These messages come up all the time on the real airplane - it's not a perfect system that flies on rails or something. The NG VNAV is actually routinely complained about on actual pilot forums from what I've seen. The bigger Boeings are better with it. There's another post I just made the other day where I explained a couple of these messages technically. It has to do with the limit angle for the path leg - you're exceeding it, simple as that - put out speedbrakes or tell ATC unable to comply - you are allowed to do that, happens all the time in the real world. They're there to accommodate your capabilities, not the other way around.

Thanks Ryan, that makes sense. I didn't question it being a problem with the NGX I just wanted to understand why it does this.

Usually I interpret this message as "I REALLY NEED DRAG!" because I can remedy to it by slowing down to vref (i have to, anyways). Once you're established on a approach config, with the plane coming in slowly, she will have time to descend.

They're there to accommodate your capabilities, not the other way around.

Some days I wish someone would tell certain controllers that!

I have just had this experience on a short flight from EGBB to EGPH. charts call for FL180 at Margo and missing the tweed intersection, the message received, "descent path unachievable" changed to VS -2500ft/min and just made the 3000' before the final turn onto finalMel Few