Lecture 7: DESCENT

PERFORMANCEAIRCRAFT WEIGHT &

PERFORMANCE

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

Cruise / En-route

DescentApproach &

LandingTake-off

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).

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.

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

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?

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.

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) / Weight1sin

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.

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

Factors Affecting the Descent performance (Descent Angle

and Rate of Descent)

AircraftConfiguration

WindSpeed Cabin Pressurization

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

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.

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.

Wind

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.

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.

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).

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.

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.

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.