AOPA Pilot Magazine - February 2013 - 22

LICENSE TO LEARN

Weber’s law

BY ROD MACHADO

Some interesting cockpit applications

ROD MACHADO

is an aviation writer and
professional speaker.
He has been contributing
to AOPA since 1978.

22 | AOPA PILOT February 2013

IF YOU CLOSED YOUR EYES, held out a cup, and asked
someone to gently pour water in it, how much liquid
would need to be added before you noticed a change in
weight? One drop? Probably not. One ounce? Maybe.
The answer is called the “discrimination threshold”
for detecting weight differences. Interestingly enough,
it depends on how heavy the cup is. By the way, don’t
try this experiment in a public place or the change you
notice will be the coins at the bottom of your cup—and
you could get arrested for panhandling.
Ernst Weber (an eighteenth-century German physician considered one of the founders of experimental
psychology) discovered that there is a relationship
between the degree of stimulation you’re receiving
and the amount it must change before you can recognize the difference. For instance, the discrimination
threshold for sensing a change in pressure on the skin
is approximately 16 percent (discrimination thresholds vary for different senses). Vary the pressure by 10
percent and people will not reliably notice the change.
This finding has some interesting cockpit applications.
Let’s assume that you’ve just entered a climb from
straight-and-level flight. After establishing the nose-up
pitch attitude, you find yourself holding so much elevator back-pressure that you look over at your passenger
and say, “Hey, spot me!” Being an astute student, you
immediately spin the trim wheel with the enthusiasm
of a Wheel of Fortune contestant, hopefully not while
yelling, “Come on, one thousand!” Suddenly, a great
deal of hand-yoke pressure disappears, which is easily
recognized because the change exceeded the discrimination threshold of 16 percent.
But what happens if you trim away less than 16 percent of the yoke pressure you’re holding? You might
not notice the pressure change. This is precisely what
happens to timid trimmers. They trim a little bit, but
not enough to notice any real difference in yoke-hand
pressure. They then release the controls, only to notice
a deviation in airplane attitude. Unfortunately, they
repeat this process several times, wasting a lot of valuable time in the process.
When you watch experienced pilots trim their airplanes, you’ll often see them make several large twists
of the trim wheel (come on, ten thousand!) so they can
easily feel the sudden decrease in control yoke pressure. With the airplane near its trimmed condition,
they’ll maintain very light hand contact with the yoke

(that’s right, they don’t let go), then trim to keep the
nose attitude and/or the VSI needle indication stabilized. Attempting to make the final trim adjustment
using only the sense of touch can be difficult since the
pressure change needed often does not exceed the
minimum threshold for pressure detection.
Discrimination thresholds apply to other areas
of flight. For instance, the discrimination threshold
for sound is about 10 percent. Our ability to detect a
change in pitch, however, is about 33 times more sensitive, with a discrimination threshold of 0.3 percent.
Holy bat talk! That’s sensitive. Now you know why
singing off key drives people away, unless it’s karaoke
night, which brings them together.
This is one reason a pilot might not notice the
sound of decreasing engine power, which could result
from an accumulation of induction-system icing or
from an aft-sliding throttle. However, he might more
easily notice a pitch change resulting from an increase
in propeller speed (perhaps caused by a loss of oil pressure in an airplane with a constant speed propeller).
The discrimination threshold for determining
differences in the perceived length of a line is approximately 1 percent, meaning that we’re quite sensitive
to detecting such visual changes. This is one reason we
can effectively use our perception of length difference
between the near and far end of the runway as a means
of detecting a deviation from a previously stabilized
glidepath. As you climb above or descend below a previously stabilized glidepath, the distance between the
near and far end of the runway increases or decreases,
respectively.
There are many variables that alter or affect
Weber’s law. One of them is time. If a stimulus changes
slowly, you might not notice the change. This helps
explain how a pilot can get into an accelerated stall
when turning from base to final with an excessive
bank, yet not notice the increase in pressure on his
derriere (i.e., increasing G force).
Here are discrimination thresholds for a few sensory stimuli in order of our decreasing sensitivity to
them: pitch 0.3 percent; length 1 percent, brightness
of lights 1.6 percent; odor 5 percent; loudness 10 percent; pressure on skin 16 percent; and saltiness of food
20 percent.
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AOPA Pilot Magazine - February 2013

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