ft. I set 400, pressed PROF, FMA displayed as per my OP, and it did descend as anticipated to 1900 ft for ZULAB, then it levelled off and I couldn't see why.It was nicely aligned etc.. and going well until it did that. I reset the altitude to GA altitude and hit the TOGA button.http://dtpp.myairplane.com/pdfs/00610RY31R.PDF

To what was your MIN PROF altitude set in the FMS?

420 ft, as per the above chart.Let me get some screenshots of the FMS, etc..Best regards,Robin.

[3-2-1-Now 126]

Hi,I re-flew it - seems I wasn't waiting long enough for it to catch up with the vertical profile.I flew towards CATOD at 3000 ft, initially in heading/altitude hold mode.As I got to within 5 nm of CATOD, I selected the MDA in the AFS, and selected NAV, FMS SPD, PROF.It captured nicely, and flew towards CATOD maintaining 3000 ft.It flew *slightly past CATOD*, still at 3000 ft. FMA at this point was reading PROF TO 400 / HOLD 3000.I monitored the vertical deviation, and I was slightly below. About 1.5 nm past CATOD it captured the vertical profile, and began its descent to ZULAB. FMA now read PROF TO 400 / PROF 1900Speed was stable and the aircraft was configured for landing.What happened next is what was catching me out. It got to ZULAB, and *levelled off* at 1900 ft. It then flew past ZULAB, level, for what seemed like forever. I monitored the vertical deviation; eventually, it captured and began the descent to 400 ft.I let it continue down to 400 ft on its own, but it was apparent that it was now too high.A few questions come from this:1) Why did it level off and not continue the approach in a continuous descent?2) Why was it high?It seemed it was targeting the center of the runway, and not the threshold.I know a RNAV procedure is not a precision approach, but I would not expect it to be high any more than if I was flying say a VOR or NDB approach.To descend 407 ft over 1.6 nm it should require an approach angle of 2.4 degrees. The only thing it looks like is that the runway was too close to the waypoint.I'll get a screenshot.Note that I have the trial version of FS Dream Team JFK installed.http://i873.photobucket.com/albums/ab300/3...pg?t=1253725340Best regards,Robin.

[downscc 1,327]

Why did it level off and not continue the approach in a continuous descent?

This is a long time complaint of mine too, the sidstar syntax we are stuck with has no way of conveying that an altitude constraint is a 'descend through' rather than a 'level at.' For this reason and a few others, I always fly non-precision approaches without PROF or VNAV preferring instead to use SPD and V/S control over the autopilot while using the vertical deviation information as a guide. There is no practical way to use the old sidstar language in today's advanced RNP environment.

[DLP084 5]

Hi, this is one of the main reasons why we need a new navdata format for the NG 2.0. The best way would be some sort of ARINC 424 compatibility. If it is possible the 747 / MD-11 should get upgraded to the new navdata format. Jan-Paul

[3-2-1-Now 126]

I noticed it was a hard altitude constraint in the FMS - maybe if that was removed it might behave itself?Best regards,Robin.

[downscc 1,327]

You want the constraint at the FAF, it and the runway elevation set the vertical path. The problem is there is only one kind of hard constraint.

[3-2-1-Now 126]

Hi,I did some more tests, and found the following:If I remove the FAF, the descent is closer to what I would have anticipated.Regardless of whether there is an altitude constraint or not, it seems to want to level off at the waypoint, even if there is no reason to do so. I have found the AP must go into HOLD XXXX ft before it will continue the descent.Between waypoints, the vertical deviation is such that you end up with level segments between waypoint. It is like the vertical path is computed such that there are steps where the waypoints are.I re-programmed the arrival with a slightly modified offset for the runway, but this made no discernible difference.The aircraft is anywhere between 400 ft and 150 ft too high on the approach.RW31R ----- FAF ----- IFThe altitude of the FAF is computed such that it intersects the path between the IF and the RW threshold. I'm fairly sure it is aiming at the threshold and not the TDZ in order to allow for level off for the flare. If it was aiming at the TDZ I'd expect it to land long. The charts also suggest this. This also makes the sim incorrect in its behavior.Other than this is seems to be working as expected, including AP disconnect at the PROF MDA.Best regards,Robin.