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SW Aviator Feb/Mar 2001
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By Timothy Lanigan

It’s a beautiful afternoon in the Albuquerque area with light winds. I am going flying! Never mind that this is an interview flight for a flight instructor position. I am just happy to be back in the Southwest after having spent the previous year sweltering in the Florida sun. I arrive for the flight; all goes well with the preflight and we are ready to fly. When we’re cleared, I add power for a short field takeoff. We’re rolling down the runway, the airspeed hits 55 knots, and I pull back briskly on the yoke to rotate. The nose comes up but the airplane doesn’t lift off and climb as I expect. Instead, the airplane laughs at me in the form of the blaring stall horn. Instantly I realize that I have made the mistake of complacency and forgotten about the effects of altitude on an airplane’s performance. This incident causes me to evaluate performance considerations, a topic each of us should periodically review.

A good place to start the review is groundspeed. Assuming no wind conditions, an airplane traveling at 90 knots indicated airspeed at sea level is going 90 knots true airspeed (and because there is no wind, 90 knots groundspeed.) That same airplane at 5,000 feet, maintaining the 90 knots of IAS, will have a true airspeed of 99 knots. (This comes from the general rule that you increase the TAS 2% per 1,000 feet in altitude.) Thus, in this situation, the airplane will have a 9-knot higher groundspeed for the same airspeed. But enough math. How does this translate to my flying? Imagine we are flying in Florida, where the highest elevation is about 300 feet MSL. On final approach, the groundspeed will seem extremely slow to those of us who are used to flying in the thin air of the Southwest. Conversely, those used to flying and landing at sea level will suddenly feel as though they are dragging behind the airplane as it whizzes along above the ground.

The second issue arising from a discussion of aircraft performance at varying altitudes is density altitude. When student pilots in Florida calculate density altitude, it turns out to be close to the airport elevation almost every time. Living in that environment, pilots can become complacent about figuring out the DA because it rarely has a major bearing on the performance of the airplane. However, the trap that many pilots from lower elevations fall into is not considering density altitude where it really matters, up here amongst the mountains. Let’s consider a typical summer day in Albuquerque using an air temperature of 25 degrees Celsius and an altimeter setting of 29.98. The airport elevation at Albuquerque is 5352’ MSL. To calculate our density altitude we need to first find the current pressure altitude. There are several ways to do this. The easiest is to set the airplane’s altimeter to 29.92 and read the altitude. The method that I use is the equation 29.92-(current altimeter setting) x 1000. I then take this number and add it to the field elevation. Using this equation we find that 29.92-29.98= -0.06. Multiply that by 1,000 and get -60. When -60 and the field elevation are added together, we get a pressure altitude of 5,292 feet. Using a density altitude table, we find that the density altitude is approximately 6,650 feet.

How does this affect the performance of an airplane? Let’s look at a Cessna 172 in the above conditions. First let’s examine the takeoff performance without factoring in density altitude. The chart for 5,000 feet gives a takeoff distance of 1,500 feet ground roll and 2,895 over a 50-foot obstacle. However, using the density altitude rather than the pressure altitude in the chart, the ground roll will be 1,850 feet and the obstacle distance will be 3,805 feet; an increase in takeoff distance of 350 and 910 feet respectively. Additionally, the pilot can expect a lower rate of climb and a longer distance to climb at higher elevations.

These brief examples present a clearer picture of the necessity for those of us flying in higher elevations to consider density altitude. Additionally, for those venturing from the flatlands, I would recommend taking some form of mountain flying course to acclimate yourself to the distinct methods of operation in and around mountainous terrain.
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The material in this publication is for advisory information only and should not be relied upon for navigation, maintenance or flight techniques. SW Regional Publications and the staff neither assume any responsibility for the accuracy of this publication's content nor any liability arising fom it
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