What is that green dashed circle on ND?

Question

I think this is a navigation display from a boeing aircraft

I searched around the internet for the meaning of the "green dashed circles" but got nothing. Thanks in advance for your answers!

Answer 1

When a point is entered into the FIX page in the FMC, the point's symbol (VOR or DME or waypoint or whatever) is displayed in green on the ND; if you enter a radial/distance then you get a circle around the point of the specified radius and a straight line on the given radial. Alternatively, one can enter just the radius (as /20, instead of 180/20) and you get just the dashed circle.

In the picture in the OP, it looks like the destination airport has been entered in the fix page, with three distance-only entries, thus giving 3 concentric circles. The 087/267 degree radial displayed appears to be the course selected with the co-located VOR tuned in.

The following picture shows the FIX page with the I-AHI ILS DME transmitter entered, and a radial/distance of 090/10 entered:

(The 328/1.6 is the aircraft's current radial/distance from the fix.)

On the ND, it looks like this:

Note that the 090 radial from the fix is shown, but the reciprocal 270 radial is not. Contrast that with the BRK VOR (near the bottom of the display) which is tuned in the #1 NAV receiver with a 165 course dialed in - that radial is shown as both the 165 and the reciprocal 345 degree courses.

This has various uses; if I get a report of turbulence "50NM south of XYZ" (XYZ not being on my route) then I can put XYZ in the FIX page, and then below it enter 180/50 and I can then see the location of the turbulence report in relation to my (magenta line) course. Alternatively, if I want to be at 10,000' and 250 knots at 36 NM from my destination, I can enter the destination (or even a runway) in the FIX page, and "/36" below it, and I'll get a 36 NM ring around the destination. Then when I'm crossing that ring I can compare my actual altitude & airspeed and have a good idea if I have more or less energy than desired.

I'll also use the ring as a reminder of some action I want to take at a given point... if I know the ride will turn choppy in the vicinity of XYZ (which may or may not be on my route), I might put a 50NM ring around XYZ as a reminder that as we approach that point, I want to turn the seatbelt sign back on & call the flight attendants to take their seats. I'll also mention that plan to the other pilot so that we're both on the same page as to how we plan to deal with the bad ride ahead.