I am reviewing autopilot requirements which mention that whenever an aircraft's altitude changes as a result of air disturbance or an out of trim condition, the inertial sensors will detect this change and an error signal is developed and fed to the relevant control channel, amplified, and applied to the servo actuator so that flight control surface can be moved. My question is what is out-of-trim condition? Is it something related to setting trim for constant speed?

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    $\begingroup$ Do you know what "trim" means in terms of aircraft? $\endgroup$ – slebetman Mar 25 '16 at 10:06
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    $\begingroup$ I am assuming the requirement is talking about pitch/roll or yaw trim. I am not clear on what it means when it refers 'out of trim'. My understanding of trim is that pilot uses trim tabs to relieve pressure on the cockpit flight controls. pilot can choose to trim the elevator (or stabilator) such that nose up attitude is maintained without pulling back on the yoke $\endgroup$ – user2927392 Mar 25 '16 at 10:09
  • $\begingroup$ Yes, basically that is correct. Put it another way - when an aircraft is "in trim" it means that the aircraft tends to fly straight when there is no pilot (it means the same thing you're saying but in layman's terms). So out of trim is obvious from this understanding: is simply means that the plane cannot maintain straight flight without the pilot. $\endgroup$ – slebetman Mar 25 '16 at 11:19

The trim system on an aircraft relieves the control pressures so that the aircraft maintains an attitude without pressure on the control system.

When hand-flying this is very noticeable in that you constantly have to be pushing, pulling, or turning (in the event you have aileron trim) to maintain an attitude. Notice how I don't say "straight and level", you can trim an aircraft to maintain a climb/descent or other attitude as long as the trim tabs have enough authority.

Autopilots detect an out-of-trim condition the same way you do. They sense that there is pressure on the controls to maintain the desired attitude, and if this is over some threshold it will display a warning light for an out-of-trim condition. Some autopilots have a trim-up or trim-down light to help the pilot determine which way to trim the aircraft.

Here is a diagram from a S-Tec System 50 Autopilot found in many GA aircraft: System 50 AutoPilot

The "out of trim" condition lights are two yellow indicators on the right side of the auto-pilot instrument.

Why is this important? Mostly for the longevity of the equipment. The autopilot works by either physically moving the cables or actuating the hydraulic system. It uses power to run motors which cause the motors to heat up and use electrical power. By trimming the aircraft, you can reduce these forces required by the autopilot so that it isn't working so hard to control the aircraft.

Another good reason is that you don't want the aircraft to do something unexpected when you hit the autopilot "off" or "disconnect" switches. This is especially dangerous in IMC when you don't have a visual reference to anything. There are also conditions which can cause the autopilot to disconnect automatically, and it takes a few precious seconds to figure out what is going on before the pilot gets positive control.

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  • $\begingroup$ So, when engine settings change which reduces airspeed then autopilot pitches aircraft down to maintain constant speed. , correct? How does autopilot know that aircraft is trimmed for constant speed. Is it because of settings in flight control panel? $\endgroup$ – user2927392 Mar 25 '16 at 16:55
  • $\begingroup$ Depending on the autopilot, it does not, it only tries to maintain altitude or heading at the expense of speed. When you trim down to maintain a speed you are setting a descent rate, which needs to be programmed into the autopilot to maintain otherwise the autopilot will adjust the controls to remain at the previously set altitude. Some autopilots have an auto-throttle system but not typically on GA aircraft. $\endgroup$ – Ron Beyer Mar 25 '16 at 17:29
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    $\begingroup$ Does being out of trim have an effect on drag? $\endgroup$ – TomMcW Mar 25 '16 at 19:20
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    $\begingroup$ @TomMcW I think that is a good topic for a new question! $\endgroup$ – Ron Beyer Mar 25 '16 at 19:27
  • $\begingroup$ Done $\endgroup$ – TomMcW Mar 25 '16 at 19:30

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