Effects of Atmospherics
Aircraft and system performance are highly dependent on the environment in which they operate, necessitating a comprehensive understanding of the effects of atmospheric conditions on their operation.
Effects of Atmospherics Introduction
- Atmospherics have both positive and negative impacts on an aircraft's performance.
Air as a Fluid
- Understanding the fluid properties of air is essential to understanding the principles of flight.
- Fluids generally do not resist deformation when even the smallest stress is applied, or they resist it only slightly.
- We call this slight resistance viscosity.
- Fluids also can flow.
- Just as a liquid flows and fills a container, air will expand to fill the available volume of its container.
- Understanding the fluid properties of air is essential to understanding the principles of flight.
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Viscosity:
- Viscosity is the property of a fluid that causes it to resist flowing.
- The way individual molecules of the fluid tend to adhere to each other determines how much a fluid resists flow.
- High-viscosity fluids are "thick" and resist flow.
- Low-viscosity fluids are "thin" and flow easily.
- Air has a low viscosity and flows easily, whereas other fluids, such as oil or grease, have a higher viscosity and do not flow as easily.
- Since air is a gas with fluid-like properties, it is viscous, meaning it resists flow around any object to some extent.
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Friction:
- Friction is the resistance that one surface or object encounters when moving over another.
- Friction exists between any two materials that contact each other.
- The surface of a wing, like any other surface, has a certain roughness at the microscopic level.
- The surface roughness causes resistance and slows the air velocity flowing over the wing. [Figure 4-1]
- Molecules of air pass over the surface of the wing and actually adhere (stick or cling) to the surface because of friction.
- Air molecules near the surface of the wing resist motion and have a relative velocity near zero.
- The roughness of the surface impedes their motion.
- The layer of molecules that adheres to the wing surface is the boundary layer.
- Once the boundary layer of air adheres to the wing, the air’s viscosity-the tendency to stick to itself-creates further resistance to the airflow.
- Drag is the cumulative force of these two forces acting together to resist airflow over a wing.
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Pressure:
- Pressure is the force applied in a perpendicular direction to the surface of an object.
- Pilots and engineers often measure pressure in pounds of force exerted per square inch, or PSI.
- An object completely immersed in a fluid will experience uniform pressure across its entire surface.
- If the pressure on one surface of the object becomes less than the pressure exerted on the other surfaces, the object will move in the direction of the lower pressure.
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Pressure Altitude:
- Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer.
- An altimeter is a sensitive barometer calibrated to indicate altitude in the standard atmosphere.
- If setting an altimeter to 29.92 "Hg SDP, the altitude indicated is the pressure altitude.
- As atmospheric pressure changes, the SDP may be below, at, or above sea level.
- Pressure altitude is essential as a basis for determining airplane performance, as well as for assigning flight levels to airplanes operating at or above 18,000 feet.
- The pressure altitude can be determined by:
- Setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude, or;
- Applying a correction factor to the indicated altitude according to the reported altimeter setting.
- Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer.
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Density Altitude:
- SDP is a theoretical pressure altitude, but aircraft operate in a non-standard atmosphere, and the term density altitude correlates with aerodynamic performance in this non-standard atmosphere. The density of air has significant effects on the aircraft's performance because, as air becomes less dense, it:
- Reduces power because the engine takes in less air.
- Reduces thrust because a propeller is less efficient in thin air.
- Reduces lift because the thin air exerts less force on the airfoils.
- Density altitude is pressure altitude corrected for non-standard temperatures and is used to determine aerodynamic performance in non-standard atmospheres.
- As the density of the air increases (at lower altitudes), aircraft performance improves; conversely, as air density decreases (at higher altitudes), aircraft performance deteriorates.
- A decrease in air density corresponds to a high density altitude, while an increase in air density indicates a lower density altitude.
- Density altitude is used in calculating aircraft performance because, under standard atmospheric conditions, air at each level in the atmosphere not only has a specific density, but its pressure altitude and density altitude identify the same level.
- The computation of density altitude involves consideration of pressure (pressure altitude) and temperature. Since aircraft performance data references air density for a standard day, this data applies to air density levels that may not match altimeter indications. Under conditions higher or lower than standard, the altimeter cannot directly determine these levels.
- Pilots can calculate density altitude by first finding pressure altitude and then correcting this altitude for non-standard temperature variations. Since density varies directly with pressure and inversely with temperature, a given pressure altitude may exist for a wide range of temperatures by allowing the density to vary. However, a known density occurs for any one temperature and pressure altitude. The density of the air has a pronounced effect on aircraft and engine performance. Regardless of the actual altitude of the aircraft, it will perform as though it were operating at an altitude equal to the existing density altitude.
- Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air, while low density altitude refers to dense air. The conditions that result in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude.
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Effect of Pressure on Density:
- Since air is a gas, it can be compressed or expanded. When compressed, a greater volume of air can occupy a given amount. Conversely, when pressure on a given volume of air decreases, the air expands and occupies a greater space. At a lower pressure, the original column of air contains a smaller mass of air. The density decreases because density is directly proportional to pressure. If the pressure doubles, the density doubles; if the pressure halves, the density halves. This statement is valid only at a constant temperature.
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Effect of Temperature on Density:
- Increasing the temperature of a substance decreases its density.
- Conversely, decreasing the temperature increases the density.
- Thus, the density of air varies inversely with temperature.
- This statement is valid only at a constant pressure.
- In the atmosphere, both temperature and pressure decrease with altitude, having conflicting effects on density.
- However, a fairly rapid drop in pressure as altitude increases usually has a dominating effect.
- Hence, pilots can expect the density to decrease with altitude.
- If a chart is not available, add 120' for every degree Celsius above the ISA provides a functional estimation:
- For example, at 3,000' pressure altitude (PA), the ISA prediction is 9° C (15° C - [lapse rate of 2° C per 1,000' x 3 = 6° C]).
- However, if the actual temperature is 20° C (11° C more than that predicted by ISA), then the difference of 11° C is multiplied by 120', equaling 1,320.
- Adding this figure to the original 3,000' provides a density altitude of 4,320' (3,000' + 1,320').
- Increasing the temperature of a substance decreases its density.
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Effect of Humidity (Moisture) on Density:
- Since air is never completely dry, a small amount of water vapor is always present in the atmosphere.
- In other conditions, humidity can become a crucial factor in an aircraft's performance.
- Water vapor is lighter than air; consequently, moist air is lighter than dry air.
- Therefore, as the water content of the air increases, the air becomes less dense, which increases density altitude and decreases performance.
- It is the lightest or least dense when, in a given set of conditions, it contains the maximum amount of water vapor.
- Humidity, also known as relative humidity, refers to the amount of water vapor present in the atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can hold at a given temperature.
- This amount varies with temperature (Warm air holds more water vapor, while cold air holds less.
- Completely arid air, which contains no water vapor, has a relative humidity of 0%. In contrast, saturated air, which cannot hold any more water vapor, has a relative humidity of 100%
- Humidity alone is typically not considered a primary factor in calculating density altitude and aircraft performance, but it is a contributing factor.
- As the temperature increases, the air can hold more water vapor.
- When comparing two separate air masses, the first warm and moist (both qualities tending to lighten the air) and the second cold and dry (both qualities making it heavier), the first must be less dense than the second.
- Pressure, temperature, and humidity have a significant impact on aircraft performance due to their effect on air density.
- There are no easy rules of thumb to apply, but the effect of humidity can be determined using several online formulas.
- Using [Figure 2], select the barometric pressure closest to the associated altitude.
- The standard pressure at 8,000 feet is 22.22 "Hg.
- Using the National Oceanic and Atmospheric Administration (NOAA) website (www.srh.noaa.gov/epz/?n=wxcalc_densityaltitude) for density altitude, enter 22.22 for 8,000 feet in the station pressure window.
- Enter a temperature of 80° and a dew point of 75°.
- The result is a density altitude of 11,564 feet.
- With no humidity, the density altitude would be almost 500 feet lower.
- Another website (www.wahiduddin.net/calc/density_altitude.htm) offers a more straightforward method for determining the effects of humidity on density altitude, eliminating the need for additional interpretive charts.
- In any case, the effects of humidity on density altitude include a decrease in overall performance in high humidity conditions.
- Learn more here.
- SDP is a theoretical pressure altitude, but aircraft operate in a non-standard atmosphere, and the term density altitude correlates with aerodynamic performance in this non-standard atmosphere. The density of air has significant effects on the aircraft's performance because, as air becomes less dense, it:
Effects of Winds
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Effects of Headwinds on Performance:
- Headwinds decrease ground speed, decreasing range.
- Headwinds increase the power required to maintain cruise performance, resulting in higher fuel consumption.
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Effects of Tailwinds on Performance:
- Tailwinds increase ground speed, increasing range.
- Tailwinds reduce the power required to maintain cruise performance, decreasing fuel burn.
Effects of Temperature
- High temperatures result in higher density altitudes.
- Higher density altitudes result in degraded engine performance.
Effects of Density Altitude
- Density altitude is the result of what the AOPA terms the Triple-H effect: high altitude and high temperature lead to high density altitude.
- Density altitude impacts takeoff, approach, and landing performance.
- Performance figures in the aircraft owner's handbook, such as the length of the takeoff run, horsepower, and rate of climb, are generally based on standard atmosphere conditions (59°F (15°C), 29.92 inches of mercury) at sea level.
- Pilots may encounter trouble when they face entirely different conditions, especially in hot weather and at higher elevations.
- Aircraft operations at altitudes above sea level and at higher than standard temperatures are commonplace in mountainous areas.
- Such operations often result in a drastic reduction of aircraft performance capabilities due to changes in air density.
- Density altitude is a measure of air density, not to be confused with pressure altitude, true altitude, or absolute altitude.
- Density altitude is not a height reference, but rather a determining criterion in the performance capability of an aircraft.
- Air density and density altitude have an inverse relationship.
- Air density, which decreases with altitude, causes an increase in density altitude.
- The further effects of high temperature and high humidity are cumulative, resulting in an increasingly high density altitude condition.
- High density altitude reduces all aircraft performance parameters.
- To the pilot, this means reduced horsepower output, reduced propeller efficiency, and a higher true airspeed is required to sustain the aircraft throughout its operating parameters.
- Reduced performance results in increased runway length requirements for takeoffs and landings, as well as a decreased rate of climb.
- Example: An average small airplane requiring 1,000' for takeoff at sea level under standard atmospheric conditions will require a takeoff run of approximately 2,000' at an operational altitude of 5,000'.
- A turbocharged aircraft engine provides sea-level horsepower up to a specified altitude above sea level.
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Density Altitude Advisories:
- At airports with elevations of 2,000' and higher, control towers and FSSs will broadcast the advisory "Check Density Altitude" when the temperature reaches a predetermined level.
- Controllers broadcast these advisories on appropriate tower frequencies or, where available, ATIS. FSSs will broadcast these advisories as part of the Local Airport Advisory.
- Air traffic facilities provide these advisories as a reminder to pilots that high temperatures and high field elevations will cause significant changes in aircraft characteristics.
- The pilot retains the responsibility to compute density altitude, when appropriate, as a part of preflight duties.
- All FSSs will compute the current density altitude upon request.
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Density Altitude Precautions:
- Fly lighter, don't carry unnecessary baggage.
- Review POH for special procedures, such as mixture position during takeoff, cruise, and landing (likely leaning until peak RPM).
- Fly indicated airspeeds (ground speed will be faster).
- Maneuver carefully and conservatively.
- Anticipate exaggerated deceleration and the effects of flight surfaces, such as flaps.
- Fly early in the day when temperatures and, therefore, density altitude are lowest.
- Recall that density altitude affects lift surfaces, the propeller, and the engine simultaneously.
Effects of Atmospherics Conclusion
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