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
Define the following:
Pressure Altitude
Altitude readout with QNE set on the altimeter sub-scale
Define the following:
True Altitude
Actual height above mean sea level
Obtained by correcting indicated altitude for temperature
Define the following:
Absolute Altitude
Actual height above terrain
Altitude readout obtained when QFE is set on sub-scale or from radio or radar altimeters.
In the standard atmosphere, True altitude = ?
Indicated altitude
In warm air, the temp lapse rate is less. True altitude is ?
Greater than indicated altitude
In cold air, the temp lapse rate is less, True altitude is ?
Lower than indicated altitude
Temp low, look out below
What is the pressure, temperature and lapse rates in the ICAO Standard Atmosphere (ISA)?
Pressure: 29.92” Hg / 1013.25 mb at MSL
Temperature: 15° C at MSL
Lapse Rates: 1.98° C per 1000ft
1” Hg per 1000’ pressure lapse rate
What two variables are required to correctly determine true altitude?
Pressure and/ or Indicated altitude
OAT
Review Questions
What is the temperature lapse rate in the ISA?
2°C per 1000ft
Review Questions
Does the change is temperature affect the pressure of a column of air?
Yes, pressure varies with temperature. Air, being a gas, expands when heated and contracts when cooled.
Review Questions
Can temperature error be corrected?
Yes, by using a Jepp or ADC
Review Questions
Define QNH
Barometric pressure in mb or “ Hg for a particular time and place, adjusted to MSL, according to ISA. Also referred to as altimeter setting.
Review Questions
What altitude will be displayed on a pressure sensitive altimeter with the QNH set when an aircraft is on the ground?
Station altitude above MSL
Review Questions
Define Absolute Altitude.
Actual height above terrain
Review Questions
When flying in an airmass colder than ISA, the altimeter will read higher or lower than the aircraft’s true altitude?
Higher, meaning the aircraft is lower than it thinks
