4. DESCENT ANALYSIS
The descent analysis is very similar to the climb analysis. In this case,
lift is also less than weight. The only difference is that a lack of thrust
exists (instead of an excess of thrust):
sin φ = (D – T) / W
R/D = TAS · sin φ W
T-D
·TASR/D =
5. DESCENT ANALYSIS
TAS
R/D
VR/D MIN
60k T
70k T
80k T
φ
BEST GLIDE ANGLE
Vφ MIN
Unlike climb speeds, Vφ MIN is always higher than VR/D MIN.
MINIMUM DRAG
6. DESCENT ANALYSIS
From the previous equation and its graph, we can make some
conclusions:
Vφ MIN is always higher than VR/D MIN.
R/D depends on weight, DA, configuration and speed. The angle of
descent (φ) depends on all of this plus the wind.
φMIN gives best glide range. This angle and its range do not depend
on weight. However, the VφMIN increases as weight increases.
VφMIN increases with headwind and decreases with tailwind.
9. DESCENT PROFILE
If the aircraft descends at a constant TAS, drag force increases, so an
increase of R/D and descent angle is produced.
However, in practise, this type of descent is never performed. The
usual descent profile is equal to the climb profile but in the opposite
way. Here is an example of an A330 (0.82 / 300 kt / 250 kt):
11. POINT OF DESCENT
Unless otherwise instructed by ATC, the flight crew will determine the
Top Of Descent (POD) for every flight.
TOD location depends on several factors, such as weight, icing
conditions, wind and cabin pressure limitations.
If an accurate determination of TOD location is required, descent
charts must be used. However, to simplify operation and reduce pilot’s
workload, a general rule for jet aircraft exists:
TOD = Flight levels to be descended · 3
Then, gross corrections (based upon experience) have to be made due
to wind, weight and icing.
