November 8, 2013
By Dan Namowitz
You rely on performance charts and lots of practice to learn what can be expected from your trainer during takeoff, landing, cruise, or when demonstrating a maneuver.
Many variables affect the published performance figures: If a runway isn’t level, or the grass isn’t dry, adjust ground-roll values for takeoffs or landing, using side notes often published along with the charted data.
How far will your aircraft glide after an engine failure? That also depends on many factors, including how promptly and precisely you establish the recommended airspeed. According to the pilot’s operating handbook for a 1978 Cessna 172N, published maximum glide-distance figures are based on four conditions: speed 65 knots indicated airspeed, the propeller windmilling, flaps up, and zero wind.
Airspeed and flaps you can control, but the likelihood of zero wind is remote. And what exactly—as your designated pilot examiner might inquire—is a "windmilling" propeller?
A propeller is windmilling, says the glossary section of the Airplane Flying Handbook, when the air moving through it "creates the rotational energy."
Here’s the catch: Not every kind of engine failure would result in a windmilling propeller; a seized engine, for example, would bring the prop to a complete stop.
So here’s another question a designated examiner might ask a flight–test applicant: Which condition is worse for glide performance—a windmilling propeller, or a stopped prop?
The answer is that windmilling is worse. That helps explain why pilots of aircraft with constant-speed propellers are taught to place the prop in the low-rpm (high-pitch) setting for a glide with a windmilling prop.
"In some high-performance singles, the decrease in drag is so dramatic that you literally feel the airplane accelerate as you pull back the propeller-pitch control (and vice versa). Don’t do this, of course, unless the engine has failed or is idling," wrote Barry Schiff in his "Proficient Pilot" column. Does it matter to glide performance if a two-bladed prop stops in the vertical or horizontal position? See the column for the answer.
Safety dictates practicing simulated engine failures at idle power and the prop windmilling. Be sure to use carb heat, and clear the engine frequently during the simulation. Accidents have resulted from overlooking carb heat, and from cases where an engine was intentionally shut down during a simulation, and could not be restarted.
Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.
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