THIS INTERESTING PRESENTATION TELLS HOW AIRCRAFTS DESCENT. Explain also why is it important to consider the descent gradient & rate of descent.

1. Lecture 7:
DESCENT
PERFORMANCE
AIRCRAFT WEIGHT &
PERFORMANCE

2. Introduction
• Descent starts at the end of the cruise phase and it ends
when aircraft will start approach for landing.
• How aircraft descent??
2
Climb
Cru is e / En -rou te
Des cent
Ap p roach
& L an d in g
Take -o f f

3. How aircraft descent???
• Aircraft descent by reducing thrust, or engine power.
• This reduces aircraft’s speed, thus creates less lift, so the
airplane slowly lowers (decreasing altitude).
• If for climb aircraft have to produce excess thrust (thrust –
drag), but for descent aircraft have to produce excess drag
(drag-thrust).

4. Top of Descent Point
• The point at which the descent is initiated from the
cruising level is called top of descent(ToD) point.
• The flight crew will have to calculate the top of descent
point to ensure that they arrive at the correct level for the
start of their approach.
• A very simple formula for determining ToD is the 3:1
method.
• A 3:1 descent plan means that the aircraft will require
three nautical miles distance for every one
thousand feet of aircraft altitude above ground.

5. Top of Descent Calculation
• For example: Aircraft Cruise at FL300 with
destination airport at sea level.
• 30,000 divide by 1000 equals 30.
• 30 multiplied by 3NM equals 90 NM required for
descent.
• 30,000 feet ÷ 1,000 feet = 30 30 x 3 NM = 90 NM

7. Introduction
• When deciding on the top of descent point, the
pilot will have to consider 2 things which are:
1.The descent gradient / angle of descent
2.The rate of descent (ROD).
Why important to consider these 2 things?

8. Why important to consider descent Gradient &
Rate of Descent
• To reduce descent distance thus reduce fuel
consumptions.
• To ensure rate of descent (rate of atmospheric
pressure changes) proportional to the rate of
change of the cabin pressure. *Note that, rapid
descents can cause trapped gas in the middle ear.
(Middle ear block).
• To ensure the safety of aircraft & passengers.
Rapid descent also can cause aircraft loss of
control & this can lead to the crash.

9. Descent gradient & Descent Angle
• Descent gradient is the ratio of height descended to
distance travelled by aircraft.
• Descent gradient depends on the difference between the
drag and thrust (the excess drag).
Descent gradient = (DRAG - THRUST) / WEIGHT
EXCESS DRAG
• The angle of descent ,  = ( Drag– Thrust) / Weight
1
sin 

10. angle of descent
• The pilot controls the angle of descent by varying
engine power and pitch angle (lowering the nose).
• If the nose is too high for the chosen power the
airspeed will decrease until eventually the aircraft stalls,
or loses lift.
• If the nose is too down, it would increase speed and
aircraft would crash to the land.

11. Rate of Descent
• The rate of descent is the vertical component of the speed,
expressed in feet per minute.
• It depends on the true airspeed (V) and the
descent gradient:
• Rate of descent = V x Descent gradient
= V x (Drag – Thrust) / Weight

12. Factors Affecting the
Descent performance (Descent Angle and
Rate of Descent)
Aircraft
Configuration
Wind
Speed Cabin
Pressurization

13. Speed
• In general, rate of descent increases with increasing speed
and increasing drag.
• Optimum speeds required for the best descent
performance.
Rate of descent = V x (DRAG - THRUST) / WEIGHT

14. WIND
• The descent angle relative to the ground will be affected by
the wind.
• Wind affects the ground speed. So, the descent gradient
will be affected as well.
• A headwind will reduce the ground speed and therefore
reduce the horizontal distance that aircraft travels in
comparison to the no wind conditions.
* Therefore a headwind gives increased descent gradient.
This important to reduce descent distance thus reduce fuel
consumptions.
• While a tailwind affects in opposite direction and gives
reduced descent gradient.

15. Wind
• But, wind has no affect on the rate of descent.
• The rate of descent is independent from the wind speed,
because it is always considered in reference to the
airspeed not the groundspeed.
• Crosswind component has no effect on the descent
gradient.

16. Wind

17. Aircraft configuration
• Aircraft configuration (flap/landing gear) affects the
aircraft’s lift and drag.
• The total drag of an aircraft will depend on its
configuration.
• When the flaps are lowered the drag is increased, resulting
in an increase in excess drag, therefore the descent
gradient is increased.
• Same thing happens when the landing gear is lowered; the
descent gradient is increased.
* Descent gradient increase, distance decrease,
save fuel.

18. Cabin pressurization
• The rate of change of the cabin pressure has to be
proportional to the rate of change of the atmospheric
pressure (rate of descent).
• The cabin pressurization has a greater affect on the rate of
descent in comparison to the rate of climb.
• As already explained, cabin pressurization systems are
designed to produce conditions equivalent to those at
approximately 8000 feet.

19. Cabin pressurization
• When the aircraft is descending, the change of cabin
pressure is proportional to the change of the ambient
pressure, in order to control the structural stress on the
fuselage from the inside.
• This is performed automatically by a sophisticated control
system that is increasing the pressure inside the cabin by
the use of compressors.
• It is important that the rate of descent is matched with a
corresponding rate of cabin pressure increase (same
structural stress).

20. Cabin pressurization
• If the rate of descent is exceeding the corresponding rate of
cabin pressure increase, the aircraft structure may
damage.
• Thus the maximum rate of descent would be limited by
this factor. Special care has to be taken by the crew during
descent and initial approach, when the cabin pressure is
manually controlled or the system is running with
degradation.
• The best passenger comfort is achieved at rates of
descent of 1500 feet per minute.

21. Plane Crash Because of Very Rapid
Descent
• EgyptAir Flight 990, Less than three minutes after leaving
cruising altitude of 33,000 feet, the aircraft crashes into the
Atlantic Ocean killing all 217 people on board.
• The aircraft subsequently dives at a rate of over
20,000 feet per minute creating weightlessness in the
cabin. ``A very rapid descent,''
• The aircraft ascends back to 24,000 feet, then dives again.
• The maneuvers cause the left engine to be damage.

22. Question Bank
a) How aircraft descent?
b) Explain how to determine the Top of Descent point? Give
reason why important to calculate it.
c) Explain why is it important to consider the descent
gradient & rate of descent.
d) Explain five (4) factors affect the performance of an
aircraft during descent.