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Terms in this set (31)
What is the atmosphere
An envelope of air that surrounds the earth and rests upon its surface
Percentages of gases that the atmosphere is comprised
Why oxygen decreases with altitude
Oxygen is a heavier element so it sinks to the surface of the earth. Most of the earth's oxygen is below 35,000 feet. Other elements are lighter so they are higher in the atmosphere.
What pressure and why are pilots concerned with this pressure
There are many kinds of pressure and atmospheric pressure is the one that pilots are most concerned with.
One of the basic factors in weather changes
Helps to lift an aircraft
Actuates some instruments - altimeter, airspeed, VSI, and manifold pressure
How does air have pressure
Air is very light but it still has mass and is effected by the attraction of gravity. Since it has weight, it has force. Since air is a fluid substance, its force is exerted in all directions. This effect on bodies within the air is known as pressure.
Weight of atmosphere under standard conditions at sea level
14.70 pounds pre square inch or 1,013.2 millibars
Weight of atmosphere at 18,000 feet and why
About half the weight
The thickness of the air is limited, therefore, the higher the altitude the less air there is above exerting force down. This is why the weight is one half.
Why was a standard atmospheric pressure and temperature determined.
What is it
Pressure changes throughout the day
59F or 15C and 29.92" of mercury or 1013.2 mb
What is the standard temperature lapse rate
2C or 3.5F per thousand feet up to 36,000 feet. Between 36,000 and 80,000 the temperature is constant.
What is the standard pressure lapse rate
1" Hg decrease per 1000 feet up to 10,000 feet
Who made these standards
International Civil Aviation Organization (ICAO)
These are often referred to as International Standard Atmosphere (ISA)
Since aircraft performance is compared and evaluated with respect to the standard atmosphere, all aircraft instruments are calibrated to the standard atmosphere
In order to account properly for the nonstandard atmosphere, certain terms must be defined
Height above a standard datum plane where the atmosphere weights 29.92" as measured by a barometer. An altimeter is a barometer.
Altitude that is indicated when pressure is set to 29.92"
Why is pressure altitude important
Basis for determining aircraft performance
Altimeter setting given to aircraft at or above 18,000 feet
2 ways to get pressure altitude
1. Set 29.92" in Kollsman window
2. Apply a correction factor to the indicated altitude according to the reported altimeter setting
Density altitude definition
Pressure altitude corrected for nonstandard temperature
What the airplane feels like it is flying at
What is density altitude used for
Correlating aerodynamic performance in the nonstandard atmosphere
Relationship between density altitude and density of air
As density of air increases, density altitude decreases
As density of air decreases, density altitude increases
Effect on aircraft performance at a low density altitude
Effect on aircraft performance at a high density altitude
3 things a lower air density reduces
1. Power because the engine takes in less air
2. Thrust because a propeller is less efficient in thin air
3. Lift because the thin air exerts less force on the airfoils
Problem with reading density altitude directly off altimeter
If it is not a standard today the reading will be incorrect
What does high density altitude refer to
Thin air - decreased performance
What does low density altitude refer to
Dense air - increased performance
Conditions that result in high density altitude
Low atmospheric pressures
Conditions that result in low density altitude
High atmospheric pressures
Effect of pressure on density
Air is a gas so it can be compressed and expanded. When air is compressed, a greater amount of air can occupy a given volume. When air pressure is decreased, the air expands and occupies a greater space.
Make a graphic with an airplane in a tube with air molecules visible
Effect of temperature on density
Increasing the temperature of a substance decreases its density. Decreasing the temperature of a substance increases its density. This is an inverse relationship.
Higher in the atmosphere both temperature and pressure decreases with altitude and have conflicting effects on density. The rapid drop in pressure as altitude increases usually has the dominating effect. Hence, pilots can expect the density to decrease with altitude
Effect of humidity on density
Previous two situations assume there is no humidity. This isn't the case though.
Water vapor is lighter than air so moist air is lighter than dry air. So as humidity increases the air becomes less dense. This increases density altitude and decreases performance.
What is relative humidity
Relative humidity is given in a percentage of how much water the air is holding and how much more it could hold at a given temperature
Comparing two air masses
Warm air can hold more moisture. Warm and moist lightens the air which increases density altitude. Cold and dry air makes the air heavier which decreases density altitude.
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