FMC Computed GP in Database approaches, curious difference B737/777

[tango4 4]

HI everyone,

I have a question regarding the way the FMC displays a GP indication in the legs page between 2 waypoints in a database approach.

The understanding I have is that normally, VNAV computes an IDLE descent path between 2 waypoints, and if not possible, then it will respect the contraints (ALT and SPD).

All that is true, EXCEPT when in approach mode, where it has to follow a published GP.

The way I understood things, the decision to have a GP 3.00° for example between 2 waypoints comes from the database, and not the FMC, because it means the procedure has been designed as such.

According to some readings, I expected to see a GP in the legs page even for an ILS approach (although of course you won't fly it in VNAV), which is clearly not the case, at least not everywhere.

I used the example of LFPG ILS26R in the screenshots below, using the latest NAVDATAPRO cycle (I had basically the the same results with an older Navigraph cycle).

I initially thought that the "issue" came from the database itself, but the curious thing is that it is not displayed in the same way in the B737 and the B777 !

Here are two screenshots of the same procedure, of course with the same common database, in the B737 and the B777:

 

https://www.dropbox.com/s/evikvadq9yid97b/2015-09-29_00001.jpg?dl=0

https://www.dropbox.com/s/cpd21eubwt4vnky/2015-09-29_00004.jpg?dl=0

 

 

In the first case, there is no Glide, and in the second case, yes (between PGS532 and RW26R).

 

Can anyone explain this ? I know these two FMS are not from the same manufacturer, but from what I read, I did not expect to see any difference here.

 

Best regards everyone.

[downscc 1,327]

 

 

but from what I read, I did not expect to see any difference here.

 

From what you read: Great, you have a reference. What is it?

 

You should expect differences between 777 and 737 FMS. There are many.  In particular, the NGX is modelled with a version of FMS software that includes IAN. See FCOM section 11.43 for descent and approach section on use of FMS.

 

I am just guess that it is the IAN feature involved here, hopefully your reference will help shed light on why there are differences.

[tango4 4]

Here is my source: (FMC 737 Bulfer guide)

https://www.dropbox.com/s/gxnnoqmo7mpjuya/bulfer.jpg?dl=0

(the first 3 paragraphs).

As you can read in my message, I AM expecting differences in those FMS.

It's just this one does not make sense to ME and I wanted to know if someone could explain it.

As mentioned in this document GP is defined in the database, so I don't understand why it would show up differently in those 2 FMS.

 

I already thought about IAN but this involves non ILS approaches, and the example I mentioned is an ILS. And as I understand it IAN is the WAY of performing the approach which is different from VNAV (the "standard" way). But IAN needs accurate data also, so that still doesn't make sense to me.

 

Charles

[downscc 1,327]

Not our navigation data, unfortunately. Our glide paths are not included in the synatax that defines our navdata. Basically, the only vertical navigation data is the altitude constraint. So for example, if the approach is non-ILS the FAF is usually in the data as something like "FIX ABCDE AT OR ABOVE XXXX" and the NGX FMS is not going to provide a GP angle. Change it from conditional to hard altitude constraint and I think the FMS might display the angle.... not positive since most my time is in the 777.

 

Bottom line, your excellent source does not apply in the simulation. We are all looking forward to the introduction of better navdata in PMDG products, but its not there yet.

[ubersu 78]

There are no gradient path angles defined in the PMDG nav data - you are seeing computed angles.

 

DJ

[tango4 4]

Thanks guys, that clearly makes sense now !

 

All the best.

 

Charles