trisho0

Trim question for PMDG 737NGX

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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?

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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...

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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.

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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

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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

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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

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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!

 

 

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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

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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.

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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.

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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.

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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.

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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

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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/

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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.

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