This is probably normal behaviour, but I'd appreciate it if you'd help me understand what's happening here.Tutorial Heathrow to Zurich:You can see from the MCDU that I've been descending from FL370 to arrive at BLM at or below FL200. (If BLM isn't legible, it's the waypoint next to LFSB.) The magenta spot has been centred on the profile indicator during the descent. What I don't understand is why now, 20 miles before BLM , the aircraft levels out at FL200, and the magenta dot sinks showing I'm above profile. Using SLEW, I found the profile passes BLM at around 14500'. So why does the aircraft not follow the profile?Thanks for your helpGeoff Kent

First, you're above profile. Below would have the magenta diamond at the top of that scale.Second, note your FMA (flight mode annunciator). It's on top of your artificial horizon (the blue/brown circle). If you look on the far right, it says PROF TO 4000 HOLD 20000, so you must not have rolled your altitude selector on your MCP (mode control panel - the AP controls) down far enough.

Thanks for that Kyle. Of course, you're right - I'm above profile. I don't know why I wrote 'below'.But on your second point, PROF TO 4000 is my manual altitude setting on the FCP.HOLD 20000, as you know, is the next limit given to the autopilot, and I can only assume it comesfrom the FMC.Because the MCDU shows the limit at BLM to be 'at or below FL200', I would expectthe FMC to allow me to follow the profile and be at 14,500' by BLM. So I'm still confused.CheersGeoff Kent

No worries on the terms, I just wanted to make sure you know that's what it meant. Chalk it up to not having coffee yet, since you did know it.You're correct in that the restriction comes from the A/P, but where it comes from depends. You can see the -FL200, which means that the FMC knows that you should be below FL200 by BLM. The profile's magenta indicator on the ND also shows that you're high. This means that the A/P is actually getting PROF info that would have it descending right now, if something else wasn't intervening. I haven't flown the MD-11 a lot recently, so my memory is fuzzy, but...I think the magenta means whatever altitudes you set in the MCP match the altitudes of the profile. So, are you sure you rolled the MCP altitude below FL200 at this point? It would show up in white if you had set an altitude in there that wasn't part of the profile, though (if I'm remembering correctly). Somewhere, somehow, something is soft-overriding the PROF mode. Having a pic of the MCP would really clear that up for me.

I definitely set the FCP altitude to 4000 during the descent from FL370. I remember seeing the PROF TO value on the FDA change as I wound the altitude down. It's colour was white most of the time, but changed to magenta each time it corresponded to any limit set in the FMC; that is at 20000, 12000, and 4000. Here's another screen shot.CheersGeoff Kent

I seem to recall some mention of problems with consecutive soft constraints with one of the payware add-ons out there. I doubt it's the MD-11, but maybe someone will be able to chime in on the matter.I'll run the tutorial when I get home to see what happens to me and see if I can think of any more ways the PROF would be overridden.

I don't know what you mean by SLEW, so I may be off track here. Here's my interpretation: your aircraft is correctly descending along a profile that will result in touchdown at your destination. Left to itself, the FMS would stay high and efficient as long as possible then begin an idle thrust descent somewhere close to the destination to glide down and attain zero altitude at the intended airport. Two things alter this situation: BLM says "no, you need to descend to FL200 right now", so the aircraft descends to FL200 but it stays there now to maximize fuel efficiency until it can rejoin the intended glide path again because it descended early and is below that slope. This is according to the plan you put into the FMS, so that's why the deviator is still centered. I'm guessing the aircraft will begin descending again once it is in idle/glide range of the airport. Look for another "top of descent" marker further along your flight plan.The other constraint on this "ideal glide plan" is your FCP altitude which is a "temporary lower limit" that the FMS will always respect. Its profile might want to continue gliding down to touchdown, but you use your FCP to tell it "No, ATC has not cleared us lower than 4000' yet". During descent phase, the FCP does not command a descent; quite the opposite, it stops any descent at a "floor". The FMS will descend when its ready per its profile and as long as it stays above your FCP floor.Does this sound like what may be occurring?

Edited February 10, 201214 yr by clarkTX

Based on the pictures, no.

Well, then maybe the "-FL" waypoint option isn't modelled correctly.

Another idea. Double check your PERF page to make sure you are in descent mode. Perhaps you never reached cruise and are still in climb mode.Clark

Hi Clarke, thanks for your help here.

I don't know what you mean by SLEW, so I may be off track here.

I just meant I hit the 'Y' key to freeze the simulation and displace the aircraft manually to see at which altitude the magenta profile spot would rise to the centre of the line.

Here's my interpretation: your aircraft is correctly descending along a profile that will result in touchdown at your destination. Left to itself, the FMS would stay high and efficient as long as possible then begin an idle thrust descent somewhere close to the destination to glide down and attain zero altitude at the intended airport. Two things alter this situation: BLM says "no, you need to descend to FL200 right now", so the aircraft descends to FL200 but it stays there now to maximize fuel efficiency until it can rejoin the intended glide path again because it descended early and is below that slope.

Check back to my original post, Clarke, and you'll see the profile passes BLM at around 14500', and not above 20,000 as you're suggesting.

This is according to the plan you put into the FMS, so that's why the deviator is still centered. I'm guessing the aircraft will begin descending again once it is in idle/glide range of the airport. Look for another "top of descent" marker further along your flight plan.

Yes, but Clarke, the aircraft shouldn't be held at 20,000' in the first place to 'begin descending again once...". The profile is well below the aircraft at BLM. The question is, why is the aircraft being held at 20,000' when the BLM altitude restriction is 'at or below' 20,000'?

The other constraint on this "ideal glide plan" is your FCP altitude which is a "temporary lower limit" that the FMS will always respect. Its profile might want to continue gliding down to touchdown, but you use your FCP to tell it "No, ATC has not cleared us lower than 4000' yet". During descent phase, the FCP does not command a descent; quite the opposite, it stops any descent at a "floor". The FMS will descend when its ready per its profile and as long as it stays above your FCP floor.

I understand, but the aircraft was held at 20,000 for so long approaching BLM, that it had no chance of losing enough altitude to land.If the 'at or below' restriction always locks the altitude AT the given value, then no one would have been able to complete the Heathrow to Zurich tutorial. So it must be a problem with my system. Anyone want to volunteer to see what happens to them at BLM?Here's the PERF page, by the way...CheersGeoff Kent

Edited February 11, 201214 yr by Papsonline

Hi GeoffI'm stumped. I don't bother with the STAR stuff much since I usually run ATC and they go by vector approached. Unfortunately I'm pretty busy the next few days but later this week I'll see if I can help repro it too. My wife is bogart'ing my PC with her Skyrim game, instead of letting me run important software like FSX. :)Clark

Thanks Clarke.But Skyrim is just a game, right? So why...?Ok, never mind. :Talk to the Hand:Geoff Kent

Not sure if this is similar, but i have been having problems with vertical restrictions "at or below" for example, last night i flew to YSSY and the star called for at or below 9000 at Boree, next waypoint after that was Ovils and i had to be around 4500 at Ovils to intercept the ils at 3000ftProblem is, between Boree and Ovils threre is only about 5nm and the autopilot aimed to be AT 9000ft at Boree and had no hope of losing 4500ft in 5 nm.The vert revision page was correct, it specifically said "at or below 9000" but it still aimed to only be AT.So this is not a navigraph issue, the data is correct, is there a technique i am missing???

It's the same issue Will. I'm still hoping someone will pick this up and suggest a solution.CheersGeoff Kent