VOR autotuning (FMC)

[Hardy Heinlin]

Good evening,

certainly not 744 specific, and rather just cosmetical: How are the autotuned VOR pairs arranged regarding the left and right receiver?

Say, the following VORs will be used in sequence along the route: AAA, BBB, CCC, DDD ...

How will it proceed?

Example 1:
AAA BBB
BBB CCC
CCC DDD

Or:

Example 2:
AAA BBB
AAA CCC
DDD CCC


Does anybody know?

Example 2 probably causes only half as many Morse ID display dropouts than example 1 does.

(I'm referring to cases where VORs are involved. I know DME-DME tuning is independent on other channels.)

Thank you!


Cheers,

|-|ardy

[Zinger]

In the absence of more defined logic, I believe the process is the following:

The FMC automatically selects a navaid which will yield the best position and cross radial navigation update information due to its position relative to the path of intended flight.
I am assuming that best goes to VOR L and next best to VOR R.

Edit: In one flight simulation document describing B777, it is explained that the FMC auto-tunes the two closest stations in its database.

More from B777 RW documentation:
VOR L and VOR R
Left and right VOR data are shown on the CDU. VOR tuning status is
shown adjacent to the VOR frequency. The automatic tuning mode
operates for procedure flying, and route operations. Enter the frequency
or identifier to manually tune a VOR. VORs are tuned automatically by
the FMC in the following priority:
· P - Procedure autotuning. The FMC selects navaids necessary for
approach or departure procedure guidance.
· R - Route autotuning. The FMC selects navaids on the active route.
The navaid must be the previous VOR or a downpath VOR within a
certain range of aircraft position.
· A - Autotuning. The FMC selects the best available valid navaid.

[Hardy Heinlin]

The distance is not necessarily the only parameter. The geometry plays a role as well. And the figure of merit, and the power etc.

What I actually wonder about is just this: When a new pair is picked up and one VOR of the pair is already tuned (was in the previous pair), will this VOR stay tuned  on its current receiver? Or will it be swapped left/right?

When a tuned station always jumps from left to right every x miles, the ND may get unnecessarily nervous, and the Morse identifier disappears in each init phase.

What would be the advantage of the swapping?


Thanks,

|-|ardy

[Hardy Heinlin]

Maybe my question wasn't clear enough.

What I mean is less complicated than it may sound. It's not about the search of VORs, it's about what happens when this search is completed.

I just like to know what goes to the left and what goes to the right side of the ND  when a new pair has been determined for best geometry while one VOR of this new pair was already in the last pair.

Normally, the pairs don't have this sequence:
left--right
AAA BBB
CCC DDD
EEE FFF

But it's rather overlapping like this:
left--right
AAA BBB
BBB CCC < both sides get a new Morse id
CCC DDD < both sides get a new Morse id

Or overlapping like this:
left--right
AAA BBB
AAA CCC  < only one side gets a new Morse id
DDD CCC  < only one side gets a new Morse id


I find the latter solution more elegant and less distracting to the crew.


Cheers,

|-|ardy