Pressure Flashcards
The atmosphere is composed of a variety of gases these are….
Nitrogen (78%)
Oxygen (21%)
Carbon Dioxide (0.04%)
Other
What is atmospheric pressure defined as?
Combined weight of gases exerting force in all directions
What is used to measure atmospheric pressure?
A Barometer
What is pressure expressed as?
Inches or Millibars of Mercury
Millibar
Hectopascal
A hectopascal is equal to…
100 newtons per square metre
What is an isobar?
The line on the map to indicate locations of the same pressure
What happens to pressure as you increase altitude?
Reduces
What is the ISA pressure at sea level?
In HPA, Hg and lb/sq.in
- 25 HPA
- 92Hg
- 7lb/sq.in
What is QNE (Question Nil Elevation)?
Setting 1013 on aircraft subscale
QNE is a pressure setting of 29.92 inches or 1013 hPa that will produce a standard atmosphere altitude and provides the basis for flight levels.
With QNE set on the altimeter, the main scale will indicate?
Height above or below 1013 HPA Pressure level
Pressure height or Pressure Altitude
At low level, the pressure reduces at a rate of…
1 HPA / 30ft of altitude increase in the troposphere
=1 Hg / 1000ft
Air density refers to….
Mass of air occupying a given volume
What 3 factors does air density depend on?
- Temperature
- Humidity
- Pressure
Decreased Pressure= …. Density
Decreased Density
Increased Temperature=
Decreased Density
Increased Humidity=
Decreased Density
Does high density give better or worse aircraft performance?
Better
Why do variations in pressure occur?
- The movement of pressure systems
- Changing the intensity of pressure systems
- Semi diurnal Pressure variation
What is semi-diurnal variation?
Rhythmic variation in pressure over 12 hour period
Where is semi-diurnal variation the greatest?
Tropics
Can change up to 5 HPA
What is standard pressure?
1013.25 HPA
What is the actual mean sea level pressure referred to as?
QNH
When QNH is set on altimeter it will read…
Altitude (Height above mean sea level)
What is local QNH?
Actual sea level pressure for a particular location
Where is Local QNH available?
Control tower or ATIS
If you set the airfield elevation on your altimeter what will it read?
Approx. Local QNH
What is area QNH?
Predicted average sea level pressure
Who forecasts area QNH?
Bureau of Meteorology
How often is area QNH updated?
Every 3 hours
Area and Local QNH can differ, but by how much?
5 HPA
In Australia, pilots cruise on local QNH up to what distance?
100nm
When cruising at or below 10,000ft
What is a transition layer?
Between A100 and FL110-FL125 depending on QNH; separates aircraft on QNH to aircraft on standard pressure
(Cruise not permitted in this layer)
Heights are expressed as..
Hundreds of feet prefixed A for altitudes on QNH; FL for those on pressure altitudes referenced to standard pressure
What is QFE?
Station level pressure
With QFE on subscale the main scale will read…
Height above or below reference pressure point
How to obtain QFE?
Set 0 on aircraft subscale
What is QFF?
Meteorological mean sea level pressure. Takes into account temperature and water vapour content.
What is the common isobar spacing?
2-4 HPA
An altimeter in an aircraft flying towards LOW PRESSURE will….
Over read
The altimeter cannot distinguish between falling surface pressure and climbing
What is a low?
A centre of low pressure surrounded by higher pressure
What is a High?
A centre on high pressure surrounded by low pressure
What is a col?
A neutral area between two highs and two lows
What is a trough?
Any elongated area of low pressure
What is a ridge?
Any elongated area of high pressure
How are pressure gradients measured?
From high to low pressure at right angles to the isobars
What does close isobar spacing indicate?
Strong gradient and strong winds
What does wide isobar spacing indicate?
Weak or shallow gradient and light winds
