trisho0

Trim question for PMDG 737NGX

Recommended Posts

Before Takeoff I try to set the trim at 6.23 following FMC info. Well.... I move the wheel via keyboard (Home/End keys) and also placing the mouse point on top of the trim wheel. A tooltip shows the current value. So, while the trim wheel is in motion the tooltip number is changing accordingly but suddenly the showing number stops while the wheel still in motion. I have to stop moving the wheel and start again until the tooltip stops showing the number again and so on until I get the expected value. Is it the way on how the Trim value procedure works? The manual doesn't answer me. 

Kyle said: All the same: when you're in the real plane, you don't get a pop up when you place your hand on the trim wheel. Eyeball it on the trim index. That's good enough. Keep in mind that the trim value is calculated based off of the assumption all of the passengers weigh the same...let's be real here. The person in 1A could be 100 pounds different from the person in 40C.

I am still confused .... My question is why the trim wheel tooltip stops changing the value while the wheel still in motion?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Just now, trisho0 said:

My question is why the trim wheel tooltip stops changing the value while the wheel still in motion?

Perhaps a limitation with the sim?

Not sure. In the end, my earlier statement rings true: eyeball it. Don't hyperfocus on a mundane detail that you're just going to have to correct for when you're in the air. It's true that the tool tip displays the trim value, but in reality, you have neither the tool tip, nor the need to be that precise.

It's like making spaghetti. The box says "add two cups of water for every box of noodles." If you want to, you can grab the measuring cup and be precise about it...or you can eyeball two cups and turn the stove on, because in the end, you're just gonna pour that water out anyway...

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, scandinavian13 said:

Perhaps a limitation with the sim?

Not sure. In the end, my earlier statement rings true: eyeball it. Don't hyperfocus on a mundane detail that you're just going to have to correct for when you're in the air. It's true that the tool tip displays the trim value, but in reality, you have neither the tool tip, nor the need to be that precise.

It's like making spaghetti. The box says "add two cups of water for every box of noodles." If you want to, you can grab the measuring cup and be precise about it...or you can eyeball two cups and turn the stove on, because in the end, you're just gonna pour that water out anyway...

I see..... so if the tooltip shows a closer expected number is OK? I mean, let's say I need to set the Trim as 6.20 then if the trim says 6.00 or 6.40 then just leave it alone?

I wonder where the Pilot check the Trim value in real.

Share this post


Link to post
Share on other sites
3 hours ago, trisho0 said:

I wonder where the Pilot check the Trim value in real.

On the trim index, right next to the trim wheel. 

B737Quadrant2.jpg

Share this post


Link to post
Share on other sites

You are the Master! Many thanks. I will see on that directly instead of tooltip from mouse. Now is really more real, I have seen that in the plane but never realized to look directly. The manual mention "tooltip" and I lived with that for a long time. WOW! definitely you gave me a so great Help! ….. better than the manual. I think now I can adjust Trim as it should.

When I do adjustments to 747 and 777 I look at the Trim index because is easy visible. Just magnify from it to read on it better. Thanks again …… Kyle

Share this post


Link to post
Share on other sites
Posted (edited)
4 hours ago, trisho0 said:

I wonder where the Pilot check the Trim value in real.

The pilots check the trim value via the little pointer and scale next to the trim wheel, but they are, as Kyle points out, aware that the load sheet recommendations and/or any FMC calculations for trim settings are based on a 'perfect world' where every passenger weighs exactly the 'standard passenger weight' and nobody is walking around the cabin, or swapping seats with their fat uncle who was sat right at the back/front, shifting the weight and centre of gravity of the airliner when doing so, and that all the baggage and cargo is conveniently of the same weight throughout its footprint on the cargo hold floor and is perfectly placed in exactly the spots indicated on the load sheet, which it never is, it's as close as it needs to be, but that is all you can really say about it.

Trim settings and potential CoG problems are more critical on a small aeroplane than on big a jetliner simply because the size of a jetliner tends to average out any minor discrepancies, for example: The weight of five people on board a tiny Cessna, as opposed to the weight of four people on board that same tiny Cessna represents a 25 percent increase in the load being carried by it and will significantly alter the CoG of that Cessna, whereas on something like a 737 800 NG, adding another (even really heavy) passenger and their heavy suitcases too, would probably be adding less than a quarter of a percent to the overall payload being carried by the airliner, so it wouldn't make a major difference in trim.

It is true that it would make a small difference, but trim values of 6.20 for the setting recommended by the FMC could just as easily be set on a figure reasonably close to that value and it would be perfectly fine since the pilot normally has to mess with the trim wheel anyway when flying manually and anyone who's ever flown in the cockpit of a 737 (or pretty much any other airliner to be honest) will know that when it is on autopilot, the trim wheel is clattering away like crazy much of the time and spinning around like a demented roulette wheel, since every second the engines are running, the trim is changing as the fuel burns away and the passengers move about..

Edited by Chock

Share this post


Link to post
Share on other sites

Agreed. One interesting point is the trim wheels are made by different manufacturers! Also, I've never found one actual aircraft where the trim markings line up on both left and right sides the same! The Noise comes from the chain tension unit on the chains that run from the wheels under the floor that drive the trimming drum. This drum winds the cable to the back of the aircraft and moves the actuator that jacks the tailplain incidence.

In reality, we line up the figures from the FMC onto the trim and confirm the setting just before take off, the 737 has a  habit of wiping the de-rated take off thrust from the FMC when power is transferred, so a last minute check is advised. In 5 years I've still found myself taking off with full power a couple of times unintentionally! 

De-rated power gives a higher trim setting. Almost every time we do a 22K temperature de-rated departure, we will trim forward once airborne. Just a quick burst, but it's a certainty!

 

 

Share this post


Link to post
Share on other sites
Posted (edited)

First, glad to see others Pilot sharing this thread and lot of thanks for all. Regarding on Trim adjustment I only set the value provided from FMC once before take-off. I don't touch Trim wheel after take-off. I don't see the needs to do more Trim adjustments on airborne. 

Edited by trisho0
correcting

Share this post


Link to post
Share on other sites
1 hour ago, trisho0 said:

I don't see the needs to do more Trim adjustments on airborne.

That's fine if you are letting the autopilot fly the airplane.  I suggest you try hand flying the aircraft until at least she is cleaned up and climbing at 250.  You'll get lots of opportunity to use trim during this brief period, at least you should be using trim.  I was taught to fly trim not pitch except when maneuvering.

Share this post


Link to post
Share on other sites
3 hours ago, downscc said:

That's fine if you are letting the autopilot fly the airplane.  I suggest you try hand flying the aircraft until at least she is cleaned up and climbing at 250.  You'll get lots of opportunity to use trim during this brief period, at least you should be using trim.  I was taught to fly trim not pitch except when maneuvering.

Thanks Dan for the suggestions. Yes I always fly with Autopilot engaged. Right after take-off I usually climb manually and by 1000 feet near the programmed Altitude I arm CMD A or B, more often CMD A. But in that period of time from take-off to AP, I never tried to move the Trim wheel. And if I have to, then at what value to change and the reason? Again, I don't see why to change trim on airborne.

Share this post


Link to post
Share on other sites

I never will understand the logic of the FMC giving the trim setting to the nearest .01 when the trim wheel pointer is at least .1 wide.  Measure it with a micrometer, cut it with an axe.

  • Like 1

Share this post


Link to post
Share on other sites
21 minutes ago, JoeDiamond said:

I never will understand the logic of the FMC giving the trim setting to the nearest .01 when the trim wheel pointer is at least .1 wide.  Measure it with a micrometer, cut it with an axe.

yep, and don't forget to take a picture (lol). I am kidding. I think no matter where is positioned that trim pointer. Simply adjust it accordingly following the FMC info. That is what I do and the plane take off nicely.

Share this post


Link to post
Share on other sites
Posted (edited)
2 hours ago, trisho0 said:

Thanks Dan for the suggestions. Yes I always fly with Autopilot engaged. Right after take-off I usually climb manually and by 1000 feet near the programmed Altitude I arm CMD A or B, more often CMD A. But in that period of time from take-off to AP, I never tried to move the Trim wheel. And if I have to, then at what value to change and the reason? Again, I don't see why to change trim on airborne.

Pat, you don't set trim using a "value" while flying, the entire reason for trim is to adjust the horizontal stabilizer or elevator such that you can take hands off the control and the aircraft will remain at the same attitude.  If you take hands off and nose drops then add nose up trim, also in real aircraft you feel control pressures when hold an attitude that the trim will nullify or "trim out."

I thought I gave you a link to the AIM... I suggest some reading on aircraft control surfaces as homework.

I also urge you to hand fly the aircraft a lot during your probationary period.

Edited by downscc

Share this post


Link to post
Share on other sites
2 hours ago, trisho0 said:

But in that period of time from take-off to AP, I never tried to move the Trim wheel. And if I have to, then at what value to change and the reason? Again, I don't see why to change trim on airborne.

This is just basic aerodynamics/airmanship.  Basically, you use elevator trim to remove the need to maintain constant pressure on the yoke; you trim so the plane will fly hands-off.  This is true and good airmanship in any aircraft, but even more important in a larger aircraft where the control forces can be significant.  

http://www.boldmethod.com/learn-to-fly/systems/how-pilots-should-use-trim-in-flight/

Share this post


Link to post
Share on other sites
16 hours ago, downscc said:

Pat, you don't set trim using a "value" while flying, the entire reason for trim is to adjust the horizontal stabilizer or elevator such that you can take hands off the control and the aircraft will remain at the same attitude.  If you take hands off and nose drops then add nose up trim, also in real aircraft you feel control pressures when hold an attitude that the trim will nullify or "trim out."

I thought I gave you a link to the AIM... I suggest some reading on aircraft control surfaces as homework.

I also urge you to hand fly the aircraft a lot during your probationary period.

I realized I should say trim adjustment instead of "value". Many thanks Dan for correcting me. I don't see AIM link. Yes, I will practice hand fly, good advice.

Share this post


Link to post
Share on other sites
16 hours ago, Stearmandriver said:

This is just basic aerodynamics/airmanship.  Basically, you use elevator trim to remove the need to maintain constant pressure on the yoke; you trim so the plane will fly hands-off.  This is true and good airmanship in any aircraft, but even more important in a larger aircraft where the control forces can be significant.  

http://www.boldmethod.com/learn-to-fly/systems/how-pilots-should-use-trim-in-flight/

I will take look at that site. I guess the trim adjustment is optional after take-off?

Share this post


Link to post
Share on other sites
Posted (edited)
1 hour ago, trisho0 said:

I will take look at that site. I guess the trim adjustment is optional after take-off?

Trimming an airplane at the simulator is hard to explain. It would be interesting what kind of controls do you use? Before I learnt to fly I had no idea what exactly „trimming“ means and as I had a joystick with a defined center position all I noticed was that when I touched the trim, the nose went up or down. So often simulator pilots tend to „stear“ the plane using the trim instead of moving the elevator.

In reality there is no such center position, at least at simple airplanes like a Cessna. The airstream moves the control surface to a position and your stick or yoke is directly connected to that surface. But this position doesn‘t mean that te airplane keeps it pitch. To do that you will have to push or pull your controls and hold them. This requires a lot of force on some airplanes. To neutralize this force, there are little trim tabs with wires attached to them that are moved by a little lever or trim wheel. In bigger airplanes, such as an airliner, there isn‘t a trim tab anymore but instead the whole stabilizer (on which te elevators are mounted) rotates up or down. This also neutralized the forces on your controls as the stab lines up with the airstream. That‘s more or less the sense of an elevator or stabilizer trim. 

 

So yes, using the trim is optional, so to say. It‘s a comfort thing but every instructor will beat the hell out of you if you have to battle your airplane to keep its attitude instead of having time to manage other important systems.

Edited by Ephedrin

Share this post


Link to post
Share on other sites
1 hour ago, trisho0 said:

I guess the trim adjustment is optional after take-off?

Use of trim while hand flying is how you should fly; however, you do not touch trim if the A/P is engaged...in fact, it is trim that the A/P generally uses to maintain pitch.  The Airman's Information Manual (AIM) is here: https://www.faa.gov/air_traffic/publications/

Share this post


Link to post
Share on other sites
2 hours ago, downscc said:

Use of trim while hand flying is how you should fly; however, you do not touch trim if the A/P is engaged...in fact, it is trim that the A/P generally uses to maintain pitch.  The Airman's Information Manual (AIM) is here: https://www.faa.gov/air_traffic/publications/

Dan, thanks again. I just flew KJFK-PHDL and after take-off I adjusted the trim and yes the nose went up and down trying to follow the horizon pink line from PFD but it won't do ILS so when I got about 1K below the programmed Altitude I had to arm AP. Otherwise, the plane won't follow ILS. So, even I learned something about trim procedures I found that really I don't have to do more trim after AP engaged. From the AIM link that info is so technical. I will check on that link again to take my reading time. I am sure I will found more to learn.

Do you agree the trim after take-off is optional?

Share this post


Link to post
Share on other sites
2 hours ago, Ephedrin said:

Trimming an airplane at the simulator is hard to explain. It would be interesting what kind of controls do you use? Before I learnt to fly I had no idea what exactly „trimming“ means and as I had a joystick with a defined center position all I noticed was that when I touched the trim, the nose went up or down. So often simulator pilots tend to „stear“ the plane using the trim instead of moving the elevator.

In reality there is no such center position, at least at simple airplanes like a Cessna. The airstream moves the control surface to a position and your stick or yoke is directly connected to that surface. But this position doesn‘t mean that te airplane keeps it pitch. To do that you will have to push or pull your controls and hold them. This requires a lot of force on some airplanes. To neutralize this force, there are little trim tabs with wires attached to them that are moved by a little lever or trim wheel. In bigger airplanes, such as an airliner, there isn‘t a trim tab anymore but instead the whole stabilizer (on which te elevators are mounted) rotates up or down. This also neutralized the forces on your controls as the stab lines up with the airstream. That‘s more or less the sense of an elevator or stabilizer trim. 

 

So yes, using the trim is optional, so to say. It‘s a comfort thing but every instructor will beat the hell out of you if you have to battle your airplane to keep its attitude instead of having time to manage other important systems.

I use Logitech Extreme 3D PRO Joystick for flying my birds. I am learning about trimming thanks to Dan in this thread.

Share this post


Link to post
Share on other sites
Posted (edited)
3 hours ago, trisho0 said:

I am learning about trimming

 

There is actually not much to learn. The theoretical description isn't worth anything as long as you don't accept a simple fact: using the elevator trim is as normal and natural on an airplane as breathing. And hey, as long as you ask if it's optional you haven't accepted it... There is no option. Sure, you can fight with your airplane, strongly grab the stick/yoke and push it to the front to make the nose stay down. But there's a point your arm will become tired. As you accelerate your airplane will pitch up. To counter this movement, you stear down (push). Release the stick and the airplane will pitch up and climb again. So you push again. To make the stick stay forward at the correct position without having to force it you trim your nose down... elevator/stab trim down. No value. No theoretical setting... just enough that you don't FEEL any force on the stick anymore. So you can take your hands off it and the plane's nose will stay were it is. This will need some practise, sure, but I doubt that there is a lot to read about. It's a very simple thing actually, as long as you don't dive into its aerodynamical background. As simple as possible: When you have to force your airplane to keep its attitude then use the available trim. That's all.

 

Why you have to set a specific trim setting on a 737 for the takeoff is a little bit different. The stabilizer actually has the aerodynamical effect of an inversed (upside-down) wing. If you closely look at its profile, it's convex on the down side in contrary to a wing... (picture below) so its "lift" is directed downwards... it will hold the back down so the plane can keep its pitch. Now on take off you have to use a trim setting that neutralizes this lift for the given CG until - at Vr - you pull back the yoke and lift up the elevator. this elevator has the same effect as a flap, but in contrary direction and the tail "flies" down.. that's actually all about the trim mistery. There isn't too much. it's just an "angle of attack" setting for the stabilizer.

 

57Tmo.jpg

Edited by Ephedrin

Share this post


Link to post
Share on other sites
7 hours ago, Ephedrin said:

Why you have to set a specific trim setting on a 737 for the takeoff 

An alternative way of thinking about this is to say the takeoff trim setting is made so that the aircraft is already in trim before flight. Once in flight you can use the control forces to trim the aircraft. Before takeoff that can’t be done. So the trim setting is calculated. If the aircraft wasn’t correctly trimmed for takeoff the result could be catastrophic. 

Share this post


Link to post
Share on other sites
Posted (edited)

Some important things to remember about trim:

1) We trim for a pitch attitude, not an airspeed or altitude

2) The trim is not used to change the pitch attitude of the aeroplane (a major fault for many simmers)

3) The trim is there to make your life easier!

Keeping the aeroplane in trim whilst hand-flying is good practice for many reasons -- not least 1) it is less tiring on the arms, 2) it will allow you to fly more accurately and 3) give you more capacity to deal with other stuff that might require you to take your hand off the control column for a second or two.

So how do we trim the aeroplane?

The usual mantra is Select (the pitch attitude required, with the elevator) - Hold (again, using the elevator to hold the nose in the required pitch attitude) - Trim (to relieve the force you are having to exert on the controls to hold the nose in the selected position). Select, hold, trim.

This is possibly more easily demonstrated in a more simple aircraft because the pitch adjustments in a large jet like the 737 are much smaller and more difficult to see -- a small pitch change results in a big difference in performance. However, if you are going to practice this in the NGX, set yourself up in level flight at, say, 7000 feet and 230 knots with nice clear skies and autopilot/autothrottle engaged. 

Look outside and observe where the horizon line is relative to the top of the glareshield. You can also look at the ADI and observe the indicated pitch attitude (look at where the aeroplane symbol -- the small box in the centre of the wing bars - sits on the pitch ladder, which is graduated in 2.5 degree steps).

Now disengage both autothrottle and autopilot so that you are in fully manual flight (thrust changes will affect the aircraft's trim). Leave the throttle alone though -- you just want to remain at that fixed thrust setting throughout. The aircraft shouldn't really move much if at all from where it is -- the autopilot should have left it nicely in trim!

Next, apply a small amount of backpressure to the yoke and raise the nose, say, 5 degrees compared to its current value (two steps up on the pitch ladder). Observe where the horizon is relative to the glareshield and try and hold that new attitude as steadily as you can. You'll find you need to hold some backpressure, increasing as the speed reduces (again - don't touch the throttle). Once the airspeed stabilises at a new lower value, the amount of backpressure you require will also stabilise. Quite uncomfortable, isn't it?

What we can do to solve this is to trim the aeroplane -- and it's best to have the trim assigned to buttons on your joystick or yoke to do this because if you have to take your hands off the stick you won't be able to feel the change in pressure and if you have to change your viewpoint you won't be able to observe the movement of the horizon.

As we are holding backpressure, we need to apply nose up trim, which in the real thing is achieved by using your thumb to click the trim switches on the yoke 'down'/towards you (i.e. it is arranged to work in the same sense that the flight controls work -- down/towards you to go up, up/away from you/forward to go down). Slowly apply some nose-up trim (the easiest way to modulate this is to hold the trim switch down for a second or so, then release, reassess and go again). As you do so, hold the attitude absolutely steady -- keep that aeroplane symbol exactly where you placed it on the ADI/the horizon line exactly where you placed it relative to the glareshield a few moments ago, whilst you trim the aircraft.

As you apply nose-up trim, of course, the amount of backpressure you need to hold to do this will gradually reduce. Eventually you will reach a point where you are not holding any backpressure but the nose is still sitting in the new attitude. Voila -- the aeroplane is now in trim! Obviously selecting a lower nose attitude works in exactly the same way except with forward pressure and nose down trim.

Of course, this is not entirely a one-time set and forget thing - atmospheric disturbances, people moving about down the back, different weights etc will all require different trim and changes in trim, and so will adjustments to airspeed, thrust and configuration. However, the principle remains the same -- select the attitude required with the elevator, hold it there until you know how much pressure is required, then trim to relieve that pressure.

As you become more proficient with practice you will get a sense of approximately how much trim is required given the amount of backpressure you are holding and/or the size of the change in pitch attitude and so will get much faster at nailing it. But as I say, remember to hold that pitch attitude rock steady whilst you are trimming -- don't select the attitude then let go of the stick whilst you trim.

Hope that helps!

Edited by skelsey

Share this post


Link to post
Share on other sites
21 hours ago, Ephedrin said:

There is actually not much to learn. The theoretical description isn't worth anything as long as you don't accept a simple fact: using the elevator trim is as normal and natural on an airplane as breathing. And hey, as long as you ask if it's optional you haven't accepted it... There is no option. Sure, you can fight with your airplane, strongly grab the stick/yoke and push it to the front to make the nose stay down. But there's a point your arm will become tired. As you accelerate your airplane will pitch up. To counter this movement, you stear down (push). Release the stick and the airplane will pitch up and climb again. So you push again. To make the stick stay forward at the correct position without having to force it you trim your nose down... elevator/stab trim down. No value. No theoretical setting... just enough that you don't FEEL any force on the stick anymore. So you can take your hands off it and the plane's nose will stay were it is. This will need some practise, sure, but I doubt that there is a lot to read about. It's a very simple thing actually, as long as you don't dive into its aerodynamical background. As simple as possible: When you have to force your airplane to keep its attitude then use the available trim. That's all.

 

Why you have to set a specific trim setting on a 737 for the takeoff is a little bit different. The stabilizer actually has the aerodynamical effect of an inversed (upside-down) wing. If you closely look at its profile, it's convex on the down side in contrary to a wing... (picture below) so its "lift" is directed downwards... it will hold the back down so the plane can keep its pitch. Now on take off you have to use a trim setting that neutralizes this lift for the given CG until - at Vr - you pull back the yoke and lift up the elevator. this elevator has the same effect as a flap, but in contrary direction and the tail "flies" down.. that's actually all about the trim mistery. There isn't too much. it's just an "angle of attack" setting for the stabilizer.

This really explained much better than the manual, well detailed. Understood about trimming. Let me say if I am right:

1. Trim setting follow CG from FMC

2. Takeoff, push back the Joystick (Yoke) to climb

3. Keep hands off and start trimming accordingly to stabilize the plane

4. Once I am 1K feet near the programmed Altitude stop trimming and arm Autopilot, no more Trim to take car of.

Am I wrong?

Share this post


Link to post
Share on other sites
13 hours ago, kevinh said:

An alternative way of thinking about this is to say the takeoff trim setting is made so that the aircraft is already in trim before flight. Once in flight you can use the control forces to trim the aircraft. Before takeoff that can’t be done. So the trim setting is calculated. If the aircraft wasn’t correctly trimmed for takeoff the result could be catastrophic. 

That is what ephedrine said

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now