Hi, would someone be so kind and explain how exactly the off path descent and the energy management circles work? I am reading about it but can't quite figure out how exactly the circles can help the pilot. I understand that the outer circle is where the descent ends in clean config and the inner circle is where the descent ends with speed brake, but why is it a circle, what happens if you make a 180 degree turn to the opposite end of the circle while descending ? And why does it have to be referenced to a waypoint and not the aircraft current location? If someone could explain how it works and how to use it I would appreciate it a lot, maybe even in a video if you have the time, thanks.

You have it backwards - it is the START of the descent profile; clean (white circle), and with drag (blue circle; speed brakes fully extended).

 

It is useful because you are descending to a point, and so you define that point, and the altitude you need to be at. The aircraft knows its position, its altitude, and its speed (specifically, ground speed). From there, it computes the performance to descend from current altitude to the target altitude at the target point, and computes clean and drag descent profiles in order to be at that altitude at that point in time.

 

If you are outside the circles, you need a lower FPA than idle. If you are ON the white circle, then assuming winds don't shift on the way down, you will hit the altitude within certain limits clean, with an idle thrust descent path.

 

Between the white and blue circles you will need some kind of drag all the way down to make it (blue circle is computed with speed brakes fully extended from the descent point all the way down).

 

If you are inside the blue circle, you are SOL, and will be high over the way point unless you can get 'S' turns to increase distance.

 

Best regards,

Robin.

Thanks Robin that makes things clearer for me too

I'm still lost but I'm a practical learner.  We don't quite have the energy OPD circles yet in my school's C152 fleet  :lol:

 

Basic example above. You're on approach to the runway shown on the left and you want to make sure you manage your energy to the FAF. On the off-path descent page you enter a fix (in this case, the FAF) along with its altitude and your desired speed. In this case we'll assume the the FAF is at 3,000ft and you want to arrive at 160kts. Lets say we've currently level at 12,000ft and 240kts following the magenta line in LNAV.

 

At this point, we're outside the white circle so if we started a descent now, we'll make the FAF on speed without speed brakes. If we wait until we're within the white circle but outside the blue circle, we can still make the FAF, but we'll need speed brakes. An important thing to note is that as altitude/speed reduces, the circles will get smaller, indicating less excess energy.

 

It's also important to note that these circles indicate the point at which you must start descending/slowing, if you're heading DIRECT to the selected point. So in this case, when following the magenta line and you move within the white circle, because you're not heading direct to the FAF at this point, you've got further to fly so could possibly delay the descent. You couldn't accept an ATC instruction to fly direct without using speed brakes though.

 

Hopefully this makes sense.

That clears up everything thank you!

now I get it.  cool feature.