Instrument Training: Airplane Trimming

Instructors teach initial instrument students to scan and interpret the instruments, and then to control and trim the airplane ? a learning sequence that should not be treated lightly. As your student becomes more proficient during the first 10 to 12 hours of training, instructors should introduce specific maneuvers and coordination exercises that reinforce these four basic elements.

Except for brief periods when you are turning, accelerating, or decelerating, the airplane must be in trim. It must fly straight and true, just like an arrow. If it doesn?t, aircraft control becomes a constant task, and you won?t have time to manage your flight properly.

The textbook solution for trimming requires an airplane equipped with three-axis trim control ? a trim control for the elevator, rudder, and ailerons. Few small general aviation airplanes have all three (trainers typically have only an adjustable pitch, or elevator, trim surface), so you must use the appropriate primary control to keep the airplane in trim. I?ll discuss how to do this, and how you can fly with your hands off the yoke and still maintain heading. It?s an unorthodox procedure, but it works well and deserves serious consideration by all instrument instructors and pilots.

The proper sequence for trimming all piston and jet airplanes is elevator, rudder, and ailerons, but you must satisfy other conditions before starting the sequence or your efforts may be in vain. During your preflight inspection neutralize the cockpit trim controls ? set them to zero ? then confirm that the trim tabs are in their neutral or streamlined position when the related primary control surface is in its neutral position. In flight, your wing fuel tanks should be balanced, and if you?re flying a multiengine airplane, power should be equal on all engines.

Initial instrument students know how to trim the elevator for visual flight, but they may need additional insights for instrument flight. Whenever you change attitude or power, rough-trim the elevator as control forces get heavy. Do not attempt to fine-trim the elevator until you?ve set the power and let the airspeed stabilize. You should check elevator trim frequently by releasing the control yoke to see if the airplane?s pitch attitude changes.

If pitch attitude changes, your natural reaction is to retrim the elevator, but that action may be incorrect. Many trim and primary control systems use cables and pulleys, and the effectiveness of these systems varies with cable tension and pulley friction. When internal resistance is high, a properly trimmed airplane may seem out of trim.

You can demonstrate this in most training airplanes. After you establish level flight and trim the elevator, release the yoke and use rudder to maintain heading. Then move the yoke forward or aft ? very slightly ? and release it. Control system resistance (or friction) may keep the yoke in either the slightly-aft or the slightly-forward position, causing the airplane to start to climb or descend. When you restore the original yoke position, the airplane should return to level flight and stay there. At no time did you touch the elevator trim control.

Atmospheric conditions can also affect elevator trim. To demonstrate this I fly students through Southern California?s Corona Pass on a day when the wind aloft is present, but not strong enough to generate turbulence. The student enters the pass and flies level with hands off the yoke. The wind flowing through the pass creates a venturi effect, and this causes the air mass ? and the airplane ? to descend. As we exit the pass, the venturi effect dissipates and the air mass and airplane climb back to the original altitude. Again, the student doesn?t move elevator trim control, and the airplane is always in trim even though the altitude changes.

All students learn that an aircraft moves with the surrounding air mass, but this phenomenon is usually observed in relation to horizontal movement (wind). Yet, air masses also move vertically.

Whenever light to moderate chop or turbulence is present during cruise flight, I demonstrate another interesting phenomenon. Again, the student flies hands-off, and every time we hit a bump, I have him punch the yoke forward with his fist. The turbulence momentarily increases the wing?s angle of attack, causing the nose to pitch up. Popping the yoke forward counteracts the pitch-up before it occurs. Again, the student doesn?t touch the elevator trim control, and the airplane maintains altitude.

The desire to maintain a properly trimmed elevator can lead to an extremely bad habit ? controlling the airplane?s pitch attitude with the elevator trim control. This habit makes you less aware of pitch attitude, increases your workload (because you?re constantly tweaking the trim control), and prevents you from feeling the elevator feedback you need to anticipate flight path deviations.

After trimming the elevator, you trim the rudder. To do it you must hold the wings dead-level using aileron input. During instrument flight the attitude indicator?s miniature airplane and the bank pointer are your primary references. During visual flight your primary references are a point that?s dead ahead on the horizon and the airplane?s wingtip position relative to the horizon.

Using rudder input to maintain a constant heading, you adjust the rudder trim to eliminate the pressure you need to maintain that heading. if your heading remains constant after you remove your feet from the pedals, you have trimmed the rudder properly. Remember, you must hold the wings dead-level during this step ? and the next step.

You trim the ailerons after you?ve trimmed the rudder. Hold the wings level with aileron input and adjust the aileron trim to eliminate aileron pressure. If your bank angle remains constant after you remove your hands from the yoke, the ailerons are trimmed properly. Now that you have trimmed all three primary controls, the airplane should fly like a well-aimed arrow ? straight and true.

If you fly a large airplane with a long wing span, you?ll need to perform the rudder and aileron trim sequence a second time. Aileron-induced yaw will occur if aileron trim displaces the ailerons from their neutral position. Therefore, you must hold the wings level once again, re-trim the rudder for constant heading, and re-trim the ailerons for wings-level flight. This procedure takes time and patience because you can easily apply excessive aileron and/or rudder trim during this procedure. If this occurs, drag increases and cruise performance decreases.

Most light airplanes don?t have aileron trim, and many don?t have rudder trim. This presents a problem during single-pilot instrument flight without an autopilot because you must keep one hand on the yoke to fly straight and level. This increases your cockpit workload because many tasks, such as opening or refolding a chart or placing a chart on a clipboard, require two hands. (In training, autopilots don?t count because the pilot is learning to fly on instruments, not how to fly an autopilot.)

Here is the trick to getting hands-free time. Trim the elevator and the rudder if rudder trim is available, then let go of the yoke and use the rudder to maintain a zero turn rate on the turn coordinator. Next. note the attitude indicator?s bank attitude and use the rudder to maintain that bank attitude and the zero turn rate. This will require a very slight amount of pressure on one rudder pedal. Barely touching the appropriate pedal is usually sufficient.

Yes, the ball is slightly off-center and the wings are not dead-level, but the airplane is almost flying itself, it?s flying straight, and your hands are free to manage charts, tune radios, and copy your clearances. This is the objective of hands-off flight.

When you fly hands-off and use the rudder to maintain bank attitude, do not apply additional rudder trim to eliminate the small rudder input that is required. This de-trims the previously trimmed rudder, and with both the rudder and the ailerons out of trim, hands-off flying becomes more difficult, if not impractical.

When your heading is under control during hands-off flight, use the throttle to pitch-trim the airplane. If the airplane starts a slight climb or descent, correct it with a slight power reduction or power increase. This will keep your altitude within tolerance while you complete the task at hand.

Most pilots, when they recognize the value of this hands-off technique, become enthusiastic, and may tend to overdo it. Remember, you use this technique only during constant-heading flight ? climbs, level flight, and descents ? so you can use both hands for a few moments.

Above all else, you must never say that you fly the airplane with the rudder. You always fly an airplane in a coordinated manner, using all three controls. You may say, however, that you can temporarily control an airplane with the rudder and the throttle during hands-off flight.

This hands-off technique generates some new rules that involve your feet and parallel the old rule of stepping on the ball to maintain control coordination:

? Step on the high wing of the attitude indicator.

? Step on the high wing of the turn coordinator.

? Step on the desired heading if the heading indicator has a vertical compass card.

Experienced pilots use the rudder to their advantage, and they always fly a trimmed-up airplane. This is why they appear to sit back, relax, and let the airplane do the flying. Their workload is low, and they have lots of time to navigate, communicate, and handle distractions ? the real secret to low stress, instrument flying.