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ATPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) practice questions

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Question 1The Atmosphere

In the ICAO Standard Atmosphere, what are the sea-level pressure and temperature values used as the reference datum?

  1. 1000 hPa and 15 °C
  2. 1013.25 hPa and 0 °C
  3. 1013.25 hPa and 15 °C Correct answer.
  4. 1013.25 mb and 20 °C

Explanation

The ICAO Standard Atmosphere fixes mean sea level pressure at 1013.25 hPa (29.92 in Hg) and temperature at 15 °C, with a density of 1.225 kg/m³. These values anchor all pressure-altitude and density-altitude computations used in aircraft performance.

Question 2The Atmosphere

What is the standard ISA temperature lapse rate in the troposphere?

  1. 6.5 °C per 1000 ft
  2. 1.98 °C per 1000 ft Correct answer.
  3. 3.0 °C per 1000 ft
  4. 2.0 °C per 1000 ft

Explanation

The ISA lapse rate is 6.5 °C per km, which converts to approximately 1.98 °C per 1000 ft (often rounded to 2 °C per 1000 ft). This gradient applies from sea level up to the tropopause at 11 km.

Question 3The Atmosphere

At what geopotential height does the ICAO Standard Atmosphere place the tropopause, and what is the temperature there?

  1. 11 km and −40 °C
  2. 16 km and −56.5 °C
  3. 8 km and −40 °C
  4. 11 km and −56.5 °C Correct answer.

Explanation

ISA places the tropopause at 11 km (about 36 090 ft), where the temperature has fallen to −56.5 °C. Above this level the model assumes an isothermal layer up to 20 km.

Question 4The Atmosphere

Which atmospheric layer contains the ozone layer that absorbs most solar ultraviolet radiation?

  1. The mesosphere
  2. The thermosphere
  3. The troposphere
  4. The stratosphere Correct answer.

Explanation

The ozone concentration peaks in the stratosphere, mainly between about 15 and 35 km, where UV absorption warms the layer and produces the temperature inversion. This warming is why temperature rises with height above the tropopause.

Question 5The Atmosphere

What is the primary reason temperature increases with height in the stratosphere?

  1. Absorption of ultraviolet radiation by ozone Correct answer.
  2. Compression of descending air
  3. Absorption of infrared radiation by water vapour
  4. Friction between air layers moving at different speeds

Explanation

Ozone in the stratosphere absorbs solar ultraviolet radiation and converts it to heat, so the layer warms with height and forms a stable inversion. This stability suppresses vertical motion and limits most weather to the troposphere below.

Question 6The Atmosphere

Which gas has the greatest proportional contribution to the greenhouse effect in the troposphere?

  1. Carbon dioxide
  2. Water vapour Correct answer.
  3. Argon
  4. Ozone

Explanation

Water vapour is the dominant greenhouse gas by contribution, absorbing and re-emitting long-wave radiation across broad infrared bands. Carbon dioxide is important and rising, but its total radiative contribution is smaller than that of water vapour.

Question 7The Atmosphere

Approximately what proportion of the atmosphere's total mass lies below 5500 m (18 000 ft)?

  1. About one half Correct answer.
  2. About one tenth
  3. About three quarters
  4. About one quarter

Explanation

Because density decreases roughly exponentially with height, about half the mass of the atmosphere lies below 5500 m. This is why most weather, moisture and aircraft operations are concentrated in the lowest few kilometres.

Question 8The Atmosphere

What is the approximate composition of dry air by volume?

  1. 50 % nitrogen, 50 % oxygen with traces of other gases
  2. 21 % nitrogen, 78 % oxygen, 1 % other gases
  3. 78 % oxygen, 21 % nitrogen, 1 % argon
  4. 78 % nitrogen, 21 % oxygen, 1 % other gases Correct answer.

Explanation

Dry air is about 78 % nitrogen and 21 % oxygen by volume, with the remaining 1 % dominated by argon and including carbon dioxide and trace gases. The composition stays essentially constant up to the homosphere.

Question 9The Atmosphere

An aircraft reports a static pressure of 900 hPa. Using the standard atmosphere, what is the approximate pressure altitude?

  1. About 9000 ft
  2. About 5000 ft
  3. About 1000 ft
  4. About 3000 ft Correct answer.

Explanation

In the lower atmosphere pressure falls by about 1 hPa per 30 ft, so a 113 hPa deficit from 1013 hPa corresponds to roughly 3400 ft; the standard low-level table gives about 3000 ft for 900 hPa. Pressure altitude is the altitude indicated when the subscale is set to 1013.25 hPa.

Question 10The Atmosphere

Which statement correctly describes the relationship between density altitude and aircraft performance?

  1. A high density altitude increases engine power but reduces lift
  2. A high density altitude increases lift but reduces engine power
  3. A high density altitude reduces both engine power and lift for a given indicated airspeed Correct answer.
  4. A high density altitude has no effect on performance provided pressure altitude is constant

Explanation

Density altitude is the altitude in the standard atmosphere corresponding to the actual air density. When air is less dense, both aerodynamic lift and normally aspirated engine power decrease, lengthening take-off runs and reducing climb performance.

Question 11The Atmosphere

Two aerodromes are at the same pressure altitude, but one is hot and humid. What is the effect on density altitude at the hot, humid aerodrome?

  1. Density altitude is higher than at the cooler, drier aerodrome Correct answer.
  2. Density altitude is lower than at the cooler, drier aerodrome
  3. Density altitude is the same because pressure altitude is identical
  4. Density altitude is unaffected by humidity

Explanation

Warm air and water vapour both lower air density, so at the same pressure altitude the hot, humid site has a higher density altitude. This degrades take-off and climb performance compared with the cooler, drier aerodrome.

Question 12The Atmosphere

Which instrument directly measures the difference between static pressure and total pressure to indicate airspeed?

  1. The Machmeter
  2. The altimeter
  3. The airspeed indicator Correct answer.
  4. The vertical speed indicator

Explanation

The airspeed indicator receives pitot (total) pressure and static pressure and displays the dynamic pressure difference as indicated airspeed. The altimeter uses static pressure alone, and the VSI uses a restricted static feed to sense rate of change.

Question 13The Atmosphere

What does the term 'QNH' represent when set on an altimeter subscale?

  1. The pressure at mean sea level with the altimeter reading zero on the ground
  2. The standard pressure of 1013.25 hPa
  3. The pressure at the aircraft's flight level datum
  4. The pressure that makes the altimeter read aerodrome elevation when on the ground Correct answer.

Explanation

QNH is the altimeter setting that causes the instrument to indicate elevation above mean sea level when the aircraft is on the ground. It is derived from station pressure reduced to sea level using the ISA.

Question 14The Atmosphere

An aircraft flies from a region of high pressure into a region of low pressure without resetting the altimeter, which remains on a constant subscale. What happens to the true altitude?

  1. True altitude equals indicated altitude
  2. The aircraft is lower than the altimeter indicates Correct answer.
  3. True altitude increases while indicated altitude falls
  4. The aircraft is higher than the altimeter indicates

Explanation

When flying from high to low pressure at a constant indicated altitude, the true altitude decreases because the pressure surfaces slope downward toward the low. The altimeter over-reads, so the aircraft is lower than shown.

Question 15The Atmosphere

How does a decrease in atmospheric pressure affect the indicated altitude of an aircraft whose altimeter subscale is unchanged?

  1. Indicated altitude decreases for the same true altitude
  2. Indicated altitude oscillates
  3. Indicated altitude increases for the same true altitude Correct answer.
  4. Indicated altitude is unaffected

Explanation

The altimeter converts static pressure into altitude assuming a fixed pressure datum. If the ambient pressure falls while the aircraft maintains the same true altitude, the instrument interprets the lower pressure as a higher altitude, so it over-reads.

Question 16The Atmosphere

What is the primary cause of the diurnal variation in surface pressure at low latitudes?

  1. Atmospheric tides driven by solar heating Correct answer.
  2. Passage of mid-latitude frontal systems
  3. Long-term changes in atmospheric carbon dioxide
  4. Variation in the Earth's orbital distance from the Sun

Explanation

At low latitudes there is a regular semi-diurnal pressure oscillation with maxima near 10 and 22 local time, caused by atmospheric tides produced by solar heating. The amplitude is small, typically a few hectopascals.

Question 17The Atmosphere

On a synoptic chart, which pressure system is associated with descending air, subsidence and generally clear skies?

  1. A frontal trough
  2. A tropical cyclone
  3. A depression
  4. A high-pressure anticyclone Correct answer.

Explanation

Anticyclones are regions of high pressure with descending air aloft, which suppresses cloud formation and gives generally fine weather. Subsidence can produce low-level inversions and haze or fog rather than deep cloud.

Question 18The Atmosphere

What causes the formation of a temperature inversion in a valley on a clear, calm winter night?

  1. Latent heat release from condensation
  2. Advection of warm air over a cold surface
  3. Subsidence warming from a high-pressure system aloft
  4. Radiational cooling of the air near the surface and drainage of cold air downslope Correct answer.

Explanation

Clear skies allow strong radiational cooling of the ground, and the air in contact cools, becoming denser and draining into the valley. The result is a shallow layer of cold air trapped beneath warmer air, forming an inversion.

Question 19The Atmosphere

Which statement best describes the variation of the tropopause height with latitude?

  1. It is constant with latitude
  2. It is highest in the tropics and lowest near the poles Correct answer.
  3. It is highest at mid-latitudes and lowest at the equator
  4. It is highest near the poles and lowest in the tropics

Explanation

Tropopause height is greatest over the tropics, around 16 to 18 km, and lowest near the poles, around 8 to 10 km, because the tropical troposphere is warmer and more deeply unstable. The tropopause also tends to be discontinuous at jet-stream latitudes.

Question 20The Atmosphere

What is the significance of the tropopause for commercial jet operations?

  1. It is the level at which engine thrust is greatest
  2. It marks the level above which temperature is nearly constant and vertical motion is suppressed Correct answer.
  3. It is the boundary below which all icing occurs
  4. It marks the level where wind speed always reaches zero

Explanation

Above the tropopause the ISA assumes an isothermal layer and the air is highly stable, so vertical motion and most weather are suppressed. This is why cruising levels are often chosen near or just above the tropopause, subject to turbulence and wind considerations.

Question 21The Atmosphere

Which process transfers heat through the atmosphere by the horizontal movement of air masses?

  1. Radiation
  2. Convection
  3. Conduction
  4. Advection Correct answer.

Explanation

Advection is the horizontal transport of heat by moving air, such as warm air being carried poleward. Conduction requires contact, radiation travels without a medium, and convection is vertical motion.

Question 22The Atmosphere

How does the specific heat capacity of water compare with that of dry air, and what is the consequence for coastal climates?

  1. Water and air have equal specific heat capacities, so there is no coastal effect
  2. Water has a higher specific heat capacity, so coastal areas have larger temperature ranges
  3. Water has a lower specific heat capacity, so coastal areas have larger temperature ranges
  4. Water has a higher specific heat capacity, so coastal areas have smaller temperature ranges Correct answer.

Explanation

Water's specific heat capacity is much greater than that of dry air, so the sea warms and cools more slowly than land. Coastal regions therefore experience smaller diurnal and seasonal temperature ranges than inland areas.

Question 23The Atmosphere

In the ISA, what is the approximate temperature at 20 000 ft?

  1. −25 °C Correct answer.
  2. −45 °C
  3. −5 °C
  4. −60 °C

Explanation

Starting from 15 °C at sea level and applying 1.98 °C per 1000 ft, 20 000 ft gives a drop of about 39.6 °C, so the ISA temperature is approximately −25 °C. The tropopause isothermal layer begins near 36 000 ft.

Question 24The Atmosphere

What is the main effect of atmospheric humidity on air density at a fixed pressure and temperature?

  1. Moist air is less dense than dry air because water vapour has a lower molecular mass Correct answer.
  2. Humidity has no measurable effect on density
  3. Moist air is denser than dry air because water vapour adds mass
  4. Moist air is denser because water vapour condenses

Explanation

Water vapour molecules have a lower molecular mass than the nitrogen and oxygen they displace, so moist air is less dense than dry air at the same pressure and temperature. This contributes to higher density altitude in humid conditions.

Question 25The Atmosphere

Which statement about the standard atmosphere's isothermal layer above 11 km is correct?

  1. Temperature rises steadily with height
  2. Temperature continues to fall at 1.98 °C per 1000 ft
  3. Temperature remains constant at about −56.5 °C while pressure continues to decrease Correct answer.
  4. Pressure remains constant while temperature falls

Explanation

In the ISA the layer from 11 km to 20 km is isothermal at about −56.5 °C, so temperature stops falling while pressure continues to decrease with height. This is the model used for pressure-altitude computations at high cruise levels.

Question 26The Atmosphere

In the International Standard Atmosphere (ISA), what is the assumed sea-level temperature and pressure?

  1. 15°C and 1000 hPa
  2. 0°C and 1000 hPa
  3. 15°C and 1013.25 hPa Correct answer.
  4. 20°C and 1013.25 hPa

Explanation

The ISA defines mean sea level conditions as a temperature of 15°C and a pressure of 1013.25 hPa. These values form the reference for all standard atmosphere calculations. Other values such as 0°C or 1000 hPa are not part of the ISA definition.

Question 27The Atmosphere

What is the approximate height of the troposphere at the equator compared with the poles?

  1. About 8 km at the equator and 16 km at the poles
  2. About 20 km at the equator and 12 km at the poles
  3. About 16 km at the equator and 8 km at the poles Correct answer.
  4. About 12 km at both the equator and the poles

Explanation

The troposphere is thicker at the equator because of stronger convective heating and a higher tropopause, typically reaching 16–18 km. At the poles, cold dense air and weaker convection result in a tropopause around 8 km. This variation is a key feature of the general circulation.

Question 28The Atmosphere

Which atmospheric layer is characterised by a temperature increase with height and contains the ozone layer?

  1. Mesosphere
  2. Troposphere
  3. Stratosphere Correct answer.
  4. Thermosphere

Explanation

The stratosphere lies above the troposphere and is marked by a temperature inversion due to ozone absorbing ultraviolet radiation. This absorption causes temperature to rise with height, making the stratosphere stable and suitable for high-altitude flight. The ozone layer is concentrated in the lower stratosphere.

Question 29The Atmosphere

What is the primary cause of the decrease in pressure with altitude in the lower atmosphere?

  1. The decrease in gravitational acceleration with height
  2. The decrease in air density with height Correct answer.
  3. The decrease in the proportion of oxygen with height
  4. The increase in temperature with height

Explanation

Pressure at any level is the weight of the air above it. As altitude increases, the density of air decreases because the air is compressed less by the overlying weight, so each unit volume weighs less. This reduction in density is the main reason pressure falls with height. Gravity changes little over the troposphere, and oxygen proportion is essentially constant.

Question 30The Atmosphere

In the ISA, what is the approximate temperature at 25 000 ft?

  1. −55°C
  2. −35°C Correct answer.
  3. −20°C
  4. −45°C

Explanation

The ISA temperature decreases at a lapse rate of about 1.98°C per 1000 ft up to 36 090 ft. Starting from 15°C at sea level, at 25 000 ft the temperature is approximately 15 − (25 × 1.98) = −34.5°C, which rounds to −35°C. This is a standard value used in performance calculations.

Question 31The Atmosphere

An aircraft is flying at a pressure altitude of 10 000 ft where the outside air temperature is −20°C. What is the approximate true altitude if the altimeter subscale is set to 1013 hPa and the terrain elevation is 0 ft?

  1. About 9 000 ft
  2. About 10 000 ft
  3. About 9 300 ft Correct answer.
  4. About 10 700 ft

Explanation

True altitude differs from pressure altitude when the actual temperature deviates from ISA. At 10 000 ft the ISA temperature is about −5°C, so the air is colder than standard. Cold air is denser, so the aircraft is lower than indicated; the true altitude is approximately 9 300 ft. The correction is roughly 4% per 10°C deviation, giving about 700 ft lower.

Question 32The Atmosphere

Which statement correctly describes the effect of high humidity on the density altitude at a given pressure altitude and temperature?

  1. Humidity increases density altitude because water vapour is less dense than dry air Correct answer.
  2. Humidity decreases density altitude because water vapour is more dense than dry air
  3. Humidity has no effect on density altitude
  4. Humidity increases density altitude only at temperatures below 0°C

Explanation

Water vapour molecules have a lower molecular mass than the average dry air molecules they displace. Therefore, moist air is less dense than dry air at the same pressure and temperature. This reduces aircraft performance and raises the density altitude, which is why humid conditions are a performance consideration.

Question 33The Atmosphere

What is the primary reason for the formation of the tropopause?

  1. The decrease in gravitational force
  2. The increase in ozone concentration
  3. The cessation of convective overturning due to a temperature inversion Correct answer.
  4. The increase in pressure with height

Explanation

The tropopause marks the boundary where the lapse rate becomes very small or reverses. In the troposphere, convection and mixing are driven by heating from below, but at the tropopause the temperature is so low that the air becomes stable and further convection is inhibited. This is primarily due to the absorption of solar radiation by ozone in the stratosphere creating a temperature inversion.

Question 34The Atmosphere

In the ISA, what is the approximate pressure at 18 000 ft?

  1. 700 hPa
  2. 600 hPa
  3. 800 hPa
  4. 500 hPa Correct answer.

Explanation

Using the standard atmosphere, pressure decreases roughly exponentially with height. At 18 000 ft (about 5 500 m), the pressure is approximately half of the sea-level value, i.e. around 500 hPa. This is a useful rule of thumb for flight levels.

Question 35The Atmosphere

Which of the following is the correct order of atmospheric layers from the surface upwards?

  1. Troposphere, mesosphere, stratosphere, thermosphere
  2. Troposphere, stratosphere, mesosphere, thermosphere Correct answer.
  3. Troposphere, stratosphere, thermosphere, mesosphere
  4. Stratosphere, troposphere, mesosphere, thermosphere

Explanation

The atmosphere is divided into layers based on temperature profile: troposphere, stratosphere, mesosphere, and thermosphere. The troposphere is where most weather occurs, the stratosphere contains the ozone layer, the mesosphere is where most meteors burn up, and the thermosphere is the outermost layer. This order is fundamental to understanding atmospheric structure.

Question 36The Atmosphere

What is the environmental lapse rate if the temperature at 2 000 ft is 10°C and at 10 000 ft is −14°C?

  1. 2.5°C per 1 000 ft
  2. 2.0°C per 1 000 ft
  3. 1.5°C per 1 000 ft
  4. 3.0°C per 1 000 ft Correct answer.

Explanation

The temperature difference is 10 − (−14) = 24°C over a height difference of 8 000 ft. Dividing 24 by 8 gives 3°C per 1 000 ft. This is the environmental lapse rate, which is steeper than the ISA lapse rate of about 2°C per 1 000 ft.

Question 37The Atmosphere

Which process is primarily responsible for the transfer of heat from the Earth's surface to the lower atmosphere during the day?

  1. Radiation
  2. Conduction
  3. Advection
  4. Convection Correct answer.

Explanation

The Earth's surface absorbs solar radiation and warms the air immediately above it by conduction. This warm air becomes buoyant and rises, transferring heat upward through convection. Convection is the dominant mechanism for vertical heat transport in the troposphere during daytime heating.

Question 38The Atmosphere

How does the height of the tropopause typically change when a deep low-pressure system approaches?

  1. The tropopause rises because the low is associated with ascending air and a thicker troposphere Correct answer.
  2. The tropopause disappears in the centre of a low
  3. The tropopause lowers because the cold air in the low is more dense
  4. The tropopause remains constant regardless of pressure systems

Explanation

A low-pressure system is associated with convergence and ascending air, which leads to a thicker troposphere and a higher tropopause. In contrast, a high-pressure system with subsidence has a lower tropopause. This relationship is important for understanding jet stream location and clear air turbulence.

Question 39The Atmosphere

What is the approximate temperature at the tropopause in the ISA?

  1. −65.0°C
  2. −45.0°C
  3. −50.0°C
  4. −56.5°C Correct answer.

Explanation

In the ISA, the tropopause is at 11 km (36 090 ft) where the temperature is −56.5°C. Above this, the stratosphere is isothermal up to 20 km. This value is a standard reference for high-altitude temperature calculations.

Question 40The Atmosphere

Which statement best explains why the atmospheric pressure at the top of Mount Everest (about 9 000 m) is approximately one-third of sea-level pressure?

  1. Because the proportion of oxygen is lower
  2. Because gravity is weaker at that altitude
  3. Because the temperature at that altitude is much lower
  4. Because about two-thirds of the atmosphere's mass lies below that level Correct answer.

Explanation

Pressure is the weight of the air above. Because air is compressible, its density decreases with height, and a large fraction of the atmosphere's mass is concentrated in the lower layers. Approximately two-thirds of the mass lies below 9 000 m, so the pressure there is about one-third of sea-level pressure.

Question 41Pressure, Density and Altimetry

In the ICAO Standard Atmosphere, what are the sea-level pressure and temperature values on which altimeter calibration is based?

  1. 1013.25 hPa and 15 °C Correct answer.
  2. 1013.25 hPa and 0 °C
  3. 1013.25 hPa and 20 °C
  4. 1000.00 hPa and 15 °C

Explanation

The ICAO Standard Atmosphere fixes mean sea-level pressure at 1013.25 hPa and mean sea-level temperature at 15 °C, with a standard lapse rate of 1.98 °C per 1000 ft. All pressure-altimeter graduations and density calculations used in aircraft performance are referenced to these values.

Question 42Pressure, Density and Altimetry

An aircraft on the ground at an elevation of 2000 ft reports a QNH of 1013 hPa. What does the altimeter read when set to 1013 hPa?

  1. 0 ft
  2. 2000 ft Correct answer.
  3. Approximately 2000 ft plus the standard correction for temperature
  4. The pressure altitude of the station

Explanation

QNH is the pressure reduced to mean sea level using the standard atmosphere, so an altimeter set to QNH indicates elevation above mean sea level. With QNH equal to 1013 hPa, the subscale matches the standard datum and the altimeter reads field elevation, 2000 ft.

Question 43Pressure, Density and Altimetry

Which pressure setting causes the altimeter to indicate height above the aerodrome reference point?

  1. QFF
  2. QFE Correct answer.
  3. QNH
  4. QNE

Explanation

QFE is the atmospheric pressure at aerodrome elevation, so an altimeter set to QFE reads zero on the ground and indicates height above the aerodrome datum. QNH, by contrast, produces an indication of elevation above mean sea level.

Question 44Pressure, Density and Altimetry

An aircraft flies from a region of high pressure toward a region of low pressure without changing the altimeter subscale. If the aircraft maintains a constant indicated altitude, what happens to its true altitude?

  1. It remains equal to the indicated altitude
  2. It increases above the indicated altitude
  3. It increases only if temperature also increases
  4. It decreases below the indicated altitude Correct answer.

Explanation

The altimeter measures pressure, not geometric height, so when actual pressure falls the aircraft must descend to keep the same indicated altitude. True altitude therefore becomes lower than the indicated value, a hazard known as flying from high to low.

Question 45Pressure, Density and Altimetry

What is the name given to the pressure level in the atmosphere at which the pressure is 1013.25 hPa?

  1. Standard isobaric level Correct answer.
  2. Transition level
  3. Density altitude
  4. Pressure altitude datum

Explanation

The 1013.25 hPa surface is the standard isobaric level used as the reference for pressure altitude. Setting the subscale to this value makes the altimeter read pressure altitude directly.

Question 46Pressure, Density and Altimetry

Which quantity does a pressure altimeter actually measure?

  1. Static pressure at the aircraft Correct answer.
  2. Rate of change of static pressure
  3. Difference between static and total pressure
  4. Total pressure at the aircraft

Explanation

The altimeter is an aneroid barometer connected to the static source, so it responds to ambient static pressure and converts it to altitude using the standard atmosphere. Total pressure and the static-to-total difference are used by the airspeed indicator, not the altimeter.

Question 47Pressure, Density and Altimetry

As altitude increases in the troposphere, how do air density and pressure behave?

  1. Pressure decreases while density remains constant
  2. Both decrease Correct answer.
  3. Density decreases while pressure remains constant
  4. Both increase

Explanation

Both pressure and density fall with height because the weight of overlying air and the compression of the air column decrease. Density decreases slightly more slowly than pressure in the lower atmosphere because temperature also falls.

Question 48Pressure, Density and Altimetry

What does the term density altitude describe?

  1. The pressure altitude corrected for temperature deviation from standard Correct answer.
  2. The altitude at which the aircraft reaches the tropopause
  3. The height of the aircraft above the aerodrome
  4. The altitude indicated on the altimeter with QNH set

Explanation

Density altitude is the altitude in the standard atmosphere at which the observed air density would occur. It is obtained by correcting pressure altitude for the difference between actual and standard temperature, and it governs aerodynamic and engine performance.

Question 49Pressure, Density and Altimetry

An aerodrome at 3000 ft elevation reports QNH 1025 hPa. Another reports QNH 1000 hPa at the same elevation. In which direction is the altimeter error when set to 1013 hPa?

  1. It under-reads at the high-pressure field and over-reads at the low-pressure field Correct answer.
  2. It under-reads at both fields
  3. It over-reads at both fields
  4. It over-reads at the high-pressure field and under-reads at the low-pressure field

Explanation

With 1013 hPa set, the altimeter treats the standard pressure as the datum. Where actual QNH is higher than 1013 hPa, the true elevation is higher than the indication, so the altimeter under-reads; where QNH is lower, it over-reads.

Question 50Pressure, Density and Altimetry

An aircraft is cleared to maintain flight level 100. What does this clearance specify?

  1. Pressure altitude with 1013.25 hPa set Correct answer.
  2. Height above the aerodrome with QFE set
  3. True altitude above the terrain
  4. Altitude above mean sea level with QNH set

Explanation

A flight level is defined as pressure altitude in hundreds of feet with the subscale set to the standard pressure of 1013.25 hPa. FL 100 therefore corresponds to 10 000 ft on the standard pressure datum, not to a true or indicated altitude above sea level.