Dear all,

 

I have a question regarding the vertical logic of the PROF descent. I was flying a fully automated descent from FL370 to ILS14 of LSZH via BLM2G STAR and GIPOL Transition. The STAR requires the following descent: BLM at or below FL200, ZH677 at or above FL120, GIPOL at or above 7000 and then to the IAF of RW14 at 4000.

 

When starting the descent, PROF logic put a HOLD 20000 constraint in order to honour BLM. The 20000 was reached well ahead of BLM. After Passing BLM at 20000, the descent continued to 12000, however, due to the PROF 20000 constraint at BLM, the 12000 at ZH677 could not be reached as well as the altitudes of the following waypoints, ending up in a situation that the MD11 was to high at the IAF and the GS couldn't be captured.

 

If PROF would have immediately continued the descent after reaching 20000, BLM would have been passed at a much lower altitude (without even violating the -FL200 requirement at BLM), BLM would have been passes at an altitude of approx 13500 and the altitudes of all subsequent waypoints/ IAF could have been reached as per the STAR and the GS of ILS14 could have been captured as per the charts (I tried this by manually manipulating the altitudes at BLM in the FMC during flying the approach).

 

Question: Why PROF interpretes -FL200 in the FMC to cross BLM at 20000 and not to cross below in order to reach the other waypoints properly? In this case, would it be better to enter the altitude of BLM with 12000 (just like ZH677, doesn't hurt the at or below requirement of BLM), to be on the safe side, taking into consideration that the descent will commence a little bit earlier?

 

Thanks,

 

Ulrich

Hi Ulrich,

 

In theory the PMDG MD-11 should have done this approach right and normally wouldn't even have hold at 20000ft as it normally would just be at 13500ft or whatever it is.

The problem which occurred seems to me, as far as I can tell, either to be a failure in the Navdata or you didn't set the right altitude on the FCP.

Example: If you set Fl 210 into the FCP the MD-11 will descend down to Fl 210 from Fl 370 and then stop at Fl 210 and ignore all SID restrictions until you don't lower the value in the FCP.

Could this be the cause of the problem? That you forgot to lower the altitude in the FCP? If not please fly the route again and provide a screenshot once the plane stops descending at Fl 200.

 

Best regards,

 

Jonathan

Jonathan,

 

the altitude at the FCP was set to 4000 to capture the GS. The altitude constraint at BLM was entered as '-FL200' at F-PLN.

 

Will post a screenshot as soon as possible.

 

Ulrich

ignore all SID restrictions until you don't lower the value in the FCP.

 

That should be "ignore all STAR restrictions until you lower the value in the FCP."

 

BLM would have been passed at a much lower altitude (without even violating the -FL200 requirement at BLM)

 

Was the altitude restriction in the FMS set to "SSS/FL200" or SSS/FL200B"? If it was the former, I think it would hold at FL200. If it was the latter and the MCP was set to 4000, I think she should continue the descent to FL120 or above.

 

In this case, would it be better to enter the altitude of BLM with 12000 (just like ZH677, doesn't hurt the at or below requirement of BLM), to be on the safe side, taking into consideration that the descent will commence a little bit earlier?

 

I don't think that would be a problem as long as you remain below FL200 and above FL120.

I had the same when i did the tutorial flight. The MCP Alt was set well below 20,000 and the FMC restriction was SSS/-FL200 (the minus sign meaning below FL200. I found myself levelling off at FL200 too.

 

Peter

I just tried it and the FMC showed -FL200 for BLM but I levelled off at FL200 a fair distance before it. I had the altitude in the MCP set for 4000. By the time I hit BLM at FL200 the PERF was showing 6300HI Path Error and then when I began the descent it descended at 300fpm to ZH677, by which time the HI Path Error was up to 7400. Ulrich is right, the FMC is reading -FL200 as a target altitude regardless of the altitude restrictions that come later.

Two possibilities:

1) speed is under FMS control (magenta cues above speed tape) but in "pitch" mode. The airplane may be descending shallow or not at all in order to reduce speed via pitch. If so you can add spoilers to help it slow down and continue descending.

 

2) speed is under direct control via the Speed knob (white speed target above the airspeed tape) and is using "pitch mode" to reach this speed override target. You could use spoilers here as well to respect your override, or push the FMS button under the speed knob to restore speed control to the FMSs.

 

Or it could be something different. Will know more with a screen shot.

 

Clark

Peter/ Fatback

 

did you experience similar behaviour with altitude restrictions during descent at other flights?

 

Clark,

 

I will post a screenshot asap, however, all settings were as per the tutorial and the HOLD 20000 was in magenta colour, so the FMC commanded it right from commencement of the descent from 37000 as a part of the vertical profile.

 

Ulrich

Clark, mine had HOLD 20000 in Magenta too even though I had the MCP set for 4000. I didn't get any Add Drag messages until much later in the descent when it was too late to correct the high approach. The speed was under FMS control but sorry, didn't notice the mode.

 

This is the first time I've experienced it and only tried it because of Ulrich's post. I thought he must have been doing something wrong so I thought I'd check it out. Sorry Ulrich...

In my experience it's unusual but not rare to have to fine tune parts of the descent in both the MD11 and the NGX, but the NGX seems a bit smarter than the MD11 in warning you if the a/c can't comply with the STAR altitude restrictions as written. Anyway, flying into Zurich you'd be following ATC directives, so the discussion here is only related to whether the MD11 can perfectly handle the descent as written. And remember it's written in 2 places: the chart and the Navigraph data, and these don't always entirely agree.

 

I'm looking at the Jeppesen chart for BLM2G dated 02Oct2011. The Descent Planning advisory on the chart reads:

 

"Expect to cross 9 nm to BLM at or above FL200, BLM between FL200 and FL150, ZH677 at or above FL120."

 

The chart then directs GIPOL at or above 7000'.

 

In the FMS enter: BLM/-9 above BLM on the F-Plan page. Set the altitude for this point to FL200. Next, click on the speed/altitude designation on the BLM line and clear the altitude restriction from the BLM page. This latter step seems logical as the a/c will begin descending from FL200 after the 9 DME point. When I did these two things the a/c was at FL200 @ 9 DME from BLM, and after this the a/c handled the descent perfectly, putting it at approximately 17000 at BLM, and hitting all of the other altitude targets on the way to the rwy 16 G/S. Depending on winds, LW, etc. you may need to apply spoilers a couple of times, but the a/c manages speed and descent with very little assistance all the way down to short final.

 

I didn't have time to try this but you may also want to simply enter the 9 DME restriction (leaving the BLM waypoint page unchanged) and see if that then prompts the a/c to begin a proper descent after that point.

 

I guess the lesson here is that you can't always expect that the FMC will handle the departures and approaches exactly as written. You have to know the capabilities of your a/c, review the specifics of the approach and plan accordingly.

I guess the old saying "the pilot is responsible" holds well here !!

Thanks Jev. I guess that is why they pay us big bucks to sit in the front seat...

I guess that is why they pay us big bucks to sit in the front seat...

 

I've always thought that PMDG (with the realism of their products) should pay us widebody/narrowbody captain pay.

During other flights with STARs, I noticed that the FMC seems always to interpret an 'at or below FLxxx' restriction as 'at FLxxx', because PROF always puts 'HOLD FLxxx' to honour the particular restriction during the descent.

 

Could someone from PMDG comment on this?

 

Thanks,

 

Ulrich

Guess I'll chime in that I had the exact same problem with this tutorial when going through it for the first time today. I do wonder if it's because of a difference in the navdata. I'm using the latest Navigraph airac, whereas maybe the STAR was different or didn't contain that 20000 restriction when this tutorial was written.

 

Michael H a tu t ian