AOPA Pilot Magazine - December 2017 - 22

PROFICIENT PILOT

Food for thought

BY BARRY SCHIFF

Lessons from engine-out training

BARRY SCHIFF

was inducted into
the National Flight
Instructor Hall of
Fame in 2003.

A JOY OF WRITING THIS COLUMN is that it provides
a gratifying opportunity to share experiences and
occasionally teach stuff that might be of value. This
business of teaching, however, is a two-way street. The
instructor occasionally becomes the student. So it was
when I recently received an email from Jake Morrel, a
retired bush pilot who lives next to his private airport
(ME32) in Maine.
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

22 | AOPA PILOT December 2017

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.
AOPA
WEB

www.barryschiff.com

http://www.barryschiff.com

AOPA Pilot Magazine - December 2017

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