Notes from FW Chief - Peter Swift
Now that we finally have summer weather, well at least sunshine and some warmth, let's review density altitude.
An aircraft is designed to operate and perform at its best under standard atmospheric conditions. These are a barometric pressure of 29.92 inches of Hg and a temperature of 59 degrees F or 15 degrees C. Think of it as the main elements of an aircraft that provide power and lift (engine and wings) are designed to "see" a certain concentration of air molecules in a known volume. As the temperature increases and/or altitude increases, the number of air molecules in a given area diminishes. This directly affects the power output of the engine, thrust generated by the propeller and the lift the wings can generate.
A classic case especially during the summer months is the "hot and high" airport. Take for example Flagstaff in Arizona, KFLG with an altitude of 7,000 ft. With a temperature of 80F, fairly normal for this time of year, the density altitude is over 10,000 ft. In other words your aircraft will operate as if it is at an elevation of 10,000 ft. Have a look at the takeoff charts for a typical C172 and you will find that with just 2 passengers and full fuel, you might not even be able to take off let alone climb to get over the mountains! Remember that as PIC it is your responsibility to know all of the variables that pertain to the flight you wish to undertake. As one would expect there is an FAA rule to this effect - 91.103.
Density altitude can easily and quickly be computed with the ever trusty E6B and their electronic equivalents, not to mention the apps now available for smart phones that will also do the calculations. Once you have computed the density altitude, use the performance tables in the POH to calculate the takeoff distance required and expected climb gradient. Remember, the POH tables are written for a new aircraft in perfect working order, operating at max efficiency and flown by a test pilot. Be conservative as you work through the numbers and it if is close, far better and safer to wait until the temperature comes down - takeoffs in the early morning will usually allow you to depart safely, but also be aware of the effect of moisture in the air. A higher humidity also drives up density altitude. Think of it as the water molecules in humid air displacing the air molecules, so again, the aircraft systems "see" less air than is optimum.
Enjoy the fine weather while it lasts, but be aware of the effect that high temperatures and mountain airports can have on the performance of your aircraft.
Fly safe
Peter Swift, Chief Instructor
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