The original question was generated because I was unsure how the FMC would calculate the top of descent if there was a blank DISCO on the legs page, I'm assuming that its assuming the points are connected regardless of the DISCO or not? Appears that way to me. 

Anyways thanks for the comments.

Jim V

17 hours ago, morpheous said:

The original question was generated because I was unsure how the FMC would calculate the top of descent if there was a blank DISCO on the legs page, I'm assuming that its assuming the points are connected regardless of the DISCO or not? Appears that way to me. 

Anyways thanks for the comments.

Jim V

That is a fair question.  In general, the FMS will calculate the vertical path up to the discontinuity.  In the context of this thread, the disco is at the terminal end of the arrival and in general an arrival will have one or more altitude constraints before the terminal so there is plenty of information available for a vertical path calculation.  There are discos that you want to deal with, such as mid-route or even the occasional arrival that ends in fix ABCD and an approach that has ABCD as an IAF.  Sometimes you need to over write the arrival ABCD over the approach ABCD to close the path.  Regardless, this does not generally create a problem for VNAV.

One of the most important things to learn about VNAV is when to stop using it.  Once you have complied with the final altitude constraint on the arrival it's typically best to switch to level change and descend as quickly as you can to get the plane down.  Once I'm established on the final approach course I will switch back into VNAV to step down to the glideslope intercept altitude.  Using VNAV while being vectored will often leave you higher than you want to be.

Exceptions exist, such as LAX where the arrival and approach link up to form a continuous path all the way to the runway.

On 12/7/2017 at 5:28 AM, headley said:

A "disco" or "discontinuity" are the five emty boxes in the LEG's page, these must always be "closed".

Leaving a disco is personal preference and can serve as a reminder. Prematurely closing off a DTW and leaving the aircraft in LNAV may cause a deviation from your clearance. Typically it involves embarrassment as the aircraft turns the wrong direction and the PF frantically engages HDG select to get the airplane turning back in the right direction.

Typically if the STAR/SID includes a heading/vector, the FMC will include the vector and the disco can be closed off. The airplane will fly the vector leg in LNAV until the vector is either deleted or you have selected another waypoint to go direct to, etc.