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July 1, 2007
By Barry Schiff
Aviation writer Barry Schiff lives in Southern California.
Earlier this year I was invited by The Ventura County (California) Chapter of the Ninety-Nines to teach a three-hour class about mountain flying to local pilots. One aspect of the course was the obviously obligatory discussion about density altitude (DA). I thought I would be able to sail quickly through the subject because surely every pilot in attendance would thoroughly understand this operational hazard. I was wrong.
I began to realize that the concept of DA was not well understood when I introduced a rule of thumb that can be used to quickly and easily determine density altitude without calculator or computer. Many can do it in their heads without pencil or paper. (See " Flying Seasons: Density Altitude," page 121.)
"Assume that we are at nearby Big Bear airport," I began, "where the elevation is almost 7,000 feet. The first thing needed to determine density altitude is the standard temperature at that airport. Can you tell me what it is?"
I asked the question in a way that suggested I wanted the answer to be said aloud. Although I heard many correct answers, I heard many more pilots call out, "15 degrees Celsius" or "59 degrees Fahrenheit." Oops. Those answers were incorrect.
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).
It is this simple misconception that often disguises the hazards associated with high density altitudes.
When a misinformed pilot prepares to depart on a day when the temperature at Big Bear is 15 degrees C, he might subconsciously say to himself, "Hey, this is a standard day. The density altitude here is the same as the elevation. No problem." But there is a problem. When the temperature is 15 degrees C at Big Bear, it is actually 14 degrees C higher than standard; DA is 8,600 feet msl, not 7,000. Flash-forward to the summer, when the temperature at a 7,000-foot-high airport might be 32 degrees C (90 degrees F), which is 31 degrees higher than the standard temperature of 1 degree (at that lofty elevation). This elevates DA to a whopping 10,500 feet msl.
Once this concept was understood by the class, I continued with the rule of thumb I had intended to discuss earlier: Every degree C higher than standard temperature for a given elevation increases density altitude by about 105 feet, but 100 feet is close enough for government work; every degree F above standard elevates density altitude by about 60 feet. This allows us to determine DA without a whiz wheel or an electronic computer.
Assume you are departing an airport at an elevation of 5,500 feet msl. We mentally calculate that the standard temperature there is 4 degrees C, right? Right. If the reported temperature is 28 degrees C, this would be 24 degrees above standard, and this results in a density altitude of 5,500 feet (elevation) plus 2,400 feet, or about 7,900 feet. In other words, density altitude is the same as it would be at 7,900 feet msl on a standard day.
If the altimeter setting is above or below 29.92 inches mercury, add another 100 feet of density altitude for each tenth of an inch below 29.92 or subtract 100 feet for each tenth of an inch above 29.92. Unless the deviation from 29.92 inches is significant, compensating for changes in altimeter setting is little more than a tempest in a teapot and can be ignored. Humidity should not be ignored but almost always is because aircraft performance charts do not include it as a factor but should. Although even the most humid air is not that much lighter than dry air, it causes reciprocating engines to lose considerable power. For example, on an 80-degree F day when the relative humidity is 90 percent, power loss resulting solely from the presence of water vapor is 6.5 percent. Higher temperatures and relative humidity can reduce power by as much as 10 percent.
Such a power loss is significant anytime but can be critical when operating at high density altitudes. Because the precise effect of humidity involves complex calculations and is difficult to determine, pilots can compensate by raising the calculated DA by 1,000 feet on hot, humid days and decreasing calculated performance by a conservative fudge factor of 10 percent. This accounts only for power loss because of the humidity.
Finally, consider that the temperature used to determine density altitude usually is the temperature broadcast over an ATIS (automatic terminal information service) or ASOS (automated surface observation system) facility, which means that it is measured in the shade. Actual temperature on the runway, which is seldom in the shade, can be substantially higher. Air temperatures taken immediately above asphalt surfaces exposed to the sun on a hot day can be as much as 40 degrees higher than temperatures measured in the shade.
This clearly demonstrates that the insidious hazards associated with operations at high density altitudes might not always be fully appreciated. It explains why experienced mountain pilots occasionally do not refuel after landing at high-density-altitude airports. They prefer instead to wait until the hour of departure to determine how much weight can be carried safely. Departures often are made early and during the coolest time of day when density altitude is at its lowest.
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