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Proficiency: Coming up short

Your best glide probably isn’t

During a recent mock checkride, I simulated an engine failure in the Piper Arrow by reducing the throttle to idle. Louis, a commercial pilot candidate, pulled his checklist from the side pouch and got to work. He used back-pressure on the yoke until the airspeed indicator read 75 knots while searching the Tennessee countryside for a field that would provide the safest site for an engine-out landing. 
P&E September
Best glide speed gives the farthest distance forward for a given altitude loss in zero wind. To achieve book numbers, however, it’s important to configure the aircraft with minimal drag, as specified in the pilot’s operating handbook.
Illustration by Charles Floyd
Best glide is calculated using a line from the origin that barely touches the glide polar (top). This calculation assumes no wind so that groundspeed is equal to airspeed. (This curve has been stretched horizontally and is not intended to accurately represent a particular model.) A 30-knot headwind shifts the glide polar to the left (center). The speed that maximizes forward distance, the speed to fly, is faster than best glide speed. With a tailwind, the speed to fly is slower than best glide speed (bottom). This example assumes a tailwind of 30 knots.

After pointing the airplane toward a relatively flat field, Louis used the elevator trim control to reduce his workload in maintaining a constant-airspeed glide. He announced and motioned through checklist procedures designed to effect a restart. Louis consulted the Piper Arrow pilot’s operating handbook (POH) and reported that at 6,000 feet above the ground, we could glide almost 10 nautical miles away. In reality, despite all checklist items properly completed, our aircraft was capable of gliding less than 6 nm. Why was his estimate so far off?

Aircraft configuration

Louis wisely used the elevator trim control to minimize his workload in maintaining a constant airspeed. With his background flying gliders, he knew that a better airspeed to use in this situation is not actually best glide airspeed but one called “speed to fly.” Estimating the tailwind component to be 15 knots, he used the slower speed of 75 knots to increase glide distance. (See sidebar “Speed to Fly”.)

Minimizing aerodynamic drag is key to achieving the longest possible glide, so Louis verified that the gear and flaps were retracted and the engine cowl flaps were closed. He kept the inclinometer ball centered because uncoordinated flight increases drag.

The Piper Arrow POH claims that, at best glide airspeed and in a minimum drag configuration, the Piper Arrow can travel approximately 1.6 nm for each 1,000 feet of altitude loss in a no-wind situation. Because the simulated engine failure occurred 6,000 feet above the terrain, we should have been able to glide 8 nm away with 1,000 feet of altitude remaining to maneuver into the wind. Our airplane was incapable of gliding anywhere close to this distance, and the culprit was the propeller configuration.

Before simulating the engine failure, the propeller control was set to 2,300 rpm. When I pulled the throttle to idle, the propeller, with a relatively fine pitch, continued to rotate at a high rpm. Multiengine pilots know how devastating the drag from a windmilling propeller can be, so their airplanes are equipped with a mechanism to coarsen the pitch and stop the rotation. The decrease in drag and the attendant forward acceleration is noticeable.

The Arrow POH emergency checklist for engine failure at altitude omits any mention of the propeller control. But the amplified emergency procedure in the POH states, “at best gliding angle, with the engine windmilling and the propeller control in FULL DECREASE RPM, the aircraft will travel approximately 1.6 miles for each thousand feet of altitude.” The glide performance chart assumes that the propeller lever is pulled to FULL DECREASE RPM. In my own experiments with the Piper Arrow, starting with 2,300 rpm, retarding the throttle but failing to pull the propeller control back as well resulted in an average 35 to 40 percent penalty in glide performance. The Arrow POH advises studying the amplified procedure in addition to the checklist. But the effect on glide performance is profound enough for the propeller configuration to be part of a checklist procedure as well. Manuals for other airplane models can be similarly perplexing.

To make matters even more confusing, the Airplane Flying Handbook offers the following advice for practicing an engine failure scenario: “To enter a glide, the pilot should close the throttle and, if equipped, advance the propeller lever forward. With back-pressure on the elevator flight control, the pilot should maintain altitude until the airspeed decreases to the recommended best glide speed.” This is the worst advice possible regarding the propeller configuration! The moral of the story is that reading these manuals with a critical eye is important in extracting the best emergency procedure.

Favorable terrain

In a perfect world, a flat field devoid of obstacles would be in easy gliding distance so that we could direct the airplane into the wind and land with the lowest possible energy. What I love most about my home in beautiful southeastern Tennessee—the rugged and forested Cumberland Plateau—makes it a far-from-perfect venue for an engine failure. The decision on which direction to glide should be made with efficiency but not so hastily as to miss a good option. Ideally, the wind would be guiding us toward the perfect field but winds and favorable terrain may be at odds. Electronic charting tools like ForeFlight Mobile and Garmin Pilot can assist in choosing a safe area. (See “Electronic Flight Bag Tools,” at right.)

If possible, plan to have 1,000 feet of altitude to spare at the landing site in order to maneuver and land into the wind. Excess groundspeed at touchdown can have a profound effect on the outcome because the energy dissipated on landing, and potential damage from hitting an obstacle, involves the square of the groundspeed.

Consider an airplane with a touchdown speed of 60 knots landing in an area with surface winds of 10 knots. Touching down downwind with a groundspeed of 70 knots (a 17-percent increase) results in a 36-percent increase (1.172 = 1.36) in energy to be dissipated. On the other hand, using that 10-knot wind to your favor by landing into it reduces that energy by 31 percent compared to no wind. That means the energy at touchdown landing into the wind is half that of landing with a tailwind. Wind matters and should play an integral role in choosing a landing site.

Maximize your own safety budget

Louis deserved much credit for his emergency descent procedure. He calmly and efficiently used his checklist to set up a glide toward favorable terrain and even used a technique from his experience flying gliders to further increase his glide distance. When he noticed that he wasn’t getting the glide performance he expected, he chose a field in easy glide distance and maneuvered for a landing into the wind to decrease the energy to be dissipated on touchdown. But the fact that he wasn’t getting anywhere close to POH glide values nagged him. I suggested that Louis make his own checklist for the airplane. Louis took his practical exam with a deeper understanding of the airplane and passed with flying colors.

In an emergency, we fall back on our training and hopefully use the balance in our safety budget wisely. Carefully reading the operating handbook and training manuals ahead of time means that starting balance is as high as possible. The process of writing my own checklist elicits questions that encourage me read the POH critically and more completely. Amplified procedures, glide polars, and energy equations aren’t helpful when actions need to be quick—unless they have been incorporated into a checklist.

Catherine Cavagnaro owns and operates Ace Aerobatic School in Sewanee, Tennessee, and is professor of mathematics at The University of the South. She is a speaker at the 2018 AOPA Fly-Ins.

Catherine Cavagnaro

Catherine Cavagnaro is an aerobatics instructor (aceaerobaticschool.com) and professor of mathematics at Sewanee: The University of the South.

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