Jump to content

FMC T/D problems...


Bluedleta737
 Share

Recommended Posts

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

 

  • Upvote 1
Link to comment
Share on other sites

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"

Link to comment
Share on other sites

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

  • Upvote 1
Link to comment
Share on other sites

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

 

  • Upvote 2
Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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 by cueceleches
  • Upvote 1
Link to comment
Share on other sites

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.

  • Upvote 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...