8. descent
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Transcript of 8. descent
8. DESCENT8. DESCENT
Performance JAR 25Performance JAR 25
DESCENTDESCENT
Descent analysis
Descent profile
Top of descend
DESCENT ANALYSISDESCENT ANALYSIS
φ
φ
D
T
L
W
Forces involved in descent
DESCENT ANALYSISDESCENT 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 · TAS R/D
DESCENT ANALYSISDESCENT 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
DESCENT ANALYSISDESCENT 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.
DESCENT ANALYSISDESCENT ANALYSIS
ALTITUDE INFLUENCE
TAS
R/D
VR/D MIN
FL200
FL 300
φ
BEST GLIDE ANGLE
Vφ MIN VR/D MIN Vφ MIN
DESCENT ANALYSISDESCENT ANALYSIS
CONFIGURATION INFLUENCE
TAS
R/D
VR/D MIN
NO FLAP
FLAP 15º
φ
BEST GLIDE ANGLE
Vφ MIN
DESCENT PROFILEDESCENT 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):
DESCENT PROFILEDESCENT PROFILE
Since the speed for best glide is usually too low, a higher speed is used.
POINT OF DESCENTPOINT 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.
DE
SC
EN
T C
HA
RT
DE
SC
EN
T C
HA
RT