AOPA Pilot Magazine - July 2016 - 18

PROFICIENT PILOT

Pulling it around

BY BARRY SCHIFF

Trim technique explained

BARRY SCHIFF

was designated as
an Elder Statesman
of Aviation by the
National Aeronautic Association in
2004.

18 | AOPA PILOT July 2016

IT HAS BEEN 24 YEARS since I wrote in this column
about the difference between a procedure and a
technique. Although the words are frequently used
interchangeably, they have different meanings, especially as we use them in aviation. A procedure is a
course of action with a specific objective. Procedures
include completing the tasks published on checklists,
making a crosswind landing, executing an instrument
approach, leaning the engine at altitude, and so forth.
The methods used by a pilot to achieve these objectives, however, are often a matter of technique.
Raising the landing gear after takeoff, for example, is a procedure, but how soon after liftoff a pilot
chooses to raise the gear is a matter of personal preference or technique.
There is one technique that I see with increasing
regularity when conducting flight reviews, one that is
rapidly becoming one of my pet peeves. This involves
pilots-including some flight instructors-who use
pitch trim to relieve back-pressure when performing
a steep turn. (The FAA apparently does not have an
official position about this use of pitch trim.)
The purpose of a steep turn as a training and
proficiency maneuver is "to develop and maintain
smoothness, coordination, orientation, division of attention, and control techniques," according to the Airplane
Flying Handbook. The maneuver also is used during
instrument training to help a pilot develop or demonstrate his ability to scan the flight instruments.
Trimming during the turn does make the maneuver
easier to perform, but defeats its purpose as a training
exercise. After all, if control wheel forces are trimmed
away and the pilot can maintain altitude with his fingertips, the exercise becomes little more challenging than
flying straight and level. This is one reason why airline
instructors typically do not allow this use of trim.
Another reason for not trimming becomes obvious when having to roll out in hurry. If the pilot cannot
retrim quickly enough, or if he forgets to add considerable forward pressure to the wheel, he might find
himself involuntarily entering a hammerhead turn.
For similar reasons, pilots should avoid trimming
during a landing flare, a practice I have observed a
number of times, particularly in aircraft equipped with
electric trim. A rejected landing at such a time could
result in a powerful, nose-up pitching moment that
could produce disastrous results.

The purpose of elevator trim is to relieve control
pressure when maintaining a prolonged condition.
Trim should not be used for relieving transitory control pressures.
When flight conditions dictate that it is imperative
for a pilot to make a steep turn (such as when used as
an escape maneuver), it would be of such short duration (180 degrees or less) that there likely would not be
enough time or advantage to justify trimming. As a result,
using trim during practice is a form of negative training.
Aircraft designers attempt to make it difficult for
pilots to inadvertently develop excessive and potentially destructive G loads. They do this by inducing stick
forces that became so heavy, it becomes more difficult
for a pilot to inadvertently overstress the aircraft.
A given lightplane, for example, might have a
designed stick force of, say, 25 pounds per G. This
means that a pilot must exert 25 pounds of pull to create one G or 100 pounds to create four Gs. To eliminate
or reduce these stick forces by trimming during a steep
turn negates the effort of the designer and makes it easier for the pilot to overstress the airplane, especially
when attempting to recover from a developing spiral
by hauling back mightily on the yoke. The best way to
recover from a spiral dive, of course, is to roll out of the
turn, raise the nose, and reduce power (if necessary).
Under no circumstances should a pilot attempt to salvage a steep turn by just pulling back on the control
wheel during a developing spiral. An instructor or an
examiner would much prefer to see you swallow your
pride, give up the turn, and recover.
Moving an airplane's center of gravity aft also reduces
the pitch force required to create a given number of Gs.
If the CG is moved excessively aft, pitch forces become
much lighter. In this condition, it is easier than you might
imagine, especially in turbulence, to inadvertently create
enough of a G load to break something-which is exactly
what has happened in a number of accidents.
Conversely, shifting the CG forward increases the
stick force required to produce a given number of Gs.
To be fair, an argument in favor of trimming during
steep turns is that this can help to prevent a high-speed
spiral from developing. That is why such trimming is
a matter of technique. Each of us has his or her own
preference.
AOPA
WEB

www.barryschiff.com

http://www.barryschiff.com

Table of Contents for the Digital Edition of AOPA Pilot Magazine - July 2016