Why don't we use elevator trim as the primary flight control?

Question

One thing you're repeatedly told as a student pilot is to fly with the stick (or yoke), not the trimmer. To select a new pitch attitude, fly to it with the stick, wait for the aircraft to settle, and then trim away constant forces.

But the position of the elevator trimmer is roughly the same for the same cruise configuration. It would be a lot easier to mark the trimmer with "level flight at 80", "gliding descent", and "best climb" positions, and just move it to those positions (before or after setting the throttle appropriately) directly. The trimmer would be the primary means of setting attitude, with the front-back axis of the stick or yoke for temporary adjustments (e.g to bring the nose up a little in a turn).

So why aren't controls designed to be used that way, and why do we fly with the stick?

Answer 1

Proper trim control technique involves exerting control pressure to attain the desired attitude then using the trim to alleviate the requirement of holding that pressure. There can be a temptation for pilots to make small adjustments using trim alone. Resist the temptation and use the proper procedure.

We use the proper technique because it's the most precise way to use trim. If adjustments are made with the trim directly we'll end up over-controlling and chasing the correct trim position.

Trim isn't just a convenience. Especially for pilots flying in instrument conditions, alleviating the load of holding control pressure and ensuring that if the pilot lets go of the controls the airplane maintains its attitude are a matter of safety.

The Airplane Flying Handbook explains Trim Control starting on 3-6. It lists the proper technique in greater detail and provides a complete rationale.

There's also the matter of doing things the same way every time. Once we've established the right procedure, we should always use it to enforce the habit. This also benefits the pilot when things get busy. Proficient instrument pilots are experts in using trim effectively.

Predefined trim positions are impractical because they vary based on weight, CG, airspeed and other factors. These factors change the amount of downforce required on the elevator and therefore the trim position. Situations like "level at 8000" or "gliding descent" won't always result in the same trim position. For other control surfaces there are similar factors. We do have predefined trim positions for takeoff, but loading often makes those undesirable and trimming is required immediately.

During flight training, good trimming technique becomes especially important during instrument training. There's a lot of good information about trim and its importance in the Instrument Flying Handbook.

Answer 2

The main reason for having trim is to modify the forces felt by the pilot to relieve strain, or as you say, 'trim away' the constant forces.

There are various reason for not using the trim as the primary control or have fixed trim positions:

• The forces on the stick and by extension the trim varies with the weight and balance of the aircraft. Though the position of the trim may be roughly same for same configuration, what if the configuration changes? What will happen if the number of passengers or luggage changes? By extension, as the fuel is consumed and the aircraft balance changes, the trim position changes. So you cannot have a fixed trim setting.

• The trim position will change depending on whether the wing configuration is different for e.g if the flaps are extended.

• The trim setting is affected by speed variations. For example, in trimmed condition, if the speed of aircraft is reduced, the stick will push the pilot's hand forward to lower the aircraft angle of attack. This is opposite of the requirement- we need higher angle of attack to maintain level flight at lower speed. The opposite happens as the aircraft is accelerated. This means that adjustment has to be made to trim,and the pilot will end up making more or less continuous adjustments to the trim.

Answer 3

The trim will behave differently with different loads, so you cannot use fixed trim settings on each flight, as each flight will be different from a load factor and center of gravity perspective.

While my trim setting for a solo flight with half full tanks will make me us best climb, the same trim setting with 3 additional passengers and full tanks will produce a different result. (Speaking from a GA/SEP perspective).