|Structure of the Atmosphere
1. How is aircraft performance significantly affected as air becomes less dense? (FAA-H-8083-25)
2. What is the standard atmosphere at sea level? (FAA-H-8083-25)
Standard atmosphere at sea level includes a surface temperature of 59°F or 15°C, and a surface pressure of 29.92 in. Hg or 1013.2 millibars.
3. What are standard atmosphere temperature and pressure lapse rates? (FAA-H-8083-25)
A temperature lapse rate is one in which the temperature decreases at the rate of approximately 3.5°F or 2°C per 1,000 feet up to 36,000 feet. Above this point, the temperature is considered constant up to 80,000 feet. A standard pressure lapse rate is one in which pressure decreases at a rate of approximately 1 in. Hg per 1,000 feet of altitude gain to 10,000 feet.
4. Define the term “Pressure Altitude”. (FAA-H-8083-25)
Pressure altitude is the height above a standard datum plane. An altimeter is a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 in. Hg Standard Datum Plane (SDP), the altitude indicated is the pressure altitude – the altitude in the standard atmosphere corresponding to the sensed pressure.
5. Why is the pressure altitude important? (FAA-H-8083-25)
Pressure altitude is important as a basis for determining airplane performance as well as for assigning flight levels to airplane operating above 18,000 feet.
6. What are two methods of determining pressure altitude? (FAA-H-8083-25)
Pressure altitude can be determined by either of two methods.
7. Define the term “density altitude”. (FAA-H-8083-25)
Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere corresponding to a particular value of air density.
8. How does air density affect air performance? (FAA-H-8083-25)
As the density of the air increases (lower density altitude), airplane performance increases and conversely, as air density decreases (higher density altitude), airplane performance decreases. A decrease in air density means a high density altitude; an increase in air density means a lower density altitude.
9. How is density altitude determined? (FAA-H-8083-25)
First find pressure altitude and then correct it for nonstandard temperature variations. Because density varies directly with pressure, and inversely with temperature, a given pressure altitude may exist for a wide range of temperatures. However, a known density occurs for any one temperature and pressure altitude. Regardless of the actual altitude at which the airplane is operating, it will perform as though it were operating at an altitude equal to the exiting density altitude.
10. What factors affect air density? (FAA-H-8083-25)
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 condition that result in 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.
11. What effect does atmospheric pressure have on air density? (FAA-H-8083-25)
12. What effect does temperature have on air density? (FAA-H-8083-25)
Increasing the temperature of a substance decrease its density. Conversely, decreasing the temperature increases the density. Thus, the density of air varies inversely with temperature. This statement is true only at a constant pressure.
13. Since temperature and pressure decreases with altitude, how will air density be affected over all? (FAA-H-8083-25)
The decrease in temperature and pressure have conflicting effects on density as you go up in altitude, but the fairly rapid drop in pressure with increasing altitude is usually the dominating factor. Hence, the density is likely to decrease with altitude gain.
14. What effect does humidity have on air density? (FAA-H-8083-25)
Water vapor is lighter than air, so moist air is higher than dry air. As the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance. It is lightest or least dense when it contains the maximum amount of water vapor. Humidity alone is usually not considered an important factor in calculating density altitude and airplane performance, but it does contribute.
15. What is the definition of the term “relative humidity”? (FAA-H-8083-25)
Relative humidity refers to the amount of water vapor in the atmosphere, and is expressed as a percentage of the maximum amount of water vapor the air can hold. This amount varies with the temperature – warm air can hold more water vapor and colder air can hold less.
16. What effect does landing at high-elevation airports have on ground speed with comparable conditions relative to temperature, wind and airplane weight? (FAA-H-8083-25)
Even if you use the same indicated airspeed appropriate for sea level operations, true airspeed is faster, resulting in a faster ground speed (with a given wind condition) throughout the approach, touchdown, and landing roll. This means greater distance to clear obstacles during the approach, a longer ground roll, and consequently the need for a longer runway. All of these factors should be taken into consideration when landing at high-elevation fields, particularly if the field is short.
B. Aircraft Performance
1. What are some of the main elements of aircraft performance? (FAA-H-8083-25)
a. Takeoff and landing distance
2. What is the relationship of lift, weight, thrust and drag in steady, unaccelerated, level flight? (FAA-H-8083-25)
For the airplane to remain in steady, level flight, equilibrium must be obtained by a lift equal to the airplane weight and power plant thrust equal to the airplane drag.
3. What are the two types of drag? (FAA-H-8083-25)
Total drag may be divided into two parts: the wing drag (induced), and drag from everything but the wings (parasite).
4. Define induced drag. (FAA-H-8083-25)
Induced drag is the part of total drag created by the production of lift. Induced drag increases with a decrease in airspeed. The lower the airspeed, the greater the angle of attack required to produce lift equal to the airplane’s weight and therefore the greater the induced drag.
5. Define parasite drag. (FAA-H-8083-25)
Parasite drag is the part of total drag created by the form or shape of airplane parts. It is the sum of pressure and friction drag due to the airplane’s basic configuration and is independent of lift. It is greatest at high airspeeds and is proportional to the square of the airspeed: if the airspeed were doubled, the parasite drag would be quadrupled.
6. How much will drag increase as airplane speed increases? (FAA-H-8083-25)
If an airplane in a steady flight condition at knots is then accelerated to 200 knots, the parasite drag becomes four times as great, but the power required to overcome that drag is eight times the original value. Conversely, when the airplane is operated in steady, level flight at twice as great a speed, the induced drag is one-fourth the original value, and the power required to overcome that drag is only one-half the original value.
7. Discuss the relationship of thrust and power as it relates to airplane climb performance. (FAA-H-8083-25)
Climb depends upon the reserve power or thrust. Reserve power is the available power over and above that required to maintain horizontal flight at a given speed. This, if an airplane is equipped with an engine that produces 200 total available horsepower and the airplane requires only 130 horsepower at a certain level flight speed, the power available for climb is 70 horsepower.
8. Define the term “service ceiling”. (FAA-H-8083-25)
Service ceiling is the maximum density altitude where the best rate-of-climb airspeed will produce a 100 feet-per-minute climb at maximum weight while in a clean configuration with maximum continuous power.
9. Will an aircraft always be capable of climbing to and maintaining its service ceiling? (FAA-H-8083-25)
No. depending on the density altitude, an airplane may not be able to reach it published service ceiling on any given day.
10. What is the definition of “absolute ceiling”? (FAA-H-8083-25)
Absolute ceiling is the altitude at which a climb is no longer possible.
11. What is meant by the terms “power loading” and “wing loading”? (FAA-H-8083-25)
Power loading is expressed in pounds per horsepower and is obtained by dividing the total weight of the airplane by the rated horsepower of the engine. It is a significant factor in the airplane takeoff and climb capabilities.
12. Define the terms “maximum range” and “maximum endurance”. (FAA-H-8083-25)
Maximum range is the maximum distance an airplane can fly for a given fuel supply and is obtained at the maximum lift/drag ratio (L/DMAX). For a given airplane configuration, the maximum lift/drag ratio occurs at a particular angle of attack and lift coefficient, and is unaffected by weight or altitude.
13. What is ground effect? (FAA-H-8083-25)
Ground effect occurs due to the interference of the ground surface is flown at approximately one wingspan above the surface. Especially with low-wing aircraft, it is most significant when the airplane is maintaining a constant attitude at low airspeed and low altitude. For example: during landing flare before touchdown, and during takeoff when the airplane lifts off and accelerates to climb speed. A wing in ground effect has a reduction in up wash, down wash, and tip vortices. With reduced tip vortices, induced drag is reduced. When the wing is at a height equal to one-fourth the span, the reduction in induced drag is about 25 percent, and when the wing is at a height equal to one-tenth the span, this reduction is about 50 percent.
14. What major problems can be caused by ground effect? (FAA-H-8083-25)
During landing – At a height of approximately one-tenth of a wing span above the surface, drag may be 50 percent less than when the airplane is operating out of ground effect. Therefore, any excess speed during the landing phase may result in a significant float distance. In such cases, if care is not exercised by the pilot, he/she may run out of runway and options at the same time.
15. What does “flight in the region of normal command” mean?
It means that while holding a constant altitude, a higher airspeed requires a higher power setting, and a lower airspeed requires a lower power setting. The majority of all airplane flying (climb, cruise, and maneuvers) is conducted in the region of normal command.
16. What does “flight in the region of reverse command” mean? (FAA-H-8083-25)
It means that a higher airspeed requires a lower power setting, and a lower airspeed requires a higher power setting to hold altitude. It does not imply that a decrease in power will produce lower airspeed. The region of reversed command is encountered in the low speed phases of flight. Flight speeds below the speed for maximum endurance (lowest point on the power curve) require higher power settings with a decrease in airspeed. Because the need to increase the required power setting with decreased speed is contrary to the “normal command” of flight, flight speeds between minimum required power setting (speed) and the stall speed (or minimum control speed) is termed the region of reversed command. In the region of reversed command, a decrease in airspeed must be accompanied by an increased power setting in order to maintain steady flight.
17. What are examples of where an airplane would be operating in the region of reverse command? (FAA-H-8083-25)
a. An airplane performing a low airspeed, high-pitch altitude, powered approach for a short-field landing.
18. Explain how runway surface and gradient affect performance. (FAA-H-8083-25)
a. Runway surface – Any surface that is not hard and smooth will increase the ground roll during takeoff. This is due to the inability to the tires to smoothly roll along the runway. Although muddy and wet surface conditions can reduce friction between the runway and the tires, they can also act as obstructions and reduce the landing distance.
19. What factors affect the performance of an aircraft during takeoffs and landings? (FAA-H-8083-25)
a. Air density (density altitude)
20. What effects does wind have on aircraft performance? (FAA-H-8083-25)
Takeoff – A headwind increases airplane performance by shortening the takeoff distance and increasing the angle of climb. However, a tailwind decreases performance by increasing the takeoff distance and reducing the angle of climb. The pilot must carefully consider the decrease in airplane performance before attempting a downwind takeoff.
21. How does weight affect takeoff and landing performance? (FAA-H-8083-25)
Increased gross weight can produce these effects:
The effect of gross weight in landing distance is that the airplane will require a greater speed to support the airplane at the landing angle of attack and lift coefficient resulting in an increased landing distance.
22. What effect does an increase in density altitude have on takeoff and landing performance? (FAA-H-8083-25)
An increase in density results in:
The various power MAP/rpm combinations are provided so the pilot has a choice between operating the aircraft at best efficiency (minimum fuel flow) or operating at best power/speed condition. An aircraft engine operated at higher rpms will produce more friction and, as a result, use more fuel. On the other hand, an aircraft operating at higher and higher altitudes will not be able to continue to produce the same constant power output due to a drop in manifold pressure. The only way to compensate for this is by operating the engine at a higher rpm.
24. What does the term 75% brake horsepower mean? (FAA-H-8083-25)
Brake horsepower (BHP) is the power delivered at the propeller shaft (main drive or main output) of an aircraft engine. 75% BHP means you are delivering 75 percent of the normally rated power or maximum continuous power available at sea level on a standard day to the propeller shaft.
25. Explain how 75%BHP can be obtained from your engine. (FAA-H-8083-25)
Set the throttle (manifold pressure) and propeller (rpm) to the recommended values found in the cruise performance chart of your POH.
26. When would a pilot lean a normally-aspirated direct drive engine? (FAA-P-8740-13)
a. Lean anytime the power setting is 75 percent or less at any altitude.
27. What are the different methods available for learning aircraft engines? (FAA-P-8740-13)
Tachometer Method – For best economy operation, the mixture is first leaned from full rich to maximum power (peak rpm), then the leaning process is slowly continued until the engine starts to run rough. Then, enrich the mixture sufficiently to obtain a smoothly firing engine.
28. Define the following airplane performance speeds. (FAA-H-8083-25)
C. Weight and Balance
1. What performance characteristics will be adversely affected when an aircraft has been overloaded? (FAA-H-8083-25)
a. Higher takeoff speed.
2. If the weight and balance of an aircraft has changed due to the addition or removal of fixed equipment in the aircraft, what responsibility does the owner or operator have?
The owner or operator of the aircraft should ensure that maintenance personnel make appropriate entries in the aircraft records when repairs or modifications have been accomplished. Weight changes must be accounted for and proper notations made in weight and balance records. The appropriate form for these changes is “Major Repairs and Alterations”. (FAA Form 337).
3. Define the term “ center of gravity” (FAA-H-8083-25)
The center of gravity (CG) is the point about which an aircraft would balance if it were possible to support the aircraft at that point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. The CG must be within specific limits for safe flight.
4. What effect does a forward center of gravity have on an aircraft’s flight characteristics? (FAA-H-8083-25)
Higher stall speed – Stalling angle of attack reached at a higher speed due to increased wing loading.
5. What effect does an aft center of gravity have on an aircraft’s flight characteristics? (FAA-H-8083-25)
Lower stall speed – Less wing loading.
6. Define the following : (FAA-H-8083-25)
Arm – The horizontal distance in inches from the reference datum line to the center of gravity of the item.
Weight * Arm = Moment
By rearrangement of this equation to the forms:
Weight = Moment / Arm
Arm = Moment / Weight
CG = Moment / Weight
With any two known values, the third value can be found.
Remember : W A M
(Weight * Arm = Moment)
8. What basic equation is used to determine center of gravity? (FAA-H-8083-25)
Center of gravity is determined by dividing total moments by total weight.
9. Explain the term percent of mean aerodynamic chord (MAC). (FAA-H-8083-1)
Expression of the CG relative to the MAC is a common practice in larger aircraft. The CG position is expressed as a percent MAC (percent of mean aerodynamic chord), and the CG limits are expressed in the same manner. Normally, an aircraft will have acceptable flight characteristics if the CG is located somewhere near the 25% average chord point. This means the CG is located one-fourth of the total distance back from the leading edge of the average wing section.
10. If the weight of an aircraft is within takeoff limits but the CG limit has been exceeded, what actions can the pilot take to correct the situation? (FAA-H-8083-25)
The most satisfactory solution to this type of problem is to shift baggage, passengers, or both in an effort to make the aircraft CG fall within limits.
11. What simple and fundamental weight check can e made by all pilots before flight? (FAA-H-8083-25)
A useful load check can be made to determine if the useful load limit has been exceeded. This check may be a mental calculation if the pilot is familiar with the aircraft’s limits and knows that unusually heavy loads are not abroad. The pilot needs to know the useful load limit of the particular aircraft. This information may be found in the latest weight and balance report, in a logbook, or on a Major Repair and Alteration Form located in the aircraft. If the useful load limit is not stated directly, simply subtract the empty weight from the maximum takeoff weight.
12. What factors would contribute to a change in center of gravity location during flight?
The operator’s flight manual should show procedures which fully account for variations in CG travel during flight caused by variables such as the movement of passengers and the effect of the CG travel due to fuel used.
13. If actual weights for weight and balance computations are unknown, what weights may be assumed for weight and balance computations? (FAA-H-8083-1)
Some standard weights used in general aviation are:
14. How is the CG affected during flight as fuel is used? (FAA-H-8083-25)
As fuel is burned during flight, the weight of the fuel tanks will change and as a result the CG will change. Most aircraft, however, are designed with the fuel tanks positioned close to the CG; therefore, the consumption of fuel does not affect the CG to any great extent. Also, the lateral balance can be upset by uneven fuel loading or burn-off. The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition.