One of summers challenges
June 1, 2019
Minnesota is strikingly beautiful no matter which season, but summer is quite simply amazing. With brilliant blue skies, diamond sparkles on the lakes, and a patchwork quilt of rolling prairie as far as the eyes can see, the beauty is seemingly endless. Even though it can get quite warm and humid, Minnesota’s summers range from tolerable to terrific and it validates why we choose to live here!
While summer flying is generally awesome, it brings about some unique meteorological challenges that pilots must be aware of and know the proper actions to take to mitigate risk. Some of the more obvious and common challenges are thunderstorms, rapidly moving fronts, and pop-up weather systems that may cover a very small or a very large area along your route of flight. One challenge that may not be as common to many pilots in the summer season is density altitude.
According to the Pilots Handbook of Aeronautical Knowledge (PHAK), density altitude (da) is pressure altitude corrected for nonstandard temperature. Because aircraft operate in a nonstandard atmosphere, the term density altitude is used for correlating aircraft performance in the nonstandard atmosphere.
Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. 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
• Thrust, because a propeller is less efficient in thin air
• Lift, because the thin air exerts less force on the airfoils.
Quite simply, density altitude is the altitude your aircraft “feels” it is flying.
The PHAK goes on to say, density altitude is determined by first finding pressure altitude, and then correcting this altitude for nonstandard 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.
The Aircraft Owners and Pilots Association (AOPA) says when you calculate density altitude, “Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F). For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows.
• 18 – 1 = 17
• 17 x 120 = 2040
• 2040 + 7000 = 9040 feet Density Altitude
This means the aircraft will perform as if it were at 9,040 feet.”
• Pressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter.
• OAT stands for outside air temperature (in degrees Celsius).
• ISA stands for International Standard Atmosphere (temperature in degrees Celsius).
For additional information, review the Aeronautical Information Manual, Chapter 7, Safety of Flight, Section 5, 7-5-6 Mountain Flying. Also see the Pilots Handbook of Aeronautical Knowledge, Chapter 4, section 4-3. Another source of information is the AOPA Safety Advisor Operations & Proficiency No. 6.
The Air Safety Institute says: “The “simple” act of taking off or landing accounts for 50 percent of all general aviation accidents.” So on those hot, humid days of summer when skies are clear and going flying is the only rational thing you can think of doing, remember to calculate density altitude before you takeoff. That way one of summers challenges won’t catch you unaware and unprepared.