Chap A: Atmosphere

Question 1

List the elements that make up the atmosphere

Answer 1

N₂ (78.1%)
O₂ (20.9%)
Ar (0.9%)

Question 2

name the 4 regions of atmosphere

Answer 2

1) Troposphere
2) Stratosphere
3) Mesosphere
4) Thermosphere

Question 3

which regions consist of the lower, middle and upper atmosphere?

Answer 3

lower: troposphere & tropopause

middle: stratosphere & mesopause

upper: thermosphere

Question 4

troposphere description

Answer 4

altitude of 8km (N/S poles) - 14.5km (Equator) ground up

highest density and pressure of all regions

air temp. decreases with lapse rate of 6.5°C per km to around 217K

greatest weather and climate changes occur here

Question 5

stratosphere description

Answer 5

extends ~50km above Earth’s surface

air is less dense and drier compared to troposphere

temp. in first 10km is constant at 217K
temp. increases after due to absorption of solar UV rays by Ozone layer

Question 6

how thick is the stratosphere?

Answer 6

35km

Question 7

where does most of the weight of air contain?

Answer 7

99% of weight contain within troposphere and stratosphere

Question 8

Mesosphere description

Answer 8

thickness of 35km, extends to 85km above SL

temp. decreases, as low as 180K (coldest in atmosphere)

Question 9

Thermosphere description

Answer 9

extends for 515 - 600km above Earth’s surface

temp. goes up as altitude increases (due to Sun’s energy)
as high as 1727°C

Question 10

chemical states in mesosphere and thermosphere

Answer 10

mesosphere: excited state as they absorb energy from Sun

thermosphere: ionized state, reaction occurs much faster than on Earth’s surface

Question 11

what is after thermosphere?

Answer 11

exosphere: hydrogen and helium are present here at very low densities

Question 12

properties of atmosphere

Answer 12

1) invisibility
ISA @ sea level:
2) density - 1.225 kg/m³
3) pressure - 1.01325 bar
4) Temperature - 15°C

Question 13

flow visualisation methods

Answer 13

1) smoke generation
2) cotton tufts
3) vaporisation
4) oil streaks

Question 14

is dry air denser than wet air?

Answer 14

yes, as atomic weight of water is 18 whereas nitrogen and oxygen is 28 & 32.

Question 15

which property of air is the most important for flight?

Answer 15

density

Question 16

when higher temp or altitude, how will pressure change?

Answer 16

higher temp: higher pressure due to particles having higher energy

higher altitude: lesser pressure as lesser air on top (stacking effect)

Question 17

ways to measure pressure

Answer 17

barometer (mercury/aneroid)
U-Tube Manometer

Question 18

how does an aneroid barometer work?

Answer 18

consist of a flexible metal bellow (or capsule) that is filled with ISA SL air

any changes outside of bellow will change the thickness of capsule (bellow will expand or contract)

higher external P_atm: bellow contract
lower external P_atm: bellow expand

Question 19

what is absolute and gauge pressure?

Answer 19

P abs: P gauge + P atm
P gauge: pressure gauge reading

Question 20

what is geometric altitude?

Answer 20

the measure of true height above ground (tape measure)

use of radar altimeter is more practical

Question 21

why is geometric altitude important?

Answer 21

for aircraft within the airport control airspace as accurate altitude and speed is required for management of all aircraft within the same space by ATC

Question 22

why is pressure altitude used over temp/density altitude?

Answer 22

pressure is the easiest to measure during flight

Question 23

why do altimeters need to be reset after climbing to a certain altitude?

Answer 23

as aircraft is presumed to have a lot of airspace such that mid-air collision is rare due to inaccurate pressure altitude measurement

Question 24

what is ISA?

Answer 24

International Standard Atmosphere, the atmospheric table that is commonly accepted table by all which states that at SL:

P = 1.01325 bar
ρ = 1.225 kg/m³
T = 15°C

values are fixed and do not include variations due to weather/seasons etc.

Question 25

why is ISA relevant for aviation?

Answer 25

provides standard calculations for aircraft, instruments and performances

Question 26

density, temperature and pressure ratio symbols

Answer 26

density: σ
temperature: θ
pressure: δ

Question 27

define viscosity

Answer 27

viscosity is the measurement of resistance of one layer of air to movement over its adjacent layers

Question 28

what is viscosity dependent on and what can it do?

Answer 28

depends on temperature

can cause eddies (turbulence) when a body moves through air and disturbs it

Question 29

how does speed of sound change relative to altitude?

Answer 29

higher altitude, slower SOS

Question 30

2 uses of ISA in aviation

Answer 30

1) basis for comparison between flight conditions
2) standardisation for aircraft instrument and performance calculations