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CFI Brief: Density Altitude

Monday’s post introduced us into the subject of aircraft performance, focusing on how the environment we fly in, specifically the atmosphere and altitude, can affect the overall performance of the aircraft. Two very important altitudes were discussed; Pressure Altitude and Density Altitude both of which are significant when determining aircraft performance. Be particularly aware when your conditions are hot, high or heavy—all indications your density altitude is going to negatively impact your aircraft performance. If you take away one thing from that post remember this: Higher Density Altitude = Decreased Performance.

Understanding the effects of density altitude is one thing but knowing how to determine the density altitude at any given point is another, so today let’s cover how to determine your density altitude. The first step is obtaining the airport data and current weather information. For this example we will use Big Bear City Airport (L35). Using the AF/D I determined an airport elevation of 6,752 feet. This piece of information alone should start to raise some concerns regarding aircraft performance—this is pretty high. Next I will need the current METAR (assuming it’s a nice summer day in August):

METAR KL35 102055Z AUTO 00000KT 10SM CLR 25/01 A3025 RMK AO2

Airport Elevation: 6,752

With the given information above we can determine the density altitude with our CX-2 flight computer, as the video below will demonstrate.

The current density altitude at Big Bear is 9,019; this is more than 2,200 feet above the already high airport elevation.

Try a few calculations on your own and see how you do, these are questions you will likely see on your Private Pilot FAA Knowledge Exam. Answers posted on Monday.

1. (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?
A—1,200-foot increase.
B—1,400-foot increase.
C—1,650-foot increase.

2. (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
A—4,600 feet MSL.
B—5,877 feet MSL.
C—8,500 feet MSL.

3. (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
A—2,000 feet MSL.
B—2,900 feet MSL.
C—3,500 feet MSL.

CT-8080-2F Figure 8.

CT-8080-2F Figure 8. (Click to enlarge!)

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One Comment

  1. Posted December 15, 2014 at 6:25 am | Permalink

    1- (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?

    C- 1,650-foot increase.

    Referencing FAA Figure 8, use the following steps:
    1. Enter the density altitude chart at 25°F. Proceed upward to intersect the 5,000-foot pressure altitude line. From the point of intersection, move left to the edge of the chart and read a density altitude of 3,850 feet.
    2. Enter the density altitude chart at 50°F. Proceed upward to the 5,000-foot pressure altitude line. From the point of intersection, move left to the edge of the chart and read a density altitude of 5,500 feet.
    3. Determine the change in density altitude:
    5,500 – 3,850 = 1,650 feet (increase)


    2- (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

    C- 8,500 feet MSL.

    Referencing FAA Figure 8, use the following steps:
    1. Since the altimeter setting that is given is not shown in FAA Figure 8, interpolation is necessary. Locate the settings immediately above and below the given value of 29.25″ Hg:
    Altimeter Setting Conversion Factor
    29.20 673 feet
    29.30 579 feet
    2. Determine the difference between the two conversion factors: 673 – 579 = 94 feet
    3. Determine the amount of difference to be added to the 29.30″ Hg conversion factor: 94 x .5 = 47 feet
    4. Add the amount of difference to the amount shown for the 29.30″ Hg conversion factor: 579 + 47 = 626 feet
    5. Add the correction factor to the airport elevation to find pressure altitude: 5,250 + 626 = 5,876 feet MSL (pressure altitude)
    6. Determine the density altitude by entering the chart at +81°F; move upward to the 5,876 pressure altitude line; from the point of intersection, move to the left and read a density altitude of 8,500 feet.


    3- (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

    A- 2,000 feet MSL.

    Referencing FAA Figure 8, use the following steps:
    1. Since the altimeter setting that is given is not shown in FAA Figure 8, interpolation is necessary. Locate the settings immediately above and below the given value of 30.35″ Hg:
    Altimeter Setting Conversion Factor
    30.30 -348 feet
    30.40 -440 feet
    2. Determine the difference between the two factors: -440 + 348 = -92 feet
    3. Determine amount of difference to be added to the 30.30″ Hg conversion factor: -92.0 x .5 = -46.0 feet
    4. Add the amount of difference to the amount shown for the 30.30″ Hg conversion factor: -348 + (-46) = -394 feet
    5. Subtract the correction factor from the airport elevation to find pressure altitude: 3,894 – 394 = 3,500 feet MSL (pressure altitude)
    6. Determine the density altitude by entering the chart at +25°F; proceed upward to the 3,500-foot pressure altitude line; from the point of intersection move to the left edge of the chart and read a density altitude of 2,000 feet.

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