As someone who has flown from Pennsylvania to Tennessee and back without an engine, veteran glider pilot Tom Knauff probably knows a thing or two about best glide speed. “We rarely use it,” he says. So much for that.
After he spends a few minutes explaining the theory of soaring, and how serious glider pilots approach their sport very technically, Knauff’s reasons for rarely using best glide speed come into focus. There’s irony in the fact that a speed pilots of powered aircraft use only when flying without power is reserved pretty much solely for aircraft with engines. Even then, Knauff makes a convincing argument that best glide speed is, in many cases, irrelevant—and even when it is used, it’s pretty squishy.
Best glide is the airspeed that gets an airplane the farthest distance through the air with the least altitude loss. It’s also known as max L/D (articulated as lift over drag). Every airplane design results in an inherent glide ratio—7 to 1 or 9 to 1, or whatever the number is. An aircraft with a 9-to-1 glide ratio can go nine feet forward for every foot of altitude lost. Competition-level gliders boast a ratio of around 60 to 1, while in the upper atmosphere the Space Shuttle was around 1 to 1—something similar to what you would get throwing a rock out a window.
Student pilots are taught that if the engine quits at altitude, the strategy is to pick a landing site, pitch for best glide speed, and then try a restart. This is a logical chain of events, but it ignores many variables—including altitude, weather, and location. Understand Knauff’s approach to soaring and those gaps start to fill in.
The purpose of flying gliders is to harness the Earth’s energy as a means of staying aloft. Sometimes that may be for only a few minutes, and sometimes it’s for many hours, thousands of miles, and at altitudes usually reserved for airliners. Given that atmospheric energy often is irregular and unpredictable, glider pilots who excel do so by maximizing altitude gain during periods of lift and minimizing altitude loss in sink—areas in which no lift exists.
With more than 50 gliding world records, and as the first person in the world to fly a glider more than 1,000 miles, Knauff knows something about the basics of his sport. In a powered airplane, he says, you select a throttle setting that results in a certain speed and fuel burn for a given altitude. But in soaring, the pilot picks the airspeed that results in a certain amount of sink or lift. “In gliders, if we want to get there, the speed turns out to be important,” he says. Knauff’s favorite tool for explaining the relationship between sink and airspeed is a polar graph—a two-axis graph that plots descent rate versus airspeed.
Although the graph is a good way to illustrate best glide speed, minimum sink, and other speeds to fly, more than anything else it shows how wind and weight affect the speed we pick to make the airplane glide a certain distance (see “Polar Graph”).
Real world. The polar graph is a great theoretical tool to help us conceptually understand the relationship of sink to airspeed, but putting it into practice isn’t always straightforward. We generally practice engine failures in flight training as if they are sudden and unexpected events, but in many cases there is some amount of warning. When those warnings start to sound, it’s clearly time to look for a place to land, whether that’s an airport or the closest farm field. But if the engine is still running and producing power, there probably isn’t a need to establish best glide speed.
When the engine does suddenly fail, it can make sense to immediately establish a best glide speed. If the failure happens close to the ground after takeoff, establishing the speed will probably involve pitching down. Higher up, it will require pitching up. But to what speed? Airplane manuals usually specify best glide at gross weight. If you’re lighter that day, the best glide speed will be slower. How much lower depends on the airplane and the weight. Regardless, it’s safe to establish the book speed as a base reference and tweak slightly from there. In an emergency, we’re not trying to win a competition. We’re just trying to get it on the ground safely.
And safety really is the key. Although we practice establishing best glide speed regardless of the distance between the airplane and the landing site, pitching to reach it sometimes won’t be necessary. Remembering that best glide is only the speed that gets us the most distance; a field 4,000 feet below and only two miles away doesn’t require a long glide. When you consider that best glide speed and a safe power-off final approach speed usually are within 20 knots of each other, and the airplane is in a low energy state, establishing best glide may actually be less safe.
The other situation in which powered pilots may use best glide speed is on final approach while trying to make the runway. Here, the polar graph comes into focus. Throughout training you may have heard to increase your approach speed by half a headwind component and decrease it by one-quarter of the tailwind component. A quick look at the graph shows why. The graph is also a perfect illustration of why stretching the glide by increasing pitch is, at a certain point, a very bad idea. As Knauff says, “There’s a strong tendency to hold back on the stick to make it stay in the air. If you understand [the polar graph], you realize it doesn’t make sense.”
It turns out that, according to Knauff, the final glide to the airport is when glider pilots will use best glide speed. The one place where it would seem logical to use it—between two thermals, or areas of lift—turns out to be a waste. In his world, as in the powered pilot’s, best glide is pretty slow. If glider one flies between two thermals at best glide, and glider two flies it 10 knots faster, glider one will arrive at the thermal higher than glider two. Over the same elapsed time glider two will gain more altitude because it has been spiraling up in the new thermal for a longer period.
In other words, consider the objective before robotically picking a speed. For Knauff, it’s to stay in the air the longest, go the farthest, or race a course the fastest. For a pilot who’s just experienced an engine failure, maybe it’s to land in a close field (a safe approach speed), or perhaps it’s to fly farther and land in a better one (best glide).
To try some engine-out glide scenarios on the AOPA Jay, download the “Engine-Out Practice” scenario from www.my-jay.com.
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