FMC T/D problems...

[Bluedleta737]

So I have a flight planned from KPDX-KABQ. everything is good except for the T/D mark which is shown way too late in my route. It wants me to start descent like 20nm from the airport and I'm cruising at FL350. I don't know what is up with that because at some airports the T/D mark is fine...

[Litjan]

Hi Bluedleta737, the VNAV descent planning can get pretty wonky - please consider it a WIP. We are going to improve on it. For now it works ok on simple routings (long enough, no weird restrictions during descent), but can be off for other cases. Just like in the real airplane, take everything the computer hands you with a grain (or tablespoon) of salt.

Cheers, Jan

 

[Bluedleta737]

13 minutes ago, Litjan said:

Hi Bluedleta737, the VNAV descent planning can get pretty wonky - please consider it a WIP. We are going to improve on it. For now it works ok on simple routings (long enough, no weird restrictions during descent), but can be off for other cases. Just like in the real airplane, take everything the computer hands you with a grain (or tablespoon) of salt.

Cheers, Jan

 

So what would be considered a "simple route"

[mmerelles]

37 minutes ago, Bluedleta737 said:

So what would be considered a "simple route"

 

Most common issue arises when the STAR arrival procedure contains orders like cross this waypoint above xxx & below zzzz. The FMC will always read them as cross them above zzzz resulting on a very late T/D.

While VNAV is rebuild just to some briefing of the ruoute and altitude constrains, if wrong, simply type correct constrain on the scratchpad then override by pressing corresponding right soft key on the LEGS page.

 

note: this also apply for SIDs. Always scan for those such kind of altitude constrains and make the corrections on the legs page. You will be fine.

 

hope this helps

[Litjan]

And in the interim/as a backup plan just calculate your descent manually. Take your altitude in FL numbers (i.e. 330) and divide this by 3.3 (=100) to get the nautical miles to fly.  It works the other way around, too, of course (i.e. at 50NM distance remaining you should be at 16.500 feet). This is valid for a normal descent schedule of M.74/280/250.

Real pilots run this calculation in their head constantly, even during a VNAV descent.

Cheers, Jan

 

[Bluedleta737]

12 hours ago, Litjan said:

And in the interim/as a backup plan just calculate your descent manually. Take your altitude in FL numbers (i.e. 330) and divide this by 3.3 (=100) to get the nautical miles to fly.  It works the other way around, too, of course (i.e. at 50NM distance remaining you should be at 16.500 feet). This is valid for a normal descent schedule of M.74/280/250.

Real pilots run this calculation in their head constantly, even during a VNAV descent.

Cheers, Jan

 

So if I was cruising at FL370 I would divide by 3.7 to get 100 or divide by 3.3 to get 112?

[mmerelles]

33 minutes ago, Bluedleta737 said:

So if I was cruising at FL370 I would divide by 3.7 to get 100 or divide by 3.3 to get 112?

by 3.3

FL 370 / 3.3 = 112NM away for your ball park descend point.

[Bluedleta737]

17 hours ago, Litjan said:

And in the interim/as a backup plan just calculate your descent manually. Take your altitude in FL numbers (i.e. 330) and divide this by 3.3 (=100) to get the nautical miles to fly.  It works the other way around, too, of course (i.e. at 50NM distance remaining you should be at 16.500 feet). This is valid for a normal descent schedule of M.74/280/250.

Real pilots run this calculation in their head constantly, even during a VNAV descent.

Cheers, Jan

 

So what would be the V/s speed for this formula to work, or would you just use VNAV?

[Morten]

I would use FLCH, just dial in the speed (M.74/280/250.) and target altitude

[Bluedleta737]

4 hours ago, Morten said:

I would use FLCH, just dial in the speed (M.74/280/250.) and target altitude

Alrighty thanks

 

[cueceleches]

One other method I use sometimes is set VS while watching the green arc which marks the bottom of your descent on the EHSI display. I then increase or decrease VS along this green arc in order to make it match my flightplan waypoints altitudes. I don’t know if I have explained it clear enough...

Edited February 10, 2018 by cueceleches

[Bluedleta737]

Just now, cueceleches said:

One other method I use sometimes is set VS while watching the green arc which marks the bottom of your descent on the MFD. I then increase or decrease VS along this green arc in order to make it match my flightplan waypoints altitudes. I don’t know if I have explained it clear enough...

Ya I use that method now and I just totally ignore the T/D mark to be quite honest.