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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.

  1. Will VNAV automatically "see" 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?

  2. 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.

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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 & altitude constraints.

Thus, the Top of Descent point will be about 3 miles sooner for a restriction to "cross XYZ at 9,000' & 210 knots" than it would be to "cross XYZ at 9,000' & 240 knots". 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 & 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.)

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  • $\begingroup$ "using Speed Intervention to increase speed" I assume you meant to write "decrease speed"? $\endgroup$ Dec 25, 2020 at 15:13
  • $\begingroup$ "VNAV will build a deceleration segment immediately prior to the point to reduce speed from 240 to 210 knots in (almost) level flight." So it will have taken the speed reduction into consideration, when defining the T/D and bring it a bit forward than otherwise? $\endgroup$ Dec 25, 2020 at 15:16
  • $\begingroup$ @StamatisVellis. "...to increase speed" is correct - it's about bleeding off excess energy, and higher speed = higher drag = more energy dissipated. Slowing early typically exacerbates a high-energy state. $\endgroup$
    – Ralph J
    Dec 25, 2020 at 16:46
  • $\begingroup$ I am confused with the "to increase speed" part, because we are looking to decrease speed, not increase it in the case examined. $\endgroup$ Dec 25, 2020 at 17:10
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The answer above is largely a complete one. I would only adf a couple of small details. FMC computations determining where to set the path are more accurate if winds aloft are entered for multiple levels on the descent forecast page.

As to speeding up or slowing down to manage energy, you would need to consider the descent airspeed in relation to where the aircraft is most efficient. The 737-NG comes down like a leaf at 270 knots +/- a few depending on weight. Cost index is a factor here too. Waiting until the last minute before diving down at 320 knots would provide a higher groundspeed and save time to the destination. The dive also requires planning to include a period of deceleration prior to the crossing restriction. Ten knots per mile works in level flight; you need more if decelerating in a descent. Calculate based on knots of groundspeed...

The mental calcs are good for situational awareness, but the FMC manages the path and speed nicely if you program the winds.

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  • $\begingroup$ Fully agree. Welcome to Av.SE! $\endgroup$
    – Ralph J
    Dec 27, 2020 at 9:11

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