Understanding Density Altitude: Its Impact on Aircraft Performance, Study notes of Aviation

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FAA–P–8740–2 • AFS–8 (2008) HQ-

Density Altitude

Federal Aviation

Administration

Density Altitude

2. Temperature. The warmer the air, the less dense it is. When the temperature rises above the standard tempera- ture for a particular place, the density of the air in that location is reduced, and the density altitude increases. Therefore, it is advisable, when performance is in question, to schedule operations during the cool hours of the day (early morning or late afternoon) when forecast temperatures are not expected to rise above normal. Early morning and late evening are sometimes better for both departure and arrival. 3. Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient tempera- tures, the atmosphere can retain a high water vapor content. For example, at 96 oF, the water vapor content of the air can be eight (8) times as great as it is at 42 oF. High density altitude and high humidity do not always go hand in hand. If high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.

Check the Charts Carefully Whether due to high altitude, high temperature, or both, reduced air density (reported in terms of density altitude) adversely affects aerodynamic performance and decreases the engine’s horsepower output. Takeoff distance, power available (in normally aspirated engines), and climb rate are all adversely affected. Landing distance is affected as well; although the indicated airspeed (IAS) remains the same, the true airspeed (TAS) increases. From the pilot’s point of view, therefore, an increase in density altitude results in the following:

• Increased takeoff distance.
• Reduced rate of climb.
• Increased TAS (but same IAS) on approach and landing.
• Increased landing roll distance.

Because high density altitude has particular implications for takeoff/climb performance and landing distance, pilots must be sure to determine the reported density altitude and check the appropriate aircraft performance charts carefully during preflight preparation. A pilot's first reference for aircraft performance information should be the operational data section of the aircraft owner's manual or the Pilot’s Operating Handbook developed by the aircraft manufacturer. In the example given in the previous text, the pilot may be operating from an airport at 500 MSL, but he or she must calculate performance as if the airport were located at 5,000 feet. A pilot who is complacent or careless in using the charts may find that density altitude effects create an unexpected—and unwelcome—element of suspense during takeoff and climb or during landing.

If the airplane flight manual (AFM)/POH is not available, use the Koch Chart to calculate the approximate temperature and altitude adjustments for aircraft takeoff distance and rate of climb.

At power settings of less than 75 percent, or at density altitude above 5,000 feet, it is also essential to lean nor- mally aspirated engines for maximum power on takeoff (unless the aircraft is equipped with an automatic altitude mixture control). Otherwise, the excessively rich mixture is another detriment to overall performance. Note: Turbocharged engines need not be leaned for takeoff in high density altitude conditions because they are capable of producing manifold pressure equal to or higher than sea level pressure.

Density Altitude

Density Altitude Charts

Density Altitude Rule-of-Thumb Chart

The chart below illustrates an example of temperature effects on density altitude.

Density Altitude Rule-of-Thumb Chart

59 oF Sea level 1,200 1,900 2,500 3,200 3,800 4, 52 oF 2,000 3,800 4,400 5,000 5,600 6,200 6, 45 oF 4,000 6,300 6,900 7,500 8,100 8,700 9, 38 oF 6,000 8,600 9,200 9,800 10,400 11,000 11, 31 oF 8,000 11,100 11,700 12,300 12,800 13,300 13,

STD TEMP ELEV/TEMP 80 oF 90 oF 100 oF 110 oF 120 oF 130 oF

Koch Chart

To find the effect of altitude and temperature, connect the temperature and airport altitude by a straight line. Read the increase in takeoff distance and the decrease in rate of climb from standard sea level values.