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