Heya all

After reading some of the comments here I decided to run a little experiment : Load a flight plan from Boston Logan to Atlanta Hartsfield, take off, and turn on doppler nav right after retracting flaps to see what would happen.

Here's what my little tests taught me about doppler nav :

In order for it to work, you'll need to :
- Turn doppler on from the O/H panel
- Select 'MAN' mode on the Doppler control panel on the central stand
- THEN load the FS flight plan
- Turn AP on on 'NAV' mode

Then the doppler will follow the flight plan and automatically switch waypoints, using whatever channel (A or B) was selected when you loaded the flight plan.
The 'selected course' box will then remain blank or not change (default 000 or whatever you entered) and only 'miles to go' will begin to count down.

Rather than turn to the heading corresponding to the active waypoint on the flightplan, stabilise its turn and fly straight forward, as it would happen with the HDG selector, the plane will stick to the flightplan exactly as it is displayed on the world map.
This means if the plane turns too slow and overshoots the active leg, it will not just fly parallel to it on the same heading (again, like it would happen with HDG mode), but it will apply corrections and automatically fall back on the planned course.

But you can also select 'virtual' waypoint manually as you go, as in 'fly this vector for this long'. In order to do this, you need to select your choosen course and the length of the leg on the dopller control panel on the central stand. If you were following the flightplan on the A channel, use the B channel, and then switch.
When you do this, the new selected leg will override the flightplan on the world map screen now only showing the new leg you just entered (heading and length)
In this case too, the doppler will make automatic corrections and fall back on the active leg if it overshoots it.

Switching between channels A and B works just like switching between NAV radios, allowing you to fly to one waypoint while the next one is already set up and ready to be activated.

10 Nm before a waypoint is reached, a red warning light will show on the panel, indicating it's time to switch waypoints.
No need to touch anything if you are still flying the FS flightplan, but if you are entering waypoint manually, this is when you need to switch the current active channel on the doppler panel.
At standard AP bank angle, this should allow a nice turn between legs without overshooting.

If no next waypoint is entered, the plane will just fly around in circles once 'miles to go' has counted down to 0.

I'm not too sure wether this accurately represents the way doppler nav systems are supposed to work, but it can certainly come in handy in some situations.
I was expecting the system to be dumb and just blindly follow the data entered, meaning if I turned doppler nav on at a different starting point than specified in the flightplan, it would just follow an offset route. Well, it does not.

I like to think this was intended to simplify navigation and simulate other crewmembers making adjustments to decrease the pilot's (your) workload during some phases of the flight. After all the 707 is supposed to be flown by a 3 members crew...  ;)

Hope this helps.

Cheers

Nico