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brivadois

Display of T/D point

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Probably, I made something wrong, but I don't understand: on the EFIS the T/D point has been reached, but not on the CDU. Everything works fine and I can descend without problem, but the EFIS display seems strange.

 

Thanks for your help.

 

I am french (nobody's perfect :blush: ) and newbie. I hope this post is correct.

 

François CHAUSSE

 

737td.jpg

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It seems to me like you forgot to change the Altitude window on the MCP.

I think the FMC will still keep displaying the T/D point, but the distance will become larger, instead of decreasing.

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Thanks for the fast reply, but (and maybe I'm wrong) I change the altitude in the MCP a little bit later when distance to T/D point in the CDU is 5 nm and everything works as usual dor the descent.

 

François

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Something's going on with your ND I think. I notice the following things:

 

The ND shows a turn, but without an intersection associated with it; that's very strange.

 

The ND is also showing you've passed the T/D, but the the FMC shows you've got 10nm to go. In addition, you're still in VNAV PTH. If you'd passed T/D, it would say VNAV ALT.

 

Does this keep happening? If so, perhaps some more screen shots, including the CRZ page, and maybe the ND set to 20nm range just to get a better overall picture.

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olli4740: yes, it seems very strange and that's why I created this topic ^_^ .

 

Flamin_Squirrel:I only noticed the fact that the T/D was passed in the ND's display and not in the CDU, but you are right: why is there a turn without intersection ?

As you suggested it, I'll send other screenshots.

 

Thanks for your help.

 

François

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Here are some new screenshots:

1-Before end of climb

 

tctd.jpg

2- 23 nm before the t/d point (with cdu display, but not  within the ND):

23beforetd.jpg

3-Ready for descent (according CDU), but I've passed the T/D point (ND Display):

descent.jpg

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olli4740: you are probably right, the flight takes 35 minutes, but there was no issue when I entered FL285 in the scratchpad for the cruise altitude.

 

François

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The FMC accepts the entry. But there should have been some FMC message 'Unable cruise alt' along the way up to cruise level.

 

Anyhoo, some rule of thumb would be to use the value of the flight distance (in nm) as a maximum FL: a 200 nm flight - no higher than FL200

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You won't get an unable cruise alt message because there IS a cruise section, albeit a tiny one!

 

That said, FL285 probably isn't a sensible choice for a flight that short, and might well be worth trying again with a lower one.

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Also note that the altitude in MCP shows 28500. Aircraft will not descend below this using A/P. If you reset the altitude after the calculated T/D point, you need to use Altitude Intervention button, or disengage/engage VNAV. 

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I think this is a small bug in how the T/D point is displayed on the ND.

 

You are turning sharply to the right, 120 deg or so. The FMC will calculate a standard rate turn, which means it needs to overshoot and intercept back on track, hence the "turn" without the waypoint.

 

I have noticed that when such an intercept point and the T/D (or T/C) point is on the same leg (ie between two waypoints), the T/D symbol "jumps" from its calculated position and sticks to the intercept point as seen in your pictures. It only applies to the generated image on the ND, as the path in the FMC remains correct. A bug, I think, but it rarely happens an isn't a problem as such.

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