Proficient Pilot: Food for Thought

As a seaplane instructor, Morrel would simulate engine failures the way most other instructors do, by simply retarding the throttle. During advanced training, however, he occasionally would fail the engine by retarding the mixture control—not a problem when practicing in a floatplane over 40-mile-long Moosehead Lake.

Students and experienced pilots alike were shocked when he did this, but not because Morrel had really killed the engine. They were stunned by how differently the airplane performed. With the engine actually shut down, Morrel says, the airplane sinks noticeably steeper; glide performance is less than when the engine is idling and contributing a modicum of thrust.

In other words, Morrel says, pilots who have genuine engine failures experience less glide performance than they have been trained to expect because their engine-out training includes the benefit of idle thrust.

Frankly, I had never stopped to consider the effect of an idling engine on glide ratio. One way to prove or disprove such a theory, however, is to conduct a simple flight test. The airplane I used was a Cessna 172SP, and my son, Brian, came along to fly and assist in gathering data.

After establishing the airplane in a normal glide at 68 knots, we timed a stabilized descent from 5,000 to 3,000 feet msl near Camarillo Airport in Southern California. Elapsed time was three minutes, 24.7 seconds; engine speed during the descent was 900 rpm.

We next attempted to do the same with the mixture control retarded to kill the engine (while directly above Camarillo’s 6,013-foot-long runway, just in case). I say “attempted” because the test didn’t go quite as planned. Windmilling speed at the beginning of the descent was only 500 rpm, much less than when the engine was idling.

As we continued the glide, however, propeller rpm began to decay until at about 200 rpm it became obvious the prop would soon stop. This was surprising because I had discovered during previous flight testing that it takes effort to stop a propeller. Usually you have to raise the nose and reduce airspeed significantly to arrest propeller rotation. I wonder now, though, if this was because I was being impatient. I have recently discovered that in many airplanes the propeller will eventually stop all by itself during a prolonged, normal glide with the engine shut down. Why, though, don’t we hear about this during discussions and reports involving actual engine failures? A propeller coming to a halt in flight is not something that a pilot would easily forget. One reason that it might not occur following an engine failure is that some pilots glide at too high an airspeed, making it easier for the prop to continue windmilling.

If a pilot has an actual engine failure and there is absolutely no hope of a restart, then allowing the prop to stop usually is a good thing. It improves the glide ratio of a typical lightplane by about 20 percent. (Another advantage of a stopped prop is that it usually stops the Hobbs meter.)

Returning to the original purpose of this flight test, is there a way to keep a propeller windmilling during a glide with the engine shut down? Well, yes, but this is going to sound a little screwy. To increase propeller windmilling speed with a dead engine, all you have to do is open the throttle and increase manifold pressure. I know. This does sound nuts. This is because we usually consider manifold pressure as a measure of power, but this is valid only when the engine is operating.

With the throttle closed, the flow of induction air into a dead engine is restricted (or choked), and the windmilling propeller must work hard to pump air out of such a “closed” engine (create a vacuum); manifold pressure and rpm are reduced. With the throttle open, induction air flows freely into the engine, and this makes it easier for a windmilling propeller to turn the engine; manifold pressure and rpm are increased.

This means that throttle position can be used to vary windmilling rpm following an actual engine failure. It is a roundabout way of saying that Jake Morrel is correct. Sometimes. Glide performance can suffer following an actual engine failure, depending on throttle position, windmilling rpm, and how well you fly the airplane.

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