Pitch, Power, Trim
As a flight instructor, the most common error I see student pilots make is failing to understand the relationship between pitch, power, and trim. To really understand it how to use these in flight, first you have to understand what each do.
Pitch refers to the elevator’s control over the lateral axis. By actuating the elevator, typically by applying a pressure forward or aft on the yoke (or control stick), the pilot changes the angle of attack of the elevator into the relative wind, creating varying amounts of lift.
Yes, that’s right, the elevator produces lift, just like the wing! Lift is a force created by air moving over an airfoil, and the elevator is an airfoil (it’s a surface designed to create lift). However, it has a negative angle of attack, which produces negative lift. In other words, it produces lift down rather than up. This is essential because the center of gravity of the plane is always in front of the center of lift created by the wings, so without this “tail-down force”, the plane would be unable to lift it’s nose, and therefore, change it’s attitude.
If the pilot changes the angle of attack of the wing by actuating the control surface, the amount of (negative) lift increase or decreases, which causes a change in the attitude of the plane. More tail-down force results in a climb, less results in a descent.
What is important to realize is that the effect of the elevator is dependent on the lift that is produced, which is affected by airspeed. When airspeed is decreased, overall lift is decreased, which results in a greater pitching down moment due to the reduced effectiveness of the elevator. Ever notice why, as you get slower, the controls get mushier and you have to push forward or pull back more to get the same effect as you did at a different speed? That’s why. This is also why trim must be applied after any airspeed change. I repeat, trim changes should occur after airspeed changes, not pitch changes.
The elevator force is entirely based on lift, and therefore, airspeed. Trim is a component, a secondary control, of the elevator. Trim sets the aerodynamic force at a specific lift configuration, which is mostly a factor of airspeed. Without adjusting the trim, the airplane will attempt to maintain it’s desired airspeed. Try it out next time you’re in the plane. Trim for 80kts, and then increase or decrease power. Your plane will attempt to catch that 80kts (It will overshoot, but since training aircraft are positively statically stable, will oscillate around this equilibrium until the damping effects stabilize it).
It is because of this that both pitch and power should be adjusted before trim, because those are the two factors that result in the aircraft's performance. And it is true that many experienced pilot’s adjust power and pitch simultaneously, but power should generally not be adjusted before pitch, as it results in airspeed loss that may be undesired. After all, most of the time when you change your pitch, you want to maintain your airspeed. If you were perhaps in a slow flight regime, you could stall the aircraft by reducing its power while maintaining its pitch attitude, as the reduced power is changing the relative wind experienced by the wings.
While you could adjust power before pitch, it often will end up resulting in multiple power changes (the first to arrest the climb or descent), then the pitch change, then a pitch and/or power change to fix the over/under application of power. Instead, by pitching to the desired flight attitude, you can adjust the power to maintain that attitude. By pitching to the desired sight picture, you can utilize a known power setting to configure the plane almost immediately to maintain that regime.
Once the new pitch attitude is established, the pilot must maintain that attitude using appropriate elevator input. It’s all too often that the pilot will allow the trim to move their pitch while they are adjusting the power or trim, which results in a continually changing airspeed, which in turn makes finding a proper power or trim setting nigh impossible. Maintain the pitch regardless of the varying control pressures you will experience while adjusting the power and trim.
It’s essential that the pilot does not fly the plane with trim. The trim is a secondary control. The professional pilot commands the plane to do what they want, and then uses the trim to maintain that.
I encourage my students to say, aloud, “pitch-power-trim”, in order, when they make elevation changes. They find that by doing it in this order, they can control the plane with far fewer deviations from their desired position, and with far fewer adjustments and effort.
