Aircraft Performance Flashcards

What effect does high density altitude, as compared to low density altitude, have on propeller efficiency and why?

Answer: Efficiency is reduced because the propeller exerts less force at high density altitudes than at low density altitudes.

High density altitude means the air is less dense, which significantly reduces propeller efficiency. With fewer air molecules for the propeller blades to interact with, the propeller exerts less thrust or force with each rotation. This decreased thrust directly translates to reduced aircraft performance, particularly in takeoff and climb.

What is density altitude?

Answer: The pressure altitude corrected for non-standard temperature.

Density altitude is a critical measure for aircraft performance, representing the pressure altitude corrected for non-standard temperature. Essentially, it is the altitude in the standard atmosphere where the air density is the same as the actual air density at a given location. Higher temperatures result in less dense air and thus a higher density altitude.

If the outside air temperature (OAT) at a given altitude is warmer than standard, the density altitude is

Answer: higher than pressure altitude.

Density altitude is pressure altitude adjusted for variations from standard temperature. If the outside air temperature (OAT) is warmer than the standard temperature for a given altitude, the air becomes less dense. This less dense air effectively makes the aircraft perform as if it were at a higher altitude, meaning the density altitude will be higher than the pressure altitude.

Which combination of atmospheric conditions will reduce aircraft takeoff and climb performance?

Answer: High temperature, high relative humidity, and high density altitude.

High temperature, high relative humidity, and consequently high density altitude all contribute to less dense air. Less dense air reduces engine power output, decreases propeller efficiency, and diminishes wing lift. This combination severely degrades an aircraft's takeoff and climb performance, requiring longer takeoff rolls and slower rates of climb.

What effect does high density altitude have on aircraft performance?

Answer: It reduces climb performance.

High density altitude indicates that the air is less dense, which negatively impacts aircraft performance in several ways. Less dense air reduces engine power, decreases propeller efficiency, and diminishes the lift generated by the wings. All these factors combine to significantly reduce the aircraft's ability to climb effectively.

(Refer to Figure 8.) What is the effect of a temperature increase from 25 to 50 °F on the density altitude if the pressure altitude remains at 5,000 feet?

Answer: 1,650-foot increase.

To determine the effect on density altitude, one must refer to a density altitude chart (Figure 8). At a pressure altitude of 5,000 feet, locate the density altitude corresponding to 25°F and then to 50°F. By comparing these two values on the chart, the increase in density altitude is found to be 1,650 feet, indicating less dense air at the higher temperature.

(Refer to Figure 8.) Determine the pressure altitude with an indicated altitude of 1,380 feet MSL with an altimeter setting of 28.22 at standard temperature.

Answer: 2,991 feet MSL.

To find the pressure altitude, you would use a pressure altitude chart (Figure 8) or apply the standard correction factor. With an altimeter setting of 28.22 inches of mercury (lower than the standard 29.92), the actual atmospheric pressure is lower, meaning the pressure altitude is higher than the indicated altitude. Using Figure 8 by aligning the altimeter setting with the indicated altitude of 1,380 feet, the corresponding pressure altitude is determined to be 2,991 feet MSL.

(Refer to Figure 8.) Determine the density altitude for these conditions: Altimeter setting.....................29.25 Runway temperature...................+81 °F Airport elevation..............5,250 ft MSL

Answer: 8,500 feet MSL.

First, calculate the pressure altitude using the airport elevation (5,250 ft MSL) and altimeter setting (29.25). Then, using a density altitude chart (Figure 8), locate this calculated pressure altitude and intersect it with the line representing the runway temperature of +81°F. Reading across to the density altitude scale will yield the approximate density altitude of 8,500 feet MSL.

(Refer to Figure 8.) Determine the pressure altitude at an airport that is 3,563 feet MSL with an altimeter setting of 29.96.

Answer: 3,527 feet MSL.

To determine the pressure altitude, compare the altimeter setting (29.96) to the standard pressure (29.92 inches of mercury). Since the altimeter setting is higher than standard, the actual atmospheric pressure is higher, meaning the pressure altitude is lower than the airport's MSL elevation. Using Figure 8 or the standard correction of 10 feet per 0.01 inch of mercury difference, the pressure altitude is calculated to be 3,527 feet MSL.

(Refer to Figure 8.) What is the effect of a temperature increase from 35 to 50 °F on the density altitude if the pressure altitude remains at 3,000 feet MSL?

Answer: 1,000-foot increase.

To find the change in density altitude, refer to a density altitude chart (Figure 8). Locate the pressure altitude of 3,000 feet MSL and find the corresponding density altitude for both 35°F and 50°F. The difference between these two density altitude values, as read from the chart, indicates a 1,000-foot increase in density altitude due to the temperature rise.

(Refer to Figure 8.) Determine the pressure altitude at an airport that is 1,386 feet MSL with an altimeter setting of 29.97.

Answer: 1,341 feet MSL.

Pressure altitude is the altitude in the standard atmosphere where the atmospheric pressure is the same as that observed. It is calculated by correcting the airport's true elevation for non-standard altimeter settings. When the altimeter setting is higher than the standard 29.92 inHg, the pressure altitude will be lower than the airport's elevation. In this case, an altimeter setting of 29.97 inHg (0.05 inHg higher than standard) results in a pressure altitude of 1,341 feet MSL, which is 45 feet lower than the airport's 1,386 feet MSL elevation.

(Refer to Figure 8.) Determine the density altitude for these conditions: Altimeter setting.......................30.35 Runway temperature.....................+25 °F Airport elevation................3,894 ft MSL

Answer: 2,000 feet MSL.

Density altitude is a crucial performance factor, representing the altitude at which the air density is equivalent to that in the standard atmosphere. It is derived by first calculating the pressure altitude and then correcting it for non-standard temperature. In this scenario, the altimeter setting is higher than standard, lowering the pressure altitude, and the ambient temperature (+25°F) is significantly colder than the standard temperature for that pressure altitude. This combination results in a density altitude of 2,000 feet MSL, indicating better-than-standard performance conditions.

(Refer to Figure 8.) What is the effect of a temperature decrease and a pressure altitude increase on the density altitude from 90 °F and 1,250 feet pressure altitude to 55 °F and 1,750 feet pressure altitude?

Answer: 1,700-foot decrease.

Density altitude is directly affected by both temperature and pressure altitude. A decrease in temperature generally leads to a lower density altitude because colder air is denser. Conversely, an increase in pressure altitude generally leads to a higher density altitude as air pressure decreases with altitude. In this scenario, the significant temperature decrease from 90°F to 55°F has a dominant effect, outweighing the increase in pressure altitude from 1,250 feet to 1,750 feet, resulting in a net decrease of 1,700 feet in density altitude.

What effect, if any, does high humidity have on aircraft performance?

Answer: It decreases performance.

High humidity means there is a greater concentration of water vapor in the air. Water vapor is lighter than the nitrogen and oxygen molecules that make up dry air. Therefore, humid air is less dense than dry air at the same temperature and pressure. This lower air density reduces engine power, propeller efficiency, and wing lift, consequently decreasing overall aircraft performance.

Under which condition will pressure altitude be equal to true altitude?

Answer: When standard atmospheric conditions exist.

Pressure altitude is the altitude in the standard atmosphere where the atmospheric pressure is the same as that observed, while true altitude is the actual height above mean sea level. These two values are equal only when standard atmospheric conditions exist. Standard conditions are defined as an altimeter setting of 29.92 inHg and a temperature of 15°C (59°F) at sea level, with a standard lapse rate.

Under what condition is pressure altitude and density altitude the same value?

Answer: At standard temperature.

Density altitude is the pressure altitude corrected for non-standard temperature. For pressure altitude and density altitude to be the same value, there must be no correction needed for temperature. This occurs precisely when the ambient temperature is equal to the standard temperature for that specific pressure altitude, meaning the air density matches the standard atmosphere for that pressure level.

Which factor would tend to increase the density altitude at a given airport?

Answer: An increase in ambient temperature.

Density altitude increases when the air becomes less dense. An increase in ambient temperature causes the air molecules to spread out, making the air less dense. This reduction in air density leads to a higher density altitude, which negatively impacts aircraft performance by reducing engine power, propeller efficiency, and wing lift.

(Refer to Figure 35.) Approximately what true airspeed should a pilot expect with 65 percent maximum continuous power at 9,500 feet with a temperature of 36 °F below standard?

Answer: 183 MPH.

To determine true airspeed, consult the aircraft's performance chart (Figure 35) for the specified power setting and pressure altitude. First, locate the 65% maximum continuous power section and the 9,500-foot pressure altitude line. Then, apply the temperature correction for 36°F below standard, which typically involves moving along the chart's temperature correction lines or interpolating. Following these steps on the chart reveals an approximate true airspeed of 183 MPH.

(Refer to Figure 35.) What is the expected fuel consumption for a 500-nautical mile flight under the following conditions? Pressure altitude........................4,000 ft Temperature...............................+ 29 °C Manifold pressure........................21.3" Hg Wind.........................................Calm

Answer: 36.1 gallons.

To determine fuel consumption, first use Figure 35 to find the true airspeed (TAS) and fuel flow (GPH) for the given pressure altitude (4,000 ft), temperature (+29°C), and manifold pressure (21.3" Hg). With calm wind, the TAS equals the ground speed. Calculate the flight time by dividing the 500-nautical mile distance by the determined TAS. Finally, multiply the fuel flow rate by the flight time to arrive at the total fuel consumption of 36.1 gallons.

(Refer to Figure 35.) What fuel flow should a pilot expect at 11,000 feet on a standard day with 65 percent maximum continuous power?

Answer: 11.2 gallons per hour.

To find the expected fuel flow, refer to the aircraft's performance chart (Figure 35). Locate the section for 65 percent maximum continuous power. Follow this line to the 11,000-foot pressure altitude. Since it's a standard day, no temperature correction is required. Reading directly from the chart at this intersection will provide the expected fuel flow, which is 11.2 gallons per hour.