Pressure and Density

Question 1

Define atmospheric pressure.

Answer 1

Reported as the QNH in the METARs, is the total weight of the column of air above the point where the pressure is being measured.

Question 2

What weight does atmospheric pressure amount to?

Answer 2

Almost 1 kg/cm2 or about 10 T/m2.

Question 3

Why do everyday objects not collapse under the weight of atmospheric pressure?

Answer 3

Air pressure exerts its force in all directions. The air pressure pushing against a cardboard box from the outside is exactly balanced by the same force pushing from the inside.

Question 4

What do the following pressure units equate to?
Hecto
Pascal
Newton

Answer 4

Hecto = 100
Pascal = 1 Newton/m2
Newton = unit of force acting on 1 kg (initially stationary) to give velocity of 1 m/s after 1 s.

Question 5

What is the unit used to measure pressure in aviation?

Answer 5

The hectopascal (hPa).

Question 6

What is an altimeter?

Answer 6

An instrument which indicates the height of the aircraft above a set reference point by measuring pressure and converting it to a height reading.

Question 7

What is a vertical speed indicator?

Answer 7

An instrument which works by measuring changes in pressure as an aircraft climbs or descends and converts these to changes to vertical speeds.

Question 8

Why is air pressure significant to in aviation?

Answer 8

Air pressure is used in the measurement of height and vertical speed. This is essential, especially in mountainous regions.

Question 9

What is the key statement relating air pressure to height?

Answer 9

Air pressure must always decrease with increasing height.

Question 10

Explain what is meant by the pressure lapse rate.

Answer 10

Air is a fluid, compressed by the weight of the air above it. Therefore, it is densest and heaviest at the earth’s surface. As we ascend, the pressure must decrease. This is known as the pressure lapse rate.

Question 11

State the approximate pressure lapse rate below 10,000 ft.

Answer 11

A 1 hPa decrease in pressure occurs for approximately every 30 ft increase in height.

Question 12

Use the example of a ‘column of air’ to describe how an anticyclone forms.

Answer 12

If more fluids (air) are added into the top of the column, the tropopause height rises, the weight of air in the column increases and therefore so does surface pressure. This is how anticyclones form.

Question 13

Define QNH.

Answer 13

Aerodrome level pressure corrected to MSL using the ISA temp lapse rate. When set on the altimeter, the instrument will read the altitude of the aircraft above MSL.

Question 14

At what height is the circuit of an airfield conducted at?

Answer 14

Flight is conducted at aerodrome level plus 1000 ft plus rounding up to the nearest 100 ft e.g. the airfield elevation at Matamata (NZMA) is 182 ft. Circuits are then conducted at 1200 ft.

Question 15

Define QFE.

Answer 15

Aerodrome level pressure set on the altimeter. When QFE is selected, the altimeter will read the height of the aircraft above the aerodrome.

Question 16

What is QNH used for?

Answer 16

The majority of aviation is conducted in QNH. It is useful for cross-country flights in that the heights of mountain ranges are given in feet above mean sea level (AMSL) and the aircraft will be flying at an altitude above MSL.

Question 17

What is QFE used for?

Answer 17

QFE is used only by parachutists and pilots conducting low-level aerobatics. This is because, in both cases, it is imperative to know the true height above the ground without the need to apply for corrections.

Question 18

Define QNE.

Answer 18

QNE is a pressure of 1013.2 hPa set on the altimeter. With QNE dialled onto the altimeter, the instrument will read the pressure altitude or flight level of the aircraft.

Question 19

What is QNE used for?

Answer 19

It is used in high level flights. This prevents the need to keep updating the QNH values over long distances. With QNE set on the altimeter, the aircraft flies at an assigned flight level above the 1013 hPa level. As the height of the 1013 hPa isobar changes (due to changing surface pressure) the aircraft will climb or descend slowly over time. Aircraft will change true altitude at the same rate and in the same direction so separation is not compromised.

Question 20

What is the transition layer?

Answer 20

It is a separation layer designed to keep aircraft flying at low altitudes with QNH set on the altimeter separated from aircraft flying at higher levels with QNE set. The layer is comprised of the transition altitude and the transition level.

Question 21

Define transition altitude.

Answer 21

The transition altitude in NZ is 13000 ft AMLS. Aircraft flying at or below 13000 ft AMSL will have QNH set on the altimeter. When climbing through 13000 ft AMSL, the altimeter must be reset to QNE (1013 hPa).

Question 22

Define transition level.

Answer 22

The transition level in NZ is normally FL150. Aircraft flying at or above FL150 will have QNE set on the altimeter. When a flight descends through FL150, the altimeter is reset to the local QNH.

Question 23

What are the two exceptions which apply to the transition level?

Answer 23

The transition level becomes FL160 if the QNH zone setting is 980 hPa or less.
The transition level also becomes FL160 when operating in IMC within 20 nm of Mount Cook centred on 43°36’S 170°09’W.

Question 24

Is flight permitted in the transition layer?

Answer 24

No. However, there are a few exceptions to this rule (CAA rule 91.425).

Question 25

Explain the importance of correct subscale setting.

Answer 25

At constant indicated altitude, flight is along a constant pressure surface. If the current QNH is set, flight towards higher pressure results in the altimeter under-reading or over-reading if heading towards a lower pressure. A difference of 10 hPa will result in a 300 ft height discrepancy.
When flying from high to low, watch out below.

Question 26

Explain how temperature changes affect the pressure lapse rate.

Answer 26

If a column of air is heated (while surface pressure remains constant), the column stretches in the vertical due to thermal expansion. This has the effect of stretching the pressure lapse rate in the vertical as well.

Question 27

Does the pressure along a flight path generally change steadily or erratically and are there exceptions to this?

Answer 27

Change of pressure generally occurs steadily. However, sometimes local, small scale effects can result in quite large changes in pressure over relatively short distances.

Question 28

List the four phenomenon which can alter local pressure.

Answer 28

Lee trough.
Thermal or heat lows.
Thunderstorms.
Diurnal variation.

Question 29

Explain how a lee trough can alter local pressure.

Answer 29

Air moving up and over the terrain builds up an area of pressure on the windward side and a consequential deficit occurs on the leeward side (as much as 20 hPa). A strong wind flow will produce a greater effect as stronger winds indicate a stronger pressure gradient.

Question 30

Explain how a thermal or heat low can alter local pressure.

Answer 30

These can form anywhere on land due to radiation from the sun heating the earth and lower atmosphere. This heated air expands and becomes less dense and lighter. Can result in differences of 10 hPa between the coast and Central Otago and less through the NI’s Central Plateau.

Question 31

Explain how a thunderstorm can alter local pressure.

Answer 31

A thunderstorm can produce fluctuations of 2 to 3 hPa as they pass, created by the large density variations driven by the up-draughting and and down-draughting air.

Question 32

Explain how diurnal variation can alter local pressure.

Answer 32

As air is heated during the day, the local pressure falls, so that a pressure minimum occurs at around 1600 local. There is a secondary minimum at 0400 as the atmosphere has a natural period of oscillation of about 12 hrs (due to the ‘atmospheric tide’ which follows the sun around the globe). Pressure maximums occur at about 1000 and 2000 local.

Question 33

Define density altitude (complex).

Answer 33

The altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at that place of observation.

Question 34

Define density altitude (simple).

Answer 34

Pressure altitude corrected for non-standard (ISA) temp variations.

In other words, the density altitude is the air density given as a height above mean sea level

Question 35

List the three factors, in order of significance, which contribute to high density altitude environments.

Answer 35

Temps warmer than the ISA.
Pressures lower than the ISA.
A high water vapour content (this increases the DA but up to 1%).

These factors are exacerbated at high altitudes but significant examples can be found at or near sea level in NZ.

Question 36

Explain the following formula:

DA = PA + CF (Ta - Tstd).

Answer 36

Formula used to calculate density altitude.
DA = density altitude (ft)
PA = pressure altitude (ft)
CF = correction factor (120 ft per 1 °C)
Ta = actual temp
Tstd = standard temp in °C (derived by subtracting 2 °C for every 1000 ft of pressure altitude from the standard MSL temp of 15 °C).

Question 37

Find the density altitude:

DA = ?
PA = 5000 ft
CF = 120 ft per 1 °C
Ta = 12 °C
Tstd = (15 - (5 x 2)) = 5 °C

Answer 37

DA = 5000 + 120 x (12 - 5)
DA = 5000 + 120 x 7
DA = 5840 ft.

Question 38

If the pressure altitude is 5000 ft and the calculated density altitude is 5840 ft, is this an example of a high or low density altitude environment and what effect does this have on aircraft performance?

Answer 38

This is a high DA environment. Although flying at the 5000 ft indicated, the atmospheric conditions in which the aircraft is flying are the equivalent of flying at 5840 ft in ISA conditions, therefore performance will be reduced.

Question 39

What is an easy way to establish the pressure altitude whilst an aircraft is still on the ground?

Answer 39

Dial in 1013 hPa on the altimeter. The altimeter will then read the height of the aircraft above the 1013 hPa level, or in effect, the pressure altitude. The difference between the airfield elevation (given on the landing plate) and the height indicated on the altimeter can then be added or subtracted to your intended cruising height.

Question 40

Explain the following formula:

PA = A + CV

Answer 40

Formula used to calculate density altitude.
PA = pressure altitude (ft)
A = QNH altitude (ft) i.e. operating altitude above MSL
CV = conversion factor = ((QNE - QNH) x 27 ft per hPa)

This formula is then fed into the DA = PA + CF (Ta - Tstd).

Question 41

Find the pressure altitude:

PA = ?
A = 8000 ft
QNH = 996 hPa
CV = (1013 - 996) x 27 ft per hPa

Answer 41

PA = 8000 + (1013 - 996) x 27
PA = 8459 ft.

Question 42

According to the Ideal Gas Law, what is air density a function of? (2)

Answer 42

Air pressure and air temp.

Question 43

What is the link between air density and aircraft performance?

Answer 43

Aircraft performance, both lift and power output, is primarily a function of air density. In higher density air, the aircraft’s performance will be enhanced and in low density air performance will be degraded.

Question 44

Explain the following formula:

P* = P/RT

Answer 44

P* = air density
P = pressure
T = temp
R = the Ideal Gas constant

Question 45

What is the greatest environmental contributor to degraded aircraft performance?

Answer 45

Temps higher than the ISA temp applicable to the altitude being flown.

Question 46

Explain the conditions for a high density altitude environment.

Answer 46

A high DA environment occurs when air density is lower than ISA, usually due to temps higher than ISA and pressures lower than ISA, at the height being flown.

Question 47

Explain what is meant by a high density altitude.

Answer 47

The air density experienced by an aircraft would be found at a higher altitude in ISA than the altitude which is being flown.

Question 48

Describe the three reductions to aircraft performance that occur in high density altitude environments.

Answer 48

The engine will deliver less power due to reduced oxygen content.
The propeller will generate less thrust as it is biting into less dense air.
The wings will generate less lift as there are less air molecules passing over them.

Question 49

In order to compensate for reduced aircraft performance in a high density altitude environment, an aircraft needs to fly at faster ground speed. Describe the four consequences to this.

Answer 49

Take-off requires more runway.
Landing ground speed is greater, so more runway is required to pull up.
The rate of climb will be markedly decreased.
The serviceable ceiling will be reduced.

Question 50

Does pressure always decrease with height?

Answer 50

YES

Question 51

Why is the transition layer in Australia lower than that in NZ?

Answer 51

Because the transition layer is generally linked to the height of the terrain, and in Australia, the highest peak, Mt Kosciuszko, is only 7,310 ft AMSL (compared to Mt Cook at 12,313 ft AMSL).

Question 52

Find the density altitude:

DA = ?
PA = 2000 ft
CF = 120 ft per 1 °C
Ta = 2 °C
Tstd = (15 - ( x )) =  °C

Is this a low or high DA?

Answer 52

DA = 1000 ft.

This low.

Question 53

Under what type of general weather pattern would negative pressure altitudes be experienced?

Answer 53

Temps lower than ISA and pressures higher than ISA, generally at a low elevation airfield.

Question 54

Find the density altitude:

Queenstown airport has an elevation of 1173 ft AMSL. If the outside air temperature is 30 °C and the current ATIS QNH is 1013.

Answer 54

DA = 3200 ft i.e. a high DA.