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Title249497952-AircraftPerformance-Keith-Williams.pdf
TagsAtmospheric Pressure Drag (Physics) Lift (Force) Wing Altitude
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Page 1

1000 Questions
ANSWERS MID EXPLANATIONS

FOR
JAR IICPL AND CPL

AIRCRAFT PERFORMAWOE

Page 220

21 2 Atmospheres

This simplifies to give 4890 = (30 x (1 01 3 - QFE))

This can be rearranged to give:

Which is -850 = - QFE

So QFE = 850 hPa.

ATMOS 21. d.
Pressure altitude is the altitude at which the existing air pressure would occur in the international standard
atmosphere. It is the altimeter indication when 1013.25 hPa is set on the altimeter sub-scale.

ATMOS 22. a.
Density is a measure of how tightiy packed the molecules of a material are, or how much mass of a
material can be held in a given volume of space. lncreasing altitude causes air pressure to decrease.
Decreasing air pressure causes air to expand such that it is less tightly packed into a given volume. This
decreases density. lncreasing temperature causes air to expand, reducing the mass that can be contained
within a given volume. This reduces density. Water vapour is less dense than air so increasing humidity
causes density to decrease. Air density is therefore decreased by increasing humidity, increasing altitude
and increasing temperature.

ATMOS 23. c.
QNH is the ambient air pressure at mean sea level. QFE is the ambient air pressure at the airfield elevation.
The two are related in that any increase in one results in an equal increase in the other. Option d is
therefore incorrect in that an increase in QNH would increase QFE. The elevation of an airfield is its
vertical distance above mean sea level. Elevation does not change with varying atmospheric condition.
Option c is therefore correct.

ATMOS 24. c.
QNH is the ambient air pressure at mean sea level. QFE is the ambient air pressure at the airfield elevation.
The two are related in that any increase in one results in an equal increase in the other. Option d is
therefore incorrect in that an increase in QNH would increase QFE. Option b is incorrect because a
change in QFE must cause a change in QNH. The elevation of an airfield is its vertical distance above
mean sea level. Elevation does not change with varying atmospheric condition. Option a is therefore
incorrect. Option c, increase QNH, is therefore correct.

ATMOS 25. a.
EAS is the Equivalent Airspeed. The movement of an aircraft through the air causes the air pressure close
to it to increase. At low speeds this increased pressure is not significant but as speed increases the
magnitude of the pressure rise increases. At high speed the pressure rise is sufficient to compress the air
slightly, thereby increasing the dynamic pressure. This process is called adiabatic compression. If left
uncorrected this would cause the airspeed indicator to become progressively less accurate, over indicating
as speed increases. The EAS is the Calibrated Airspeed corrected for the effects of this adiabatic
compression of the air.

ATMOS 26. c.
An airspeed indicator gives an indication proportional to dynamic pressure. That is to say the indication
will always be the same for any given dynamic pressuie, so climbing at constant IAS or CAS means

Page 221

Atmospheres 21 3

climbing at constant dynamic pressure. But dynamic pressure is equal to %pV2, where p is air density and
V is TAS. So when climbing at constant IAS, the value of TAS must increase to compensate for decreasing
density, in order to maintain a constant dynamic pressure and airspeed indication. TAS therefore increases
as altitude increases in a constant IAS climb.

ATMOS 27. d.
Mach number is the speed of an aircraft as a fraction of the local speed of sound. Mach 1 for example
means a speed equal to the local speed of sound, whereas mach 0.5 means a speed only half of the local
speed of sound. The speed of sound in the atmosphere is not constant but is proportional to the square
root of absolute temperature (LSS = 38.94 TM Absolute temperature ). This means that if air temperature
decreases, the local speed of sound also decreases. But if the local speed of sound decreases, then the
TAS equating to any given mach number also decreases. Decreasing temperature therefore causes the
TAS at any given mach number to decrease. As pressure altitude increases up to the tropopause at 36000
ft, the air temperature decreases at a rate of approximately 1.98 degrees per 1000 ft. Above 36000 ft the
temperature remains constant. This means that as pressure altitude increases up to 36000 ft, the local
speed of sound decreases, then remains constant at higher altitudes. So when climbing at constant Mach
number TAS will decrease up to 36000 ft, then remain constant.

ATMOS 28. d.
As pressure altitude increases up to the tropopause at 36000 ft, the air temperature decreases at a rate of
approximately 1.98 degrees per 1300 ft. Above 36000 ft the temperature remains constant. So as altitude
increases the temperature decreases up to 36000 ft then remains constant above this altitude.

ATMOS 29. d.
An airspeed indicator gives an indication (IAS) proportional to dynamic pressure. That is to say the indication
will always be the same for any given dynamic pressure, so descending at constant IAS means descending
at constant dynamic pressure. Airspeed indicators are susceptible to individual instrument errors and
errors due to minor inaccuracies in the pressure sensing systems. The Calibrated Airspeed (CAS) is the
IAS corrected for these minor errors. So descending at constant IAS or CAS means descending at constant
dynamic pressure.

ATMOS 30. a.
Density altitude is the altitude at which the prevailing air density would occur in the International Standard
Atmosphere (ISA). As altitude increases, the air pressure decreases allowing the air to expand. This
expansion causes density to decrease, so density decreases with increasing altitude. This means that
anything which causes the density to decrease causes the density altitude to increase. Increasing air
terr~perature also causes the air to expand, thereby reducing its density and increasing the density altitude.
So increasing temperature decreases density but increases density altitude.

ATMOS 31. a.
In an inversion the normal relationship between temperature and altitude is reversed, so descending
causes temperature to decrease. The local speed of sound (LSS) is determifled by temperature alone, so
decreasing temperature causes the local speed of sound to decrease. Mach number represents TAS as
a fraction of LSS, so as the LSS decreases, the Mach number at any given TAS increases. So when
descending through an inversion at any given TAS, the temperature and LSS decrease, causing the mach
number to increase. This is the opposite to what happens in a normal atmosphere. So option a is correct
and options b and c are incorrect.

Page 439

Landing 431

LAND 53. d.
The weight, altitude, temperature (WAT) limited !anding mass is the maximum mass at which an aircraft
can achieve the minimum acceptable climb gradient in the event of a decision to abort a landing and go-
around again. JAR 25.121 specifies that this minimum gradient is 2.1% for a twin engine class A aircraft.
But this is increased to 2.5% or the published aerodrome minimum gradient, whichever is greater, when
conducting instrument approaches with decision heights below 200 ft.

LAND 54. c.
An Airbus A320 is a twin engine class a aircraft. JAR 25.121 specifies that this minimum gradient is 2.1%
for a twin engine class A aircraft. But this is increased to 2.5% or the published aerodrome minimum
gradient, whichever is greater, when conducting instrument approaches with decision heights below 200
ft. JAR 25.1.510 states that if this gradient cannot be achieved, the decision height must be increased to
at least 200 ft.

LAND 55. a.
When gusty or turbulent conditions require an increase in VAT, the LDR must be recalculated. The new
LDR can be calculated for changes in VAT using the following equation:

New LDR = Old LDR x (VAT2 I (VREF 9 7)2)

This gives New LDR = 4000 m x ( 130' 1 (1 10 9 7)2)

Which is New LDR = 4939 m.

LAND 56. b
JAA regulations define a steep approach as one in the descent gradient is 4.5O or greater.

LAND 57. c.
JAR OPS 1.51 5 states that if an operator is unable to comply with the forecast wind requirements at the
destination field. but can comply with the still air requirements, the aircraft can still be despatched provided
an alternative field is available at which all of the landing requirements can be met.

I

, LAND 58. c.

I
The measured landing distance is horizontal distance between an aircraft actually attaining screen height
and the point at which it comes to a full stop when landing on a hard dry surface. This distance is then

I
I multiplied by suitable factors to take account of actual conditions when calculating the landing distances
I required (LDR) for any given field.

I
LAND 59. b.
JAR OPS 1.550 states that for a class b aircraft using a wet grass surface, the LD must be not more than
50.7% of the LDA. This means that the LDA must be 1.97 times the LD. Questions of this type can be
solved by using the inverse function on a pocket calculator. That is to say 11.507 is 1.97.

LAND 60. c.
A declared safe area is an area free of obstructions immediately before the threshold. Its maximum
permitted length is 90 metres, minimum width is twice that of the runway, and between 5% upslope and
2% downslope. It is required for short landings, when the screen is considered to be at the beginning of
the declared safe area.

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