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TOD ignored while in PROF

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Hi gents, I am searching for some info regarding the MD-11Xs logic when in CRZ, on PROF, approaching TOD.

 

I have seen the MD ignoring the T/D, thus not start descent when crossing T/D with lower altitude set in MCP with PROF active. Does this have something to do with not setting minimum for the approach?

 

Could it be that IF a RWY is selected in STAR before T/D, the minimum have to be set for it to start descending? Sounds a bit far fetched, but still it is the only explanation I have for what I have seen...

 

I've been reading the FCOM and FCTM but can't find anything about this issue.

 

Thanks,

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Page 27 of the Heathrow to Zurich tutorial,  "Descent" phase starts at the top of the page "on the FCP lower the altitude to 4000 ft", read the complete section for all the details.

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It's sad that I can not say exactly what holds your MD11 away from starting desc., but setting minimums (like DH) has nothing to do with this. If you see the arrow-marked T/D point, it has to start descending without specific minimums.  

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If you inadvertently select a higher altitude in the FCP than you wanted for cruise, easily done, the FMC accepts this as a new cruise altitude target.  No caution message is given and no confirmation is required. The FMA will however show the cruise level it is expecting to go to over the usual altitude mode data. The MD-11 won't climb above the FCP selected altitude but it will be in altitude hold mode, not profile cruise. So as you approach TOD the FMC is still expecting a further climb, rather than a descent. The FMA shows the FMC selected cruise level reducing towards the actual flight level as you approach TOD, because the distance left to climb before TOD is reducing. However the FMC remains in a climb mode and does not trigger the descent automatically as a result. Whether this is a programming bug or correct aircraft behavior isn't clear from aircraft manuals.

 

So if the FMA profile information after the last step climb is not all magenta you should check the CDU INIT page to see what the selected cruise level is, and correct it if necessary. If PROF and the cruise altitude are both magenta the AP will descend at TOD in profile mode, assuming you select a lower altitude of course.

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Hmm.... as long as PERF mode is active, it should descend AFAIK (assuming you selected a lower altitude in the MCP).

 

As Kevin says - check the INIT page. I have found it does odd things if the CRUISE altitude shows ---/---/---/---/---/---

 

If that is the case, just enter the CURRENT altitude for cruise, as it seems to kick the FMS into entering the correct phase.

 

Another thing to check is the PERF page. In cruise, it should say CRZ but sometimes it will be in CLB (it does this when it displays all dashes). This is also a source of CTD during go-around.

 

Best regards,

Robin.

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If you inadvertently select a higher altitude in the FCP than you wanted for cruise, easily done, the FMC accepts this as a new cruise altitude target.  No caution message is given and no confirmation is required.

I had this problem this weekend, and this was very helpful. Does anyone know: is this a quirk of the sim, or does the real bird do this?

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I had this problem this weekend, and this was very helpful. Does anyone know: is this a quirk of the sim, or does the real bird do this?

I can't see PMDG adding this feature without good reason. I certainly don't think they would have done it this way by mistake. So I suppose the real aircraft AP works like this, at least in the way it updates cruise altitude. Its only a problem in the sim, especially the VC, because of the way the FCP clickspots have been designed.

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Hmm.... as long as PERF mode is active, it should descend AFAIK (assuming you selected a lower altitude in the MCP).



As Kevin says - check the INIT page. I have found it does odd things if the CRUISE altitude shows ---/---/---/---/---/---

It could also read

 

XXX/---/---/---/---/--- where XXX is the higher flight level you dialled in by mistake.

 

If PROF and the cruise altitude are both magenta on the FMA then it will descend at TOD when you select a lower altitude. If the altitude is white then check the INIT page and correct cruise altitude as necessary..

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Thanks Robin and the rest of you for pitching in.

 

After watching the Martinair MD-11 DVD it is apparent that they check every action that changes something in the FMA for a good reason. When some of the altitude info is presented in white you should be aware that it will not follow the vertical profile.

 

Cheers,

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On any aircraft like this, the FMA should be the first point of reference. In the early days, failure to check the FMA caught a lot of crews out.

 

Best regards,

Robin.

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