Descent and glide
Navigation · Chapter 55 · DGCA CPL and ATPL
At idle, weight does the work. Heights are stretched here so you can see the slopes.
Remember
- Descent gradient = (D − T) ÷ W.
- Distance = height × L/D.
- Best glide at VMD.
- Weight changes time, not distance.
- Wind changes distance, not time.
Going down
Thrust to idle, at the top of descent, some 80 to 120 miles out. Now weight does the work.
Climbing, part of the weight pulled you back. Descending, the same part pulls you forward. It is what keeps you moving.
The same formula as the climb, turned around.
Descent gradient % = D − T / W × 100
Airliners bring it down at about 2 600 ft/min high up, slowing to 700 ft/min low down. Flaps and gear go out below about 5 000 ft.
The most economical descent is continuous, at idle, until flaps and gear.
How far it glides
Now no thrust at all. How far does it go? Only one number decides: lift over drag.
Glide distance = height × L/D
L/D is best at one speed. VMD, the bottom of the drag valley. Fly faster or slower and you come down steeper.
Best glide: highest L/D, at VMD.
Trying to stretch the glide by raising the nose? You just slow below VMD. More drag, and a steeper glide. It is the opposite of what feels right.
What changes it
This lesson is part of the Navigation plan. See plans