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kmanning

Auto-Pilot is NOT Engaging

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2 hours ago, rsrandazzo said:

Ken,

The correct phrase here is that the airplane should be "in trim."  What we mean when we say this, is that you can take your hands off the controls and the airplane won't show a strong tendency to change pitch or roll from where you have it currently.

Yes, that's basically what it does when I have the flight director bar and airspeed on cue, that is V2 + 20.

2 hours ago, rsrandazzo said:

Obviously, if you are climbing, it will eventually lose a bit of airspeed and the pitch will naturally lower

It doesn't lose airspeed. It will maintain V2 + 20 as long as I follow the flight director bar commands. This is usually when I engage the auto-pilot.

2 hours ago, rsrandazzo said:

 but what we are talking about is that being "in trim" means the airplane is naturally going where you have it pointed currently- you do not have to hold control pressure to keep it on the desired pitch/roll.

Does that help?

Yes, I understand. But what I'm confused about now is that I'm being told that I need to trim the aircraft after take-off, and I've never had to do that in the past. I've always just pull back on the yoke at Vr, pitch the airplane to about 15 degrees and follow the flight director commands to maintain a climb of V2 + 20. Then I was able to engage the auto-pilot. I went back and reviewed all of the manuals for this phase of flight, including the tutorials, and none of them mentions anything about trimming the aircraft during this phase of the flight. It just mentions the procedures I've just mentioned above. I've always assumed that the trim control automatically adjusts to different phases of flight, and that's the noise that one hears in the cockpit. The yoke has an electric trim control but it was my understanding that the FCTM discourage pilots from using them, and recommend that pilots use the yoke during this phase of flight.

Ken.

 

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Trimming doesn‘t need tonbe mentioned in a manual, it‘s basic part of initial flight training, long before an airliner‘s FCOM comes into account. 

And no, pilots definitely are not discouraged to trim. They are only done so to use the trim instead of pulling/pushing the yoke to move the nose up or down. Basically the trim is used to neutralize the forces that have to be applied. Holding back a yoke of a 737 or 744 is exhausting. There are really heavy forces on your muscles if you have to do that over a longer period. 

And in manual flight no boeing will move the stab trim itself. That‘s not an airbus. You don‘t command the airplane, you fly it. Different philosophy.

as you say you could engange the AP like this in the past I could imagine that the „easy“ engagement was activated accidentally.


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1 hour ago, Captain Kevin said:

if you're still having to move your stick just to maintain the pitch that you're trying to maintain, you're not in trim.

Hi Kevin,

No, I'm not having to move my joystick just to maintain the pitch. The only time I'm moving the stick is when I rotate to raise the nose up and then pitch the airplane so that it follow the flight director commands. Once I have achieved that, it stays there for the most part. In other words, I'm not having to move the joystick back and forth just to maintain a certain pitch.

 

1 hour ago, Captain Kevin said:

If the plane's flying the desired pitch even with your hands off the controls, then you're in trim.

That's basically what the airplane is doing.

Ken.

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7 hours ago, kmanning said:

Hi Kevin,

No, I'm not having to move my joystick just to maintain the pitch. The only time I'm moving the stick is when I rotate to raise the nose up and then pitch the airplane so that it follow the flight director commands. Once I have achieved that, it stays there for the most part. In other words, I'm not having to move the joystick back and forth just to maintain a certain pitch.

 

That's basically what the airplane is doing.

Ken.

I’m not clear as to whether your joystick has a centering spring to return the stick to a neutral position when you let go of it. Most do, but if yours does not, that is probably part of the problem, as it makes it difficult to “feel”when the aircraft is out of trim.

If the joystick pitch axis is accurate, it would probably be quite easy to match (and hold) the flight director command bars in the initial climb. But if the horizontal stab is out of trim, you would be commanding up elevator deflection to hold your desired pitch, and that is what the autopilot does not “like”. What the autopilot wants to “see” is the elevators at neutral before it will connect. The only way to have the elevators at neutral in a climb is to have the proper amount of nose up stabilizer trim.

The FMS-derived trim reference preset that is calculated after you enter your weights, flap setting and CG on the PERF Init pages is only a starting point. That setting is valid for rotation and liftoff, but once the aircraft is airborne, the pilot will have to manually and continuously adjust the trim to relieve pressure on the yoke as the aircraft accelerates. If you watch any YT cockpit video of an Boeing airliner takeoff, you will see the pilot making continuous brief adjustments of the yoke trim switch during climb. 

The fly by wire system on an Airbus is different in this regard.

Once established in climb, let go of the joystick or yoke completely and see what the pitch does. Does it stay put, or does the nose want to rise or drop? Typically it will drop, meaning you need more nose-up trim. What you want to achieve is the ability to let go of the yoke with the aircraft holding pitch attitude by itself, which is an indication that the elevators are at neutral.

Again, if your joystick has no centering spring, being able to trim by feel may be very difficult, in which case you may have to de-select the realistic autopilot engagement option in the PMDG options menu. I know you don’t want to do that, but with no spring in the joystick or yoke to provide a degree of trim feedback, it may be the only way to  get the AP to engage reliably.

Once the autopilot is engaged, it will automatically control pitch trim itself, but you need to be in trim initially to get the autopilot to connect.


Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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2 hours ago, JRBarrett said:

Again, if your joystick has no centering spring, being able to trim by feel may be very difficult, in which case you may have to de-select the realistic autopilot engagement option in the PMDG options menu. I know you don’t want to do that, but with no spring in the joystick or yoke to provide a degree of trim feedback, it may be the only way to  get the AP to engage reliably.

I agree with Jim.. I couldn't use a joystick that didn't self-center.  A student pilot learns that it is control pressure that counts, not control movement.  We fly the airplane balancing the control pressures with trim.  We never look down at the stick or yoke or trim wheel to see where it is at (except when setting takeoff trim), it is all done with feel.  A floppy stick?


Dan Downs KCRP

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22 hours ago, Ephedrin said:

Trimming doesn‘t need tonbe mentioned in a manual, it‘s basic part of initial flight training, long before an airliner‘s FCOM comes into account. 

Yes, it needs to be mentioned, if that's part of the actual procedure. Being a basic part of the initial training does not cut it. I'm not saying that a 737 pilot doesn't use the trim after take-off, but a new comer flying the PMDG 737NGX for the first time is not going to trim the airplane if it's not mentioned in the manual. He's going to use only the technique that's mentioned in the manual.

Ken.

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On 10/6/2018 at 1:00 AM, kmanning said:

I went back and reviewed all of the manuals for this phase of flight, including the tutorials, and none of them mentions anything about trimming the aircraft during this phase of the flight. It just mentions the procedures I've just mentioned above

Yeah. Mostly because from the foundations of ever getting into airplanes, pilots are taught to re-trim as necessary. So far, you have at least three real world pilots telling you to re-trim, so...re-trim, and make sure you’re passing the plane to the AP in a manner in which it will accept it.

Something is wrong with the way you’re giving the plane to the AP. If you’ve been getting lucky up to this point, then you’ve been getting lucky. 


Kyle Rodgers

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15 hours ago, JRBarrett said:

I’m not clear as to whether your joystick has a centering spring to return the stick to a neutral position when you let go of it. Most do, but if yours does not, that is probably part of the problem, as it makes it difficult to “feel”when the aircraft is out of trim.

Well, it does have a return to center, but it does not use springs. Anyway, I have that turned off because the return to center action is not realistic to a real yoke in a actual aircraft. In a real aircraft, you can freely move the yoke back and forth and there's no return to center force, but that only applies when you're sitting on the ground. When you're in flight, you have aerodynamic forces acting on the control surfaces that put force on the control wheel. Therefore, the aerodynamic forces that some of you refer to are only being simulated by that return to center force. 

 

15 hours ago, JRBarrett said:

Once established in climb, let go of the joystick or yoke completely and see what the pitch does. Does it stay put, or does the nose want to rise or drop? Typically it will drop, meaning you need more nose-up trim. What you want to achieve is the ability to let go of the yoke with the aircraft holding pitch attitude by itself, which is an indication that the elevators are at neutral.

Yes, it stays on put when I have achieved guidance cues by following the flight director bars and a V2 + 20. This tells me that the airplane should be in trim. But let me mention that when I release the yoke, it is not centered because I'm in the climb. The yoke is pulled back, as it would be in the real aircraft.

15 hours ago, JRBarrett said:

Again, if your joystick has no centering spring, being able to trim by feel may be very difficult, in which case you may have to de-select the realistic autopilot engagement option in the PMDG options menu. I know you don’t want to do that, but with no spring in the joystick or yoke to provide a degree of trim feedback, it may be the only way to  get the AP to engage reliably.

Yes, that's true that I cannot feel if the plane is in trim, but I could look at the PFD flight director bar and that should give me some cue as to how much to trim. If I continue to have issues with the auto-pilot not engaging, I'll just turn the return to center position back on so that I can feel the trim. But I've always used the flight director bars as a guidance as suggested in the manual.

Ken.

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don't  know  if  you have  done  this,  have  you  tried   disconnecting  your  stick to see if  you have  the  same  issues 


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Peter kelberg

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2 hours ago, kmanning said:

Yes, it needs to be mentioned, if that's part of the actual procedure. Being a basic part of the initial training does not cut it. I'm not saying that a 737 pilot doesn't use the trim after take-off, but a new comer flying the PMDG 737NGX for the first time is not going to trim the airplane if it's not mentioned in the manual. He's going to use only the technique that's mentioned in the manual.

So by that logic, you need the manual to tell you that you need to release the parking brakes in order to be able to taxi the plane?

1 hour ago, kmanning said:

Well, it does have a return to center, but it does not use springs. Anyway, I have that turned off because the return to center action is not realistic to a real yoke in a actual aircraft. In a real aircraft, you can freely move the yoke back and forth and there's no return to center force, but that only applies when you're sitting on the ground. When you're in flight, you have aerodynamic forces acting on the control surfaces that put force on the control wheel. Therefore, the aerodynamic forces that some of you refer to are only being simulated by that return to center force. 

 

1 hour ago, kmanning said:

Yes, it stays on put when I have achieved guidance cues by following the flight director bars and a V2 + 20. This tells me that the airplane should be in trim. But let me mention that when I release the yoke, it is not centered because I'm in the climb. The yoke is pulled back, as it would be in the real aircraft.

This sounds like your problem. On the actual aircraft, you would be in trim if you're able to maintain your desired pitch with the yoke centered, but if you turned off return to center, you wouldn't have any idea if your yoke was centered or not. In other words, if your yoke is pulled back as you just described, you are NOT in trim, hence autopilot not engaging. The whole idea of trim is so you don't have to have the yoke pulled back during the climb since on the actual 747, that's a lot of force you're having to apply to the yoke just holding it back.


Captain Kevin

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Air Kevin 124 heavy, wind calm, runway 4 left, cleared for take-off.

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1 hour ago, kmanning said:

Anyway, I have that turned off because the return to center action is not realistic to a real yoke in a actual aircraft. In a real aircraft, you can freely move the yoke back and forth and there's no return to center forcebut that only applies when you're sitting on the ground.

Sry but this is wrong. The yoke column will return to the center position, it actually snaps back pretty fast when you release it. Very similar to what we got in our Saitek, CH or Thrustmaster yokes and joysticks. 

However you are correct if you compare it to a GA airplane without any hydraulics. The yoke of a C172 will be fully forward and in whatever left/right position you left it on the ground. But not in a Boeing airplane. Helicopters for example don‘t have centering forces. Their stick will fall to whatever side. 

As a conclusion, aftrr you have removed the centering springs/rubbers/function there is no chance for you to correctly and properly fly the Boeing and as long as the joystick is not in its center position the AP will definitely not engage in realistic settings. 

 

Edited by Ephedrin

,

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4 hours ago, Ephedrin said:

Sry but this is wrong. The yoke column will return to the center position, it actually snaps back pretty fast when you release it. Very similar to what we got in our Saitek, CH or Thrustmaster yokes and joysticks. 

However you are correct if you compare it to a GA airplane without any hydraulics. The yoke of a C172 will be fully forward and in whatever left/right position you left it on the ground. But not in a Boeing airplane. Helicopters for example don‘t have centering forces. Their stick will fall to whatever side. 

As a conclusion, aftrr you have removed the centering springs/rubbers/function there is no chance for you to correctly and properly fly the Boeing and as long as the joystick is not in its center position the AP will definitely not engage in realistic settings. 

 

that's  why I suggested  for the op  to  disconnect  his  joystick  to  see  if  his issues disappear


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Peter kelberg

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6 hours ago, kmanning said:

Well, it does have a return to center, but it does not use springs. Anyway, I have that turned off because the return to center action is not realistic to a real yoke in a actual aircraft. In a real aircraft, you can freely move the yoke back and forth and there's no return to center force, but that only applies when you're sitting on the ground. When you're in flight, you have aerodynamic forces acting on the control surfaces that put force on the control wheel. Therefore, the aerodynamic forces that some of you refer to are only being simulated by that return to center force. 

 

Yes, it stays on put when I have achieved guidance cues by following the flight director bars and a V2 + 20. This tells me that the airplane should be in trim. But let me mention that when I release the yoke, it is not centered because I'm in the climb. The yoke is pulled back, as it would be in the real aircraft.

Yes, that's true that I cannot feel if the plane is in trim, but I could look at the PFD flight director bar and that should give me some cue as to how much to trim. If I continue to have issues with the auto-pilot not engaging, I'll just turn the return to center position back on so that I can feel the trim. But I've always used the flight director bars as a guidance as suggested in the manual.

Ken.

The fact that the aircraft symbol in the attitude director is aligned with the command bars does not indicate that the aircraft is in trim in the pitch axis! It only indicates that you are applying the proper amount of pitch to maintain the climb angle that the FMS/Flight Director  requires at the moment. 

If the pitch is being maintained by holding the elevators in the “up” position, the autopilot will not engage in a real aircraft. The autopilot elevator servo is not designed to apply a high degree of torque. It “wants” the elevators to be neutral to relieve tension on the servo.

The servo contains a force sensor, and if it detects that the elevator is deflected from neutral it will not engage.

Yes, you need to use the elevators to initially rotate into climb attitude of course, but once the aircraft is established in the climb, you must trim the horizontal stabilizer to relieve the “pull” that you are applying to the yoke, so that the aircraft will maintain its pitch angle without any elevator deflection. The hstab is a far more powerful flight control than the elevators when it comes to pitch.

Yes, in a Cessna you can move the yoke freely on the ground, but even then, it will have a “preferred” neutral position. If you pull the yoke fully aft in a Cessna on the ramp, and let go of it, it will move forward on its own because the elevators move down under their own weight.

In a jet transport aircraft with no hydraulics applied, you cannot move the yoke at all. It will be rigidly locked in place in both pitch and roll. Once the hydraulics are pressurized - (by starting engines or activating electric aux pumps) the yoke “comes alive”, and can be moved - and it absolutely does have a neutral position in both pitch and roll. If you push it forward, or pull it back, or apply left or right roll, and then let go it will move back to the current neutral point.

You have correctly stated that the controls in a Cessna “firm up” in flight, and that is due to the air passing over the control surfaces. Since the yoke is directly connected to the ailerons and elevators with cables and pulleys, you can directly “feel” the control forces in flight - you are directly moving the control surfaces using muscle power alone.

In a large jet, with hydraulic-actuated controls, there is no “feedback” of control surface deflection because they are being moved by powerful hydraulic actuators. For that reason, most jets with purely hydraulic controls use an “artificial feel” unit in each axis, to produce resistance to yoke deflection just like occurs naturally in a Cessna with directly linked mechanical controls.

Guess what the artificial feel units use to provide the feedback resistance? In many aircraft, they use springs! That’s right - springs - just like in a desktop flight yoke or joystick. Now, the springs in an aircraft artificial feel unit are much more sophisticated than “just” the fixed spring used in an FS yoke, and depending on the aircraft model, they may be in the form of actual coiled springs, or tension bungees. Some aircraft use a system of cams and pushrods. In any AFU though, the unit provides artificial resistance, just like the spring in an FS yoke.

The tension in a real aircraft AFU is continuously varied by an associated hydraulic actuator that responds to changes in airspeed, to give the sense of “loose” controls at low air speeds and “firm” controls at high airspeeds.

In the pitch axis, the artificial feel unit is also linked to the horizontal stab actuator. If you sit in the cockpit with hydraulics applied, and run the hstab trim through its entire range from full nose down to full nose up, you will see the yoke move with the trim - i.e. the neutral point varies with the applied stab trim.

The long and short is this: if your flight sim yoke has some kind of automatic return to neutral function - whether a spring or otherwise - you need to enable it. That return to neutral force is the only way you can feel when you are out of trim. It may not be fully accurate to how a Cessna yoke works when not moving through the air, but it is actually quite close to how the yoke works on an aircraft with hydraulic flight controls.

I cannot emphasize this final point too much: Just because you have matched the Attitude Director pitch to the flight director command bars in climb, it does not automatically follow that you are in trim.

Edit:

I should emphasize that this discussion applies specifically to Boeing 737/747 aircraft, or any aircraft that uses a similar flight control system.

Airbus aircraft are very different, in that there is no mechanical connection between the sidestick and the flight controls. The flight controls are positioned by computers. In an Airbus, once you command a specific pitch in climb, you can return the sidestick to neutral, and the pitch will remain “where you put it”. Whether hand flying or on autopilot, the stab trim will be positioned automatically to relieve force on the elevators. 

Not so in Boeings. There is no “in trim” indicator - the “trim indicator” is biological - i.e. the pilot’s fingers, wrists and forearms - if the pilot has to exert muscle power to hold the yoke in a specific position, the trim switch will be actuated to relieve any need to push or pull on the yoke.

Once the autopilot is successfully connected, then the stabilizer trim will automatically be changed to relieve elevator deflection force, but not when flying manually. 

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Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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18 hours ago, kmanning said:

Well, it does have a return to center, but it does not use springs. Anyway, I have that turned off because the return to center action is not realistic to a real yoke in a actual aircraft. In a real aircraft, you can freely move the yoke back and forth and there's no return to center force, but that only applies when you're sitting on the ground. When you're in flight, you have aerodynamic forces acting on the control surfaces that put force on the control wheel. Therefore, the aerodynamic forces that some of you refer to are only being simulated by that return to center force. 

 

Yes, it stays on put when I have achieved guidance cues by following the flight director bars and a V2 + 20. This tells me that the airplane should be in trim. But let me mention that when I release the yoke, it is not centered because I'm in the climb. The yoke is pulled back, as it would be in the real aircraft.

Yes, that's true that I cannot feel if the plane is in trim, but I could look at the PFD flight director bar and that should give me some cue as to how much to trim. If I continue to have issues with the auto-pilot not engaging, I'll just turn the return to center position back on so that I can feel the trim. But I've always used the flight director bars as a guidance as suggested in the manual.

Ken.

Do you by chance have a force feedback joystick?

Your comment about having the yoke pulled back in the climb is a clue why you can’t engage. That is not normal. Your yoke/joystick should be nearly centered soon after takeoff (if trimmed properly). Disabling the centering function on your joystick will prevent you from engaging the autopilot. I am quoting from experience. 

Edited by MattS

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13 hours ago, JRBarrett said:

The fact that the aircraft symbol in the attitude director is aligned with the command bars does not indicate that the aircraft is in trim in the pitch axis! It only indicates that you are applying the proper amount of pitch to maintain the climb angle that the FMS/Flight Director  requires at the moment. 

If the pitch is being maintained by holding the elevators in the “up” position, the autopilot will not engage in a real aircraft. The autopilot elevator servo is not designed to apply a high degree of torque. It “wants” the elevators to be neutral to relieve tension on the servo.

The servo contains a force sensor, and if it detects that the elevator is deflected from neutral it will not engage.

Yes, you need to use the elevators to initially rotate into climb attitude of course, but once the aircraft is established in the climb, you must trim the horizontal stabilizer to relieve the “pull” that you are applying to the yoke, so that the aircraft will maintain its pitch angle without any elevator deflection. The hstab is a far more powerful flight control than the elevators when it comes to pitch.

Yes, in a Cessna you can move the yoke freely on the ground, but even then, it will have a “preferred” neutral position. If you pull the yoke fully aft in a Cessna on the ramp, and let go of it, it will move forward on its own because the elevators move down under their own weight.

In a jet transport aircraft with no hydraulics applied, you cannot move the yoke at all. It will be rigidly locked in place in both pitch and roll. Once the hydraulics are pressurized - (by starting engines or activating electric aux pumps) the yoke “comes alive”, and can be moved - and it absolutely does have a neutral position in both pitch and roll. If you push it forward, or pull it back, or apply left or right roll, and then let go it will move back to the current neutral point.

You have correctly stated that the controls in a Cessna “firm up” in flight, and that is due to the air passing over the control surfaces. Since the yoke is directly connected to the ailerons and elevators with cables and pulleys, you can directly “feel” the control forces in flight - you are directly moving the control surfaces using muscle power alone.

In a large jet, with hydraulic-actuated controls, there is no “feedback” of control surface deflection because they are being moved by powerful hydraulic actuators. For that reason, most jets with purely hydraulic controls use an “artificial feel” unit in each axis, to produce resistance to yoke deflection just like occurs naturally in a Cessna with directly linked mechanical controls.

Guess what the artificial feel units use to provide the feedback resistance? In many aircraft, they use springs! That’s right - springs - just like in a desktop flight yoke or joystick. Now, the springs in an aircraft artificial feel unit are much more sophisticated than “just” the fixed spring used in an FS yoke, and depending on the aircraft model, they may be in the form of actual coiled springs, or tension bungees. Some aircraft use a system of cams and pushrods. In any AFU though, the unit provides artificial resistance, just like the spring in an FS yoke.

The tension in a real aircraft AFU is continuously varied by an associated hydraulic actuator that responds to changes in airspeed, to give the sense of “loose” controls at low air speeds and “firm” controls at high airspeeds.

In the pitch axis, the artificial feel unit is also linked to the horizontal stab actuator. If you sit in the cockpit with hydraulics applied, and run the hstab trim through its entire range from full nose down to full nose up, you will see the yoke move with the trim - i.e. the neutral point varies with the applied stab trim.

The long and short is this: if your flight sim yoke has some kind of automatic return to neutral function - whether a spring or otherwise - you need to enable it. That return to neutral force is the only way you can feel when you are out of trim. It may not be fully accurate to how a Cessna yoke works when not moving through the air, but it is actually quite close to how the yoke works on an aircraft with hydraulic flight controls.

I cannot emphasize this final point too much: Just because you have matched the Attitude Director pitch to the flight director command bars in climb, it does not automatically follow that you are in trim.

Edit:

I should emphasize that this discussion applies specifically to Boeing 737/747 aircraft, or any aircraft that uses a similar flight control system.

Airbus aircraft are very different, in that there is no mechanical connection between the sidestick and the flight controls. The flight controls are positioned by computers. In an Airbus, once you command a specific pitch in climb, you can return the sidestick to neutral, and the pitch will remain “where you put it”. Whether hand flying or on autopilot, the stab trim will be positioned automatically to relieve force on the elevators. 

Not so in Boeings. There is no “in trim” indicator - the “trim indicator” is biological - i.e. the pilot’s fingers, wrists and forearms - if the pilot has to exert muscle power to hold the yoke in a specific position, the trim switch will be actuated to relieve any need to push or pull on the yoke.

Once the autopilot is successfully connected, then the stabilizer trim will automatically be changed to relieve elevator deflection force, but not when flying manually. 

Now if THAT wasn‘t clear... then IDK...

great explanation!


,

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