It's a point called the Geo-Something-Point on the arrival. Once past that point you can open up the speed window and it will stay in PTH. Prior to that point it will revert to SPD.
The fix for this is as follows:
Change the first point on the arrival with a BETWEEN crossing restriction to a Hard Altitude or AT OR BELOW. That will create the new Geo-Point right there and you can open up the speed window and stay in PTH from that point on.
If your first fix (TRUPS ,for example) looks like this:
TRUPS 280/FL300 FL240
and you open up the speed window just past that fix, you revert to VNAV SPD and it goes idle.
So you would change it to this:
TRUPS 280/FL300B
Or
TRUPS 280/FL270
Now you can open up the speed window past TRUPS and remain in VNAV PTH at whatever speed you want.
Works great, we do this all the time coming in to Dfw, lax, DCA, etc... All those arrivals begin, and continue for a while, with BETWEEN altitudes. It is essential to do this, because we are constantly being cleared to "descend via the XXX1 Arrival except maintain 310kts." Well, it is a royal PITA to go leg by leg and change the speeds manually...so using this fix takes care of your descent speed from the first point on: you just open up the speed window and stay in PTH. Voila!
Last edited by aa73; 7th Dec 2015 at 15:03.