How to deal with both a speed and an altitude crossing restriction while in VNAV PTH descent (Boeing 737NG)? Suppose you are in VNAV PTH descent and are approaching the IAF for your Approach, The IAF has both an altitude crossing restriction and a speed restriction.
Assume that your VNAV PTH descent is at 240 kt at that moment and the speed restriction is 210 kt.

Will VNAV automatically &quot;see&quot; the speed restriction and reduce the descent rate (pitch up) to meet the lower speed? If yes, won't it then miss the altitude crossing restriction, as the planned path is modified?

If the PIC uses SPD INTV to set the limiting speed, won't VNAV switch to VNAV SPD and thus once again automatically pitch up to reduce the speed? Again, this may result in not meeting the altitude restriction, as you will end up higher than your intended path.


So, the question is, what should you do in a situation like this?
My guess is, revert to V/S and MCP SPD, use Speed Brakes and arrange to be over the IAF at the correct speed and altitude, and then re-engage VNAV.
With both restrictions entered on the LEGS page, VNAV will build a deceleration segment immediately prior to the point to reduce speed from 240 to 210 knots in (almost) level flight. There is no need, typically, to come out of VNAV, use the speed brakes, or do anything else like that in order to have VNAV meet both speed &amp; altitude constraints.
Thus, the Top of Descent point will be about 3 miles sooner for a restriction to &quot;cross XYZ at 9,000' &amp; 210 knots&quot; than it would be to &quot;cross XYZ at 9,000' &amp; 240 knots&quot;. It takes about 1 NM to slow 10 knots in level flight at idle.
The exception to not needing to come out of VNAV is if you end up with excess energy - perhaps unexpected tailwinds. In that case, you have to lose the energy, which is most easily done with the speedbrake. Switching between modes doesn't bleed energy, although it may affect the tradeoff between kinetic &amp; potential energy. (If you have enough distance, using Speed Intervention to increase speed CAN bleed off excess energy by increasing drag. BUT, you then have a longer decel segment... so to gain from this you either have to spend several minutes at the higher speed, or be descending to a point with only an altitude restriction, but no associated speed.)