Density Altitude, Part Two


In the last issue of Ops Bull, we discussed density altitude (DA) and its affect on your maneuver size, your true airspeed, and your need to plan ahead. In this section, we will discuss how to calculate performance loss in a high DA, planning your high DA routine, and other factors to consider at a high DA.

So how do you know how much performance you’ll lose? The Koch Chart calculates performance loss for climb and takeoff under normal (non-aerobatic) conditions. If we make the assumption that climb performance loss equals performance loss through powered maneuvers, then we can use the chart to indicate the increase in altitude necessary for these maneuvers. Looking at the chart, we see that at 3000 feet MSL and 85°F, we will experience a 50 percent reduction in climb performance. This indicates that you should double your start altitude for powered maneuvers. At 6000 feet MSL and 100°F, we get a 75 percent reduction in climb performance, indicating the necessity to quadruple your start altitude!

For pilots at a high DA show, there are several things to try to help counter the adverse affects of decreased performance.  Change the choreography of your sequence slightly. Consider removing descending pull maneuvers, such as reverse Cubans and square loops. Soften your maneuvers. Barreling your rolls slightly or pulling one less G in a loop can save lots of energy. Leave out a few snaps and tumbles. Make larger reversing turns. Since each routine is different, only the PIC will be able to tell exactly what routine modifications with which he is able to cope.

High DA doesn’t affect only the airplane. Other factors to consider at high DA shows are reduced pilot G tolerance, which can substantially reduce your ability to cope with unexpected situations. Poor engine cooling in the thin air may require a reduction of power or a break to allow temperatures to return to normal. Hilly terrain in the air show box can be a subtle killer. Inconsistent or high horizon can cause you to misinterpret your attitude, and rotor downwash downwind of ridges can cause turbulent, descending air. All of these situations are common in the mountain west.

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