PRESSURE & DENSITY

Question 1

Define atmospheric pressure

Answer 1

The weight of a column of air above the point at which the pressure is being measured.
Includes water vapour. Approx 1kg/cm3.

Question 2

Hectopascal meaning

Answer 2

Hecto: 100
Pascal: 1 Newton/m3

Question 3

Newton?

Answer 3

Newton = unit of force acting on 1kg to give a velocity of 1m/s after 1 second.

Question 4

Why is pressure significant to aviation?

Answer 4

Allows us to measure aircraft height above a set reference point.
Altimeter measures pressure to provide this.
VSI also uses changes in pressure as a/c climbs or descends and converts these to vertical speeds

Question 5

Define pressure lapse rate
RoT?

Answer 5

Air is densest & heaviest at the earths surface.
As we climb, pressure always decreases.
30ft per 1hpa.

Question 6

Describe pressure with relevance to cyclones & anti cyclones

Answer 6

An anticyclone is an area of relative high pressure, caused because air has been added to the vertical column = increase in surface pressure.
A cyclone has had air removed, surface pressure decreases & tropopause lowers

Question 7

QNH

Answer 7

Aerodrome level pressure corrected to MSL using the ISA temp lapse rate. When set on the altimeter, it will read ALTITUDE AMSL. (Set below transition)

Question 8

QFE

Answer 8

Aerodrome level pressure set on the altimeter.
Reads the HEIGHT above aerodrome level.

Question 9

QNE

Answer 9

1013.25 hpa set on altimeter, instruments will read the PRESSURE ALTITUDE or FLIGHT LEVEL.
Set above transition .

Question 10

Reason for QNE

Answer 10

Pilots would otherwise constantly be changing their QNH for local pressure variations (Lows & Highs).
QNE allows for separation of aircraft over long distances as pressures change. (The actual level the aircraft is flying at will increase & decrease with varying pressure!)

Question 11

Transition altitude/level & why we have it

Answer 11

Descent through FL150: Set QNH
Climb through A130: Set QNE
Separates aircraft flying on QNH vs. QNE

Question 12

Exceptions to transition level in NZ

Answer 12

when surface pressure <980hpa
When operating IMC within 20nm Mt Cook.

Transition level is F160

Question 13

Flight towards lower pressure from high pressure?

Answer 13

Flying high to low, watch out below.

Question 14

How does temperature affect pressure lapse rate

Answer 14

Increase in temp = expansion of air particles in the vertical column.
Therefore when the temps are hotter, the aircraft’s pressure level is HIGHER than if temps are colder. (as long as MSL pressures remain constant)

Question 15

What causes localised pressure variations?

Answer 15

• Lee trough
• Thermal/heat lows
• TS
• Diurnal variation

Question 16

How does a Lee trough cause localised pressure variations?

Answer 16

Air moving up & over terrain builds an area of high pressure on the windward side & deficit results on the leeward side. Stronger the wind, the greater the affect

Question 17

How does a thermal/heat low cause localised pressure variations?

Answer 17

In hot spells when sun heats the land, air expands & becomes less dense = low pressure.

Question 18

How does a TS cause localised pressure variations?

Answer 18

Can alter local pressure due to large density variations driven by Up & Down drafts

Question 19

How does diurnal variation cause localised pressure variations?

Answer 19

Essentially due to heating from the sun during the day, and loss of heat into the atmosphere at night.
Higher temps during the day = expansion, reduced density and therefore pressures & vice versa at night.
Oscillation follows 1600 & 0400 min pressures & 1000/2200 max pressures

Question 20

Define Density altitude (DA)

Answer 20

Pressure altitude corrected for non-standard (ISA) temperature variationsW

Question 21

Why are high DAs ‘bad’?

Answer 21

Reduced performance: longer T/O roll, reduced climb rates (less air particles going over the wings), less power available (due less oxygen), faster GS on appch & longer landing rolls. Service ceiling is also reduced.

Question 22

What factors contribute to high DA environments

Answer 22

1. Temps warmer than ISA
2. Pressures lower than ISA
3. High water vapour content

Question 23

How to calculate PA?

Answer 23

PA = A+CV

A = QNH alt in ft
CV = conversion factor ((1013-QNH) x27ft per hpa)

Question 24

How to calculate DA from PA?

Answer 24

DA = PA + CF (Ta - Tstd)

CF = correction factor (120ft per deg C)
Ta = Actual temp
Tstd = standard temp in deg C (subtract 2deg for every 1000ft of PA from ISA temp of 15)

Question 25

What is the greatest contributor to degraded perf is?

Answer 25

Temperatures higher than ISA (applicable to altitude being flown)

Question 26

Why is “high” DA worse?

Answer 26

Higher DA simply means the aircraft is flying in an environment where the air density is LESS than what it should be on an ISA day (i.e. the air density currently being experienced should be found at a higher altitude).