Nav 20-21 - Altimetry Flashcards
What are two characteristics of the atmosphere?
It is a mixture of gases
It is compressible
Where is the greater mass of the atmosphere located?
Near the earth’s surface, 50% of it’s mass is below 18 000’
What is MSL as defined by ISA?
- 7 lbs/ sq. in
- 3 kPa
- 92” Hg
- 2 mb
Pressure in the atmosphere can be likened to what?
The weight of a column of air
Pressure decreases as altitude increases:
1” Hg = ____ ft
1 mb = __ ft
1000 ft
30 ft
Describe the relationship between pressure, temperature and altitude.
Pressure varies with temperature
Temperature decreases with altitude
Thus, the rate of pressure change with height depends on air temperature and the change is less in warm than in cold air.
What is the Q code designator for standard pressure?
QNE
QNE altimeter sub-scales are set at what?
The ISA pressure of 29.92” Hg
If an altimeter is set to the ISA pressure, what will it display?
Pressure altitude, which is the altitude above MSL under ISA conditions
What is the Q code for observed pressure at an aerodrome corrected for temperature?
QFE
If QFE is set on the altimeter sub-scale, what will it read on the ground? When altitude is gained?
It will read zero on the ground and display the height as the aircraft gains altitude.
What is the Q code designator for Altimeter Setting?
QNH
Instead of reading zero on the ground, a QNH altimeter will read?
The height above MSL
How do we get QNH?
You will already know the surface pressure at the aerodrome
You must also know the elevation of the aerodrome ASL
An imaginary column of ISA air from ground to sea level is added to surface pressure
The derived pressure is QNH
Define the following:
Indicated Altitude
Altitude readout with QNH set on the altimeter sub-scale
