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Questions on Descent Performance

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PostPosted: Tue Apr 17, 2012 3:55 pm    Post subject: Questions on Descent Performance Reply with quote

Arrow Regarding the best angle and best rate of descent speeds, "A heavier aircraft must descend at a faster airspeed in order to maintain the same slope as a similar lighter aircraft"

Arrow How does an increase in aircraft mass affect the gliding range? "No Effect"

Arrow What factors affect the descent angle in a glide?

a) Configuration and angle of attack <-- Correct
b) Configuration and altitude
c) Mass and altitude
d) Mass and configuration

Arrow An aeroplane executes a steady glide at the speed for minimum glide angle. If the forward speed is kept constant, what is the effect of a lower mass on Rate of descent, glide angle and CL/CD ratio?

Rate of descent - Increases
Glide angle - Increases
CL/CD ratio - Decreases

Lower mass = Less lift requirement = Less induced drag = Drag curves moves left = Lower Vmd

But if a higher Vmd is maintained then drag increases.

Descent Angle is Sin gamma = D-T/W

Normal descent is flown as a glide (idle throttles).

Thrust being zero in a glide the formula for glide angle becomes:

Sin gamma = D / Weight

Lift can be taken equal to weight (though it is actually slightly less), the foumula becomes:

Sin gamma = Drag / Lift

Using the co-efficients of drag and lift.

Sin gamma = CD/CL

With more drag:

CD/CL increases = Glide angle increases = Rate of descent increases.

CL/CD decreases for obvious reasons.

Arrow Two identical aeroplanes at different masses are descending at idle thrust. Which of the following statements correctly describes their descent characteristics?

a) At a given angle of attack, both the vertical and the forward speed are greater for the heavier aeroplane <-- Correct
b) There is no difference between the descent characteristics of the two aeroplanes
c) At a given angle of attack the heavier aeroplane will always glide further than the lighter aeroplane
d) At a given angle of attack the lighter aeroplane will always glide further than the heavier aeroplane

Arrow Two identical aircraft, one with a light load and one with a heavy load are in an idle power descent, from the same height. Both experiencing the exact same atmospheric conditions. The heavy aircraft will:

a) need to use a faster speed in order to achieve the same descent angle as the light aircraft <-- Correct
b) have the same descent range and endurance but using a faster speed

Arrow Which of the following combinations basically has an effect on the angle of descent in a glide? (Ignore compressibility effects).

a) Configuration and angle of attack <-- Correct
b) Mass and altitude
c) Altitude and configuration
d) Configuration and mass

Arrow With all engines out, a pilot wants to fly for maximum time. Therefore he has to fly the speed corresponding to:

a) the minimum power required <-- Correct
b) the minimum angle of descent
c) the maximum lift

Vmp is the point where the product of speed and drag are equal and is less than Vmd. Glider pilots try and stay airborne for as long as possible and this is the speed they would attempt to fly but it is very speed unstable. We call it gliding for endurance. Basically we are trying to keep the forward speed to a minimum and the ROD to a minimum, its done at Vmp.

Source: (http://www.atpforum.eu/showthread.php?t=4596)

Arrow Which of the following would give the greatest gliding endurance?

a) Flight close to CL MAX <-- Correct
b) Flight at the best CL/CD ratio
c) Flight at VMD
d) Flight at 1.32 VMD

The speed used during the descent is the rate of movement of the aeroplane along the descent path "V" and can be divided into two components: horizontal and vertical. The horizontal component is the aeroplane's forward movement across the ground, its groundspeed, and the vertical component is its rate of descent.

It is essential to reduce the vertical component to a minimum to produce the maximum endurance. This means that the descent speed of the aeroplane and the rate of descent must be as low as possible throughout the descent. The distance travelled during the descent is of no consequence. Therefore, the speed to fly is less than VMD at VMP.

V in the figure is the descent TAS in feet per minute and the rate of descent in fpm is V x sine gamma, where gamma is the glide angle. Therefore the glide angle must be the minimum possible.

The power required is the product of the TAS and the drag. Therefore, the lowest rate of descent is attained at the speed for which the least power is required, which is close to CLMax. For any given mass this speed occurs at the lowest point on the power-required curve that may be determined by the point at which a horizontal line is tangential to the curve.

Arrow What gives one the greatest gliding time?

a) Lower mass <-- Correct
b) A head wind
c) A tail wind

Arrow The drift down procedure specifies requirements concerning the: "obstacle clearance after engine failure"

Arrow In the drift down for an aeroplane in Performance Class B the net gradient of descent is assumed to be gross gradient of descent "Increased" by "0.5%"

Arrow If the TAS is 100 kts on the glide slope of 3 deg, what is the Rate of Descent?

Ans: 500 ft/min

Based on trigonometry

Tan of angle = Opposite / Adjacent

Tan of angle = Rate of Descent / Horizontal Distance

Rate of Descent = Tan of angle x Horizontal Distance

Tan of Angle = Glide sLope Angle

Horizontal Distance = Ground Speed

Since we want Rate of Descent in feet per minute, we will have to convert Ground Speed from nm/hr to ft/min by divinding it by 60 and multiplying by 6080.

Rate of Descent (ft/min) = Tan of glide slope angle x GS/60 x 6080

A Rule of Thumb for 3 deg glide slope

Multiply your GS by 5 and that will give a rough rate of descent.


Half the speed and add a Zero

e.g. If GS is 140 then

140/2 = 70 and add a zero (i.e. multiplying by 10) to make it 700 ft/min.

If glide slope is not 3 deg

For every 0.25 of a degree difference above or below the standard 3 deg glide slope, add or subtract respectively 10kts to your Ground speed before using it in the rule of thumb calculation.

e.g. If the GS is 140 and glide slope is 3.5 deg then GS to be used for rule of thumb calculation is 140 + 20 = 160.

Arrow With respect to en-route diversions (using drift down graph), if you believe that you will not clear an obstacle do you must:

a) assess remaining fuel requirements, then jettison fuel as soon as possible <-- Correct
b) jettison fuel from the beginning of the drift down

Arrow (Refer to CAP 698 figure 4-24)Why does the curve for an equivalent weight of 35000 kg, only start 4 mins after engine failure?

a) At that weight the aircraft takes longer to slowdown to the optimum drift down speed <-- Correct
b) At that weight the aircraft has a higher TAS and therefore more momentum
c) At that altitude the engine takes longer to spool down after failure

Arrow When in a gliding manoeuvre, in order to achieve maximum endurance the aircraft should be flown at:

a) the speed for min. power <-- Correct
b) the speed for max. lift
c) the speed for min. drag
d) the speed for max. lift/drag

Arrow In a twin engined jet aircraft with six passenger seats, and a maximum certified take off mass of 5650 kg. What is the required en-route obstacle clearance, with one engine inoperative during drift down towards the alternate airport?

a) 2000 ft <-- Correct
b) 1000 ft

When stabilised in a level flight, the minimum clearance en-route is 1,000 feet with a positive gradient, 5nm either side of track. Regarding clearance during the drift down the requirement is that the aircraft should not descend below the minimum altitude that gives 2,000 feet clearance 5nm either side of track.

Arrow An aeroplane carries out a descent from FL 410 to FL 270 at cruise Mach number, and from FL 270 to FL 100 at the IAS reached at FL 270. How does the angle of descent change in the first and in the second part of the descent? Assume idle thrust and clean configuration and ignore compressibility effects.

"Increases in the first part; is constant in the second"

Descent angle = D - T / W

With idle thrust and Lift almost equal to weight

Descent angle = D / L

Thus Increase in Drag will increase the descent angle

At constant mach descent IAS and TAS are increasing which inceases the total drag (due to profile drag)

So in the first part angle increases.

In the second part we are descending at constant IAS, so the drag remains whatever it is unless we increase speed.

So in the second part angle remains the same as long as IAS is held constant.

Arrow During a descent at constant Mach number, the margin to low speed buffet will:

a) increase, because the lift coefficient decreases <-- Correct
b) increase, because the lift coefficient increases

Air speed increases - AoA decreases - CL decreases (see the next question)

Arrow The lift coefficient decreases during a glide with constant Mach number, mainly because the:

a) IAS increases <-- Correct
b) glide angle increases
c) aircraft mass decreases
d) TAS decreases

Arrow When gliding into a headwind airspeed should be: "higher than the max. range glide speed in still air"

Arrow During a glide at constant Mach number, the pitch angle of the aeroplane will:

a) decrease <-- Correct
b) increase at first and decrease later on
c) increase
d) remain constant

Pitch angle (Theta) is the angle between the aircraft's longitudinal axis and the horizontal. An element of that pitch angle is alpha. As we descend at a constant Mach the IAS increases which means alpha must be decreased, therefore a more nose down attitude, which means the angle between the longitudinal axis and the horizontal has increased- an increase in pitch angle.

Now the interpretation. If we start from the cruise pitch attitude of say 2-3 degrees and enter the descent we put the nose down decreasing the pitch. As the nose passes through the horizon the pitch angle starts to increase again so we've gone from decreasing to increasing.

It can be decreasing in a sense that going from e.g. +2 to -4 is a Decrease.

Source: (http://www.atpforum.eu/showthread.php?t=9057)

Arrow Which of the following will not increase the minimum glide angle relative to the ground:

a) increased weight <-- Correct
b) lowering the landing gear
c) increased flap angle
d) increased headwind

Headwind will increase the angle "relative to the ground" not otherwise.
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