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I'm now studying a Flight Management System (FMS), and it says that when the FMC predicts that the airplane will not reach an ALT constraint, the CDU will display "UNABLE NEXT ALT" and then we have to manually set a lower airspeed to provide a steeper climb.

But, I've learned that when we pitch up, the aircraft slows down because of more drag.

I also remember that when an aircraft increases speed, pitch goes up because of the increase in thrust and more lift from the wings.

So why is a decrease in speed needed to make the FMC altitude constraint, rather than an increase?

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  • $\begingroup$ Do you fly this airplane with a FMS? $\endgroup$ – Michael Hall Nov 28 '20 at 16:26
  • $\begingroup$ When you choose a slower airspeed, aren't you choosing a speed that is closer to Vx? Are you familiar with what Vx means? $\endgroup$ – quiet flyer Nov 29 '20 at 20:51
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I think you have it backwards, it is not that a slower airspeed produces a steeper climb, it it that a steeper climb, given the same power, will cause a slower airspeed.

If the only constraint you're setting the FMS is the airspeed to maintain for a constant thrust, then it will pitch up more, increasing the rate of climb, which will slow the airspeed.

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Initially, pitching up will trade airspeed for altitude. Going from a climb at 300 knots to a climb at 250 knots means pulling back & increasing the rate of climb while the speed decreases.

For a close-in fix to the climb profile, this is often sufficient to rapidly gain several hundred feet above the FMC prediction and solve the situation.

For a steady climb, the slower speed is typically a steeper sustained angle, mainly because the parasite drag goes up with airspeed (and at typical climb speed ranges, parasite drag is the larger contributor to total drag than induced drag is). So for a case when you're many miles from the restriction, the improved climb angle at the lower speed will help you meet the constraint. (Note that this only works to a point, getting slower than the max angle of climb speed, other things come into play & you're then hurting your climb performance. But that Max Angle speed is typically well below normal climb speeds.)

Finally, as a matter of planning, it takes time -- and therefore distance -- to accelerate. Delaying that acceleration until past the constraint can be helpful too. There is a departure out of KBWI with an altitude constraint at or above 11,000' that can be hard to make under certain conditions if you let the FMC accelerate the aircraft from 250 (speed below 10,000') to a fast climb speed (say, 300 knots) before getting very far above 10. The easy solution is to keep the speed at 250 until through 11,000, which makes the restriction easily, and then accelerate.

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If you keep a constant power setting then the way to decrease your speed is to pitch up. Simply, this will increase your rate of climb as more of your total vector is ‘upward’ rather than ‘forward’. The CDU is assuming a constant power setting during the climb so will naturally calculate a higher rate of climb for a lower airspeed.

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