Situation: Descending out of FL280, VNAV enabled w/1700fpm descent for a STAR into Heathrow. I set A/S hold to 240KIAS. It’s holding 240 steady so I step away for a minute, return and find 3 A/S violations for >250kias.
Question: if it can’t maintain both parameters, does AP prioritize making the descent profile (5,500 ft at BOZZO) or holding the A/S I set?
My ego took a hit today with those 3x violations 
The current “version” of VNAV solely focuses on meeting the descent altitudes set. Speed is down to you to manage, putting spoilers into FLIGHT mode if necessary.
I would agree that the autopilot system in general needs a serious revamp.
Gotcha, yea I had spoilers in flight at it could hold 240kias at a low power setting so I thought I was safe. Now I know to keep a better eye on it
Hi, only autothrottle responds to maintaining speed at the moment, but that’s only as far as thrust is maintaining speed.
But as in your case, nothing prevents speed from increasing in descent after autothrottle is already at idle, simply from too high a descent rate.
So, you are left with the responsibility to manually decrease descent rate to prevent overspeed (beyond adding drag such as airbrakes or, if absolutely necessary, lowering gear)
The feature to be added to VNAV to prevent this would be VNAV automating pitch control to limit airspeed increase.
This was a recent related topic:
This was the referenced feature request in that topic: