The benefits of getting high

On one, I’ll be flying my Beechcraft Bonanza, Niky, from Tennessee to California to spend time with my family and friends where I grew up.

Once I have a route, electronic flight bags like ForeFlight make quick work of refining those plans and ensuring Niky is up to the mission. For example, if forecast winds are available, it’s easy to select an altitude that minimizes flight time and ensures that I won’t go below my conservative fuel reserve minimums. If headwinds become stronger with altitude, lower might be better. But sometimes it’s worth slowing down a bit and using more than wind information in altitude selection.

For this discussion, we’ll assume that a normally aspirated airplane like Niky flies in calm winds on a standard day.

Efficiency: I’ve heard for years the general rule that, for airplanes like the Bonanza, altitudes around 8,000 feet msl offer a sweet spot in terms of efficiency. But what’s meant by efficiency isn’t clear. The chart in my pilot’s operating handbook (POH) that gives pressure altitude versus true airspeed—what I call the knee diagram (see Figure 1)—shows that, for each fixed power setting, the associated true airspeed increases with altitude until the associated manifold pressure can no longer be sustained. Above the altitude where the maximum occurs, the true airspeed begins to decrease.

I’m often more concerned about fuel efficiency, especially if it means avoiding a fuel stop. Although the knee diagram is silent on fuel efficiency, we can tease the information out by adding another column to the POH’s Cruise Power Setting table. For each configuration, dividing true airspeed (knots) by fuel flow (gallons per hour) will yield nautical miles per gallon. These calculations demonstrate that fuel efficiency increases with altitude all the way up to the highest detailed altitude of 16,000 feet msl. The change is more dramatic with higher power settings, even when accounting for the extra fuel it takes to get to those higher cruise altitudes (see Figure 2).

For a 650 nm leg flying 65 percent power in Niky, flying at 8,000 feet msl requires bumping up against my personal fuel minimums, so a fuel stop might be necessary. At 12,000 feet, the trip takes just 15 minutes longer but provides an acceptable fuel buffer (see Figure 3). If the fuel stop is necessary, then the higher altitude is both time and fuel efficient.

Greater protection in turbulence: For airplanes that don’t list a gust penetration airspeed, flying below maneuvering speed, V_A, is a good practice in turbulent conditions. The idea is that your airplane is better protected from structural damage when maneuvering aggressively below V_A, then the aircraft structure is similarly protected in gusty conditions below that airspeed. For my Bonanza, V_A is 134 KIAS or 132 KCAS. The version of speed that pertains to aircraft stresses is calibrated airspeed (or equivalent airspeed if compressibility effects are at play). At sea level, calibrated airspeed is the same as true airspeed but, as altitude increases, the gap between the two widens. For example, using 75 percent power at 6,000 feet msl, the calibrated airspeed is 157 KCAS (172 KTAS) while, at 14,000 feet, the calibrated airspeed is 131 KCAS (163 KTAS), below maneuvering speed. At higher altitudes, we can enjoy high true airspeeds with far less concern about the structural damage that turbulence can impose.

Improved glide distance: Glide distance is proportional to height above terrain so being twice as high above the ground translates to gliding twice as far. To reach the flattest terrain that is free of obstacles for a forced landing, it can make sense to look in every direction. So, the area accessible in such an emergency increases as the square of the airplane’s height above the ground, meaning that doubling altitude will provide four times as many options (see Figure 4).

Cooler temperatures: When I departed Southern California last summer headed east back to Tennessee, the Golden state was experiencing a heat wave. When I landed in Visalia, in the late afternoon, the temperature was 116 degrees Fahrenheit. Since Niky has no air conditioning, flying high was a relief. I had leveled off at 9,000 feet when ATC asked me to climb to 11,000 feet. I said, “I’ll take 13,000 feet!” On a hot day like that, the temperature at my cruise altitude was pleasant.

It’s important, of course, to review the rules for supplemental oxygen use here. Above 12,000 feet pressure altitude for more than 30 minutes necessitates its use, and above 14,000 feet, it should always be used. But the ill effects of hypoxia can happen at altitudes much lower than these, so I like to use my own oxygen concentrator when I fly above 9,000 feet.

Altitude provides better fuel efficiency, less stress on the airplane, and increased options in an emergency situation—all things that help my adventure be as safe and enjoyable as possible.