ISA, Basic Altimetery Flashcards
What 3 variables describe the state of the atmosphere under ISA?
- temperature
- pressure
- density
What is the standard atmosphere conditions, and the average tropopause height?
Temp. 15 c or 59 F
Pressure 29.92 or 1013.25
Tropopause Standard Height = 36,000’
What is pressure defined as?
F = Mass * Gravity / Area
Define: Station Pressure
A measurement of the weight of the atmosphere above the instrument
What are the different types of barometers?
mercurial barometer (more accurate) aneroid barometer (more portable)
What is virtual temperature?
The temperature the air would be if all the water vapor were removed and its density remained constant (always warmer than ambient)
What temperature should you use in density altitude calculations for more accuracy?
virtual temperature
What is the altimeter setting, and how is it found?
It is the station pressure (for an elevation 10 feet above the mean runway elevation) reduced to sea level, using ISA. (DC-3 altimeter is 10 feet above ground)
station pressure + [0.0011 * height above sea level]
When is the true and indicated altitude different?
Difference is greatest when temp. is below standard
What is QNE?
Standard (set at 29.92) - positive separation by pressure level
What is QNH?
Altimeter setting - should read field elevation on the ground
What is QFE?
Using station pressure - should read “0” on the ground
What is the lowest usable FL if altimeter seeting is less than 29.92?
Above FL180, FL180 if altimeter setting is above 29.92
For every 11Cc that the temperature varies from standard, the altimeter will be in error by how much?
4%
High density altitude affects aircraft performance in what three major ways?
- Reduced power
- Reduced thrust
- Reduced lift
How is stability defined?
The faster the atmosphere “gets cold” with increasing altitude, the more unstable it is. Low-level moisture, underlying dry air aloft, enhances instability.
What is the lifted index?
The temperature of the environment at 500mb minus the temperature of the parcel of air at 500 mb
What is precipitable water?
amount of water vapor from surface to 500mb and condense it in a beaker the depth of that water in the beaker. 1” of water equals 10-12” of snow
