VINTAGE INSTRUCTOR
THE
BY DOUG STEWART
The “DA” The abbreviation “DA” means different things to different folks. For people who get involved with court proceedings, it brings to mind a state or county prosecutor. For those who grew up in the ’50s, it might evoke a hairstyle resembling the posterior of a waterfowl. But for pilots it should mean only one thing: density altitude. Unfortunately, I have found— not only as an examiner asking an applicant to describe what density altitude is during a practical test, but also as an interested pilot perusing the National Transportation Safety Board (NTSB) accident records—that many pilots really don’t understand what density altitude is. And without that understanding, many are getting themselves into bad situations because they fail to recognize the ramifications of highdensity altitudes. It’s true that many pilots can give the “official” definition of density altitude: pressure altitude adjusted for nonstandard temperature. But when asked how they might describe DA to a young child, they are at a loss. Before I offer my simple description of DA, let’s look at the definition first. We’ll begin with pressure altitude. The easiest way to explain pressure altitude is to say that it is indicated altitude on the altimeter, when the altimeter is set to the standard pressure of 29.92 inches Hg. Thus, the higher the atmospheric pressure, the lower the pressure altitude, and
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vice versa. We now adjust this altitude for nonstandard temperature. You will remember that standard temperature is 15°C (59°F). As the temperature rises above this standard, so will the density altitude.
And without the understanding, many are getting themselves into bad situations . . . But what if you are as numerically challenged as I am, or not of a scientific bent? We certainly don’t want to see our aircraft get bent, but it (your airplane) might very well end up rolled up in a ball if you fail to comprehend this important concept and its impact on aircraft performance. (You will, on the other hand, recognize its impact on the ground.) So I offer this simple explanation that even a young child could understand: Density altitude is the altitude that
your airplane “thinks” it’s at. If an aircraft were a sentient being capable of thoughts and feelings, it would factor in the mean sea altitude it was at, the barometric pressure, the temperature, and the humidity (which plays a major part in affecting aircraft performance, even though it is not factored into density-altitude calculations) and come up with a “feels like” altitude. The higher this “feels like” altitude, the more cautious we, as pilots, need to be. I hope we all know that the higher we go, the less dense the air gets, and the less dense the air gets, the poorer the aircraft performance gets, especially when it comes to takeoff, landing, and climb performance. This is really aeronautical knowledge 101. That being said, why is it that over the last five years (May 2003 through May 2008) the NTSB records show that there were 138 airplane accidents, including 79 fatalities, in which density altitude played a major part? (I’m sure if you asked an insurance underwriter whether he concurred with these numbers, he would come up with even more claims, as many accidents that might involve density altitude do not necessarily have to be reported to the NTSB.) As I looked at mentions of “probable cause” in the NTSB reports, I came up with the following statistics: • In 41 (29.7 percent) of these accidents, “poor pilot planning” (in many cases, no planning whatso-
ever) was a contributor to the event. • Twenty-eight of the reported accidents (20.2 percent) occurred at airports or in high mountainous areas. • Twenty-three of them (16.6 percent) happened in “vintage” (manufactured before 1968) airplanes. • Thirteen (9.4 percent) of them happened during the attempted go-around. • In 11 cases, an instructor was on board. • In 10 of them, the aircraft exceeded the maximum certified gross weight. It was interesting to note several recurring themes in these accidents. Many times pilots neglected to configure the airplane appropriately. Improper use of flaps was often a contributing factor. There were numerous takeoff and go-around accidents where pilots failed to properly lean the engine. But the thing that most stood out was that in many instances pilots had sufficient time to take proper action—such as aborting a takeoff, initiating an early go-around, or turning before they impacted a mountain ridge— but failed to do so. Let’s look at a couple of them. The first was the crash of a Taylorcraft BC12-D, which caused one fatality: “According to several witnesses in the area, the pilot had been attempting to land to the west on a grass strip. The pilot had made approximately five attempts to land prior to the accident. During the sixth approach, the airplane touched down approximately midfield, the pilot added power, and the airplane became airborne again. Witnesses stated that the airplane struck a road embankment at the end of the runway, continued in a steep climb, and then struck several 60-foot-high aspen trees approximately 150 feet west of the end of the runway. The airplane rolled off hard to the right and impacted the southbound lane of a county road in a nose-low attitude. Airport elevation was approximately 7,800 feet mean sea level. Density altitude was calculated to be 10,063 feet. The air-
port runway is surrounded on every side by vegetation, and terrain elevation rises dramatically in all directions. According to the owner of the airport, it is recommended that pilots land to the east and depart to the west due to the obstacles and terrain near the airport.” The second accident involved a Stearman and led to two minor injuries: “The pilot reported that he was concerned with the field elevation, airplane weight, heat, and humidity
It was interesting to note several recurring themes in these accidents. before the flight. During the takeoff roll, the airplane had a slower acceleration and longer takeoff roll than normal. The pilot considered aborting the takeoff twice, but was concerned that there was not enough available runway to land and felt that he would be able to out-climb the terrain located at the end of the runway. After bouncing twice on the runway, the airplane began to climb in ground effect, about 100 feet per minute. When the pilot realized that he would not clear the terrain, he lowered the nose in an attempt to gain airspeed. He located an area of lower terrain, made a shallow right turn, and attempted to fly through the area; however, the airplane sank into the trees and rolled. The pilot stated that the engine sounded as if it was producing full power, and that he was unfamil-
iar with the airplane’s high-densityaltitude performance capabilities.” It would appear that not only in these two accidents, but also in so very many others, the pilot, rather than relying on sound decisionmaking and preflight performance planning, relied more on hope. Now, hope is a wonderful thing! It certainly has its place in presidential campaigns. But there is absolutely no room for hope when it comes to aviation. “Hope” will not increase airplane performance as the departure end of the runway gets closer and closer, and the airplane still won’t rotate. “Hope” won’t increase an airplane’s rate of climb as obstacles loom in the windscreen. “Hope” won’t get an airplane flying again when one has waited too long before initiating a go-around. “Hope” won’t save the life of someone who has been fatally injured in an airplane accident. However, I do hope that the following suggestions will help prevent you from having an accident in which density altitude plays a role. You don’t have to be at high field elevations to become a victim of this. Anytime the density altitude is 2,000 feet or more above your field elevation it’s time to pay attention. It’s time to have as much information available as possible. Certainly the first place to look for information is in your airplane’s pilot’s operating handbook (POH). (Unfortunately, for many vintage aircraft, a POH might not even exist.) Find out the manufacturer’s recommendations for aircraft configuration and engine-leaning requirements in high-densityaltitude environments. Refer to the takeoff, climb, and landing performance charts to get an idea of expected performance. Keep in mind that the performance figures in the POH were obtained in a brand-new aircraft flown by a highly qualified test pilot. Factor that in as you adjust the figures to reflect an accurate and realistic expectation of the performance you will get in your airplane, with you at the controls.
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If the density altitude is at 5,000 feet or higher, leaning the engine for maximum power is highly recommended. Again, the POH will rule on this, but in the absence of a POH, here are some suggestions. For start-up and taxi, lean at 1000 rpm (all propeller combinations) until the rpm peaks; then enrich slightly. Before takeoff, go to full throttle and lean the mixture. With a fixedpitch prop, lean to maximum rpm and then enrich slightly. With a variable-pitch prop, on carbureted engines, lean to engine smoothness. If you have an EGT gauge, lean to +100°F on the rich side of peak. With a fuel-injected engine, lean to the correct fuel-flow setting according to the POH for your specific airplane (often, though not always, marked on the fuel-flow gauge). To ensure maximum available power in the event that you need to make a go-around at a high-altitude airport, do not apply full rich mixture as part of your landing checklist. Keep in mind that departures will have a greater rate of success when made during cooler times, such as early in the morning or late in the evening. Remember that you might have to reduce your takeoff weight by draining fuel or leaving behind the mother-in-law (she’ll tell you to leave behind the big bucket of chicken and the cooler filled with beverages). Don’t forget that what worked in the depths of the winter or at the seaside airport might not work at all at the height of summer or in the mountains. Now that it’s summertime, please be especially aware of the DA. No, not when you have a court appointment. No, not when you’re going to have your hair cut. Be aware of the density altitude every time you are beckoned aloft by…blue skies and tail winds. Doug Stewart is the 2004 National CFI of the Year, a NAFI Master Instructor, and a designated pilot examiner. He operates DSFI Inc. (www. DSFlight.com) based at the Columbia County Airport (1B1).
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