Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Trim technique

Tech Log The very best in practical technical discussion on the web

Trim technique

Old 11th Nov 2018, 02:32
  #1 (permalink)  
Thread Starter
 
Join Date: Sep 2016
Location: USA
Posts: 458
Trim technique

A brief discussion came up and quickly died down in the Lion Air thread, which I thought might be interesting to continue, but don't want to derail that thread. It was centered mostly around whether to fly with the trim as primary pitch control, or keep to using the elevator for maneuvering and taking the stick forces out later with trim. As with many internet discussions, both sides' succinct statements were misunderstood and overextended into meanings/situations probably not intended. Overall my position is that it's a nuanced issue with room for application of both philosophies in different situations; and that any hard-line blanket policy is likely to miss situations where it would be ill-applied.

The basic premise of trim, of course, is that it's there to remove long-term stick forces that are already developed. And in many cases where someone is new and/or nervous, they tend to overtrim, back and forth, greatly increasing their workload and destabilizing the airplane. I saw it in myself as a light plane student, saw it in my own light plane students as an instructor, and saw it in myself as a new jet FO in the CRJ.

Especially given that the CRJ's stick forces are way higher than almost everything I'd flown before, the importance of trim naturally impressed itself upon me. Since it was prevalent in my brain, any time I was task saturated and some light turbulence started, I'd immediately trim against it. But since there was no long term state change (this trim setting has held a neutral stick force at this speed, thrust, and flap setting for the last 20 seconds, hello McFly! So why the change?) it only leads to having to correct back the other way, which I do but by the time I do it I've lost some speed and gained some altitude, all of which needs to be corrected at the same time as well; and as I PIO my way through all that, whatever little task capacity I started with, has shrunk to nothing.

Things got a lot better once I realized this problem, and forced myself to not trim against every stick input. Anytime I'm in bumps I fly only with the stick for at least a few seconds, and then ask myself if the numbers of up and down inputs have been about the same, or there's a preponderance of one or the other; and only in the second case would I retrim. After that, wait a few more seconds, etc.

---

However, there still are some cases for which I actively fly the airplane with the trim. What they all have in common is that they're phase transitions where I know about the long-term trim change ahead of time, so why not A) lead with it instead of B) doing the thrust and elevator first, letting a strong stick force develop, and then fixing it with the trim? Doing B can lead to some big stick forces (especially since it's a high engine jet with a reverse thrust-pitch couple, and with artificial feel that makes it hard to make delicate changes around neutral) that can lead to some pitch roughness as I fix it. These situations are:

- Beginning a climb or descent from level. I know that the thrust change is gonna try to do the opposite thing with the nose relative to what I intend, so (in beginning a climb) a long-ish blip of nose-up trim as I increase thrust kills the nose-down tendency before it even starts, starts the nose going up smoothly, and then I manage slight deviations with the elevator until it settles into the new attitude, whereupon I do some final clean-up trimming.

- Accelerating before/after flap retraction, and after 10,000 feet. At 10,000, a nice long press of nose down trim starts the nose down very smoothly, it initially settles right around a 1000 fpm climb rate, and then a little blip every few seconds keeps it at that rate as I accelerate until I reach the final climb speed. On a smooth day, not one elevator input has been made! This is by far the smoothest way I can fly this phase, and a hardline elevator-then-trim philosophy would have me making pointless elevator inputs that I would have to release later when making the trim inputs that I'm making anyway!

All this reminds me of my last airplane the King Air, (normal thrust-pitch couple) where I got to a point where in levelling off from a descent, I'd start a power increase (nice and early and slow), finish with a blip of nose-down trim right as I came level at my target altitude, and not once make an input to the elevator. The smoothness, efficiency, and... "one-ness," if I may, gave me such a pleasure...
Vessbot is offline  
Old 11th Nov 2018, 03:51
  #2 (permalink)  
 
Join Date: Jun 1999
Location: Oztrailia
Posts: 2,714
I always thought Elevator was a Primary flight control and trim, well secondary........

Keep it simple.
ACMS is offline  
Old 11th Nov 2018, 04:08
  #3 (permalink)  
 
Join Date: Nov 2010
Location: Denver
Age: 52
Posts: 36
Originally Posted by Vessbot View Post
A brief discussion came up and quickly died down in the Lion Air thread, which I thought might be interesting to continue, but don't want to derail that thread. It was centered mostly around whether to fly with the trim as primary pitch control, or keep to using the elevator for maneuvering and taking the stick forces out later with trim. As with many internet discussions, both sides' succinct statements were misunderstood and overextended into meanings/situations probably not intended. Overall my position is that it's a nuanced issue with room for application of both philosophies in different situations; and that any hard-line blanket policy is likely to miss situations where it would be ill-applied.

The basic premise of trim, of course, is that it's there to remove long-term stick forces that are already developed. And in many cases where someone is new and/or nervous, they tend to overtrim, back and forth, greatly increasing their workload and destabilizing the airplane. I saw it in myself as a light plane student, saw it in my own light plane students as an instructor, and saw it in myself as a new jet FO in the CRJ.

Especially given that the CRJ's stick forces are way higher than almost everything I'd flown before, the importance of trim naturally impressed itself upon me. Since it was prevalent in my brain, any time I was task saturated and some light turbulence started, I'd immediately trim against it. But since there was no long term state change (this trim setting has held a neutral stick force at this speed, thrust, and flap setting for the last 20 seconds, hello McFly! So why the change?) it only leads to having to correct back the other way, which I do but by the time I do it I've lost some speed and gained some altitude, all of which needs to be corrected at the same time as well; and as I PIO my way through all that, whatever little task capacity I started with, has shrunk to nothing.

Things got a lot better once I realized this problem, and forced myself to not trim against every stick input. Anytime I'm in bumps I fly only with the stick for at least a few seconds, and then ask myself if the numbers of up and down inputs have been about the same, or there's a preponderance of one or the other; and only in the second case would I retrim. After that, wait a few more seconds, etc.

---

However, there still are some cases for which I actively fly the airplane with the trim. What they all have in common is that they're phase transitions where I know about the long-term trim change ahead of time, so why not A) lead with it instead of B) doing the thrust and elevator first, letting a strong stick force develop, and then fixing it with the trim? Doing B can lead to some big stick forces (especially since it's a high engine jet with a reverse thrust-pitch couple, and with artificial feel that makes it hard to make delicate changes around neutral) that can lead to some pitch roughness as I fix it. These situations are:

- Beginning a climb or descent from level. I know that the thrust change is gonna try to do the opposite thing with the nose relative to what I intend, so (in beginning a climb) a long-ish blip of nose-up trim as I increase thrust kills the nose-down tendency before it even starts, starts the nose going up smoothly, and then I manage slight deviations with the elevator until it settles into the new attitude, whereupon I do some final clean-up trimming.

- Accelerating before/after flap retraction, and after 10,000 feet. At 10,000, a nice long press of nose down trim starts the nose down very smoothly, it initially settles right around a 1000 fpm climb rate, and then a little blip every few seconds keeps it at that rate as I accelerate until I reach the final climb speed. On a smooth day, not one elevator input has been made! This is by far the smoothest way I can fly this phase, and a hardline elevator-then-trim philosophy would have me making pointless elevator inputs that I would have to release later when making the trim inputs that I'm making anyway!

All this reminds me of my last airplane the King Air, (normal thrust-pitch couple) where I got to a point where in levelling off from a descent, I'd start a power increase (nice and early and slow), finish with a blip of nose-down trim right as I came level at my target altitude, and not once make an input to the elevator. The smoothness, efficiency, and... "one-ness," if I may, gave me such a pleasure...
Agree completely, I still think Airbus should have kept trim manual to keep the pilots more connected in manual flight.
hans brinker is offline  
Old 11th Nov 2018, 04:14
  #4 (permalink)  
Thread Starter
 
Join Date: Sep 2016
Location: USA
Posts: 458
Originally Posted by hans brinker View Post
Agree completely, I still think Airbus should have kept trim manual to keep the pilots more connected in manual flight.
That may be so, but trim isn't some extraneous feature that they can decide to include or leave out of the C* control law... the lack of it is a fundamentally inherent behavior. I can't imagine what a C* control with trim would even... be.
Vessbot is offline  
Old 11th Nov 2018, 04:17
  #5 (permalink)  
 
Join Date: Jul 1999
Location: Out of a Suitcase
Posts: 135
Originally Posted by hans brinker View Post
Agree completely, I still think Airbus should have kept trim manual to keep the pilots more connected in manual flight.
The autotrim works perfectly fine in the airbus. You have a wheel that moves and has graduations on it so you can easily see where the trim is.
Eric Janson is offline  
Old 11th Nov 2018, 04:50
  #6 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,856
Originally Posted by Vessbot
That may be so, but trim isn't some extraneous feature that they can decide to include or leave out of the C* control law... the lack of it is a fundamentally inherent behavior. I can't imagine what a C* control with trim would even... be.
I don't understand the C* lingo, nor do I fly an Airbus or any of these new-fangled FBW machines, but doesn't the Airbus just automatically trim the speed out ie it's pitch stable (slow down, no backtrim required), instead of Boeings being speed stable (slow down, back trim required)?

In any case, with other underslung aeroplanes eg 777, doesn't the FBW remove any thrust-pitch coupling using I imagine first the elevators then trim with the pilot even knowing it's happening?

I'm with ACMS. Control a "normal" aeroplane with the elevators, then trim away any residual force. You can use Vesspot's technique of anticipating trim requirements (flaps extension say) but you run the real risk of subconsciously doing it more and more. This is bad.

Pulling and pushing in the sim I say to myself occasionally "Bloggs, the speed and config hasn't changed, stop trimming!!".
Capn Bloggs is offline  
Old 11th Nov 2018, 19:29
  #7 (permalink)  
 
Join Date: Nov 2010
Location: Denver
Age: 52
Posts: 36
Originally Posted by Eric Janson View Post
The autotrim works perfectly fine in the airbus. You have a wheel that moves and has graduations on it so you can easily see where the trim is.
Okay, so another thing that you have to look for as opposed to having tactile feedback...
I really feel it would be safer to feel it in the flight controls if your speed drops down ( I know, eventually you will).
hans brinker is offline  
Old 11th Nov 2018, 19:39
  #8 (permalink)  
 
Join Date: Nov 2010
Location: Denver
Age: 52
Posts: 36
Originally Posted by Capn Bloggs View Post
I don't understand the C* lingo, nor do I fly an Airbus or any of these new-fangled FBW machines, but doesn't the Airbus just automatically trim the speed out ie it's pitch stable (slow down, no backtrim required), instead of Boeings being speed stable (slow down, back trim required)?

In any case, with other underslung aeroplanes eg 777, doesn't the FBW remove any thrust-pitch coupling using I imagine first the elevators then trim with the pilot even knowing it's happening?

I'm with ACMS. Control a "normal" aeroplane with the elevators, then trim away any residual force. You can use Vesspot's technique of anticipating trim requirements (flaps extension say) but you run the real risk of subconsciously doing it more and more. This is bad.

Pulling and pushing in the sim I say to myself occasionally "Bloggs, the speed and config hasn't changed, stop trimming!!".
Yes no pitch trim required on the A320, hand flew level, reduced the speed from 320 to 210, never had to pull back on the stick.

As far as waiting till you need trim, I flew the F50. When going from flaps 15 to flaps 25 it would balloon up, and would require 2 full revolutions of the trim wheel. If you waited till you felt the pressure you would need two hands on the controls to be able to keep the aircraft level, and would need to ask the other pilot to trim for you. There is definitely a time and place for anticipatory trimming.
hans brinker is offline  
Old 11th Nov 2018, 20:02
  #9 (permalink)  
 
Join Date: Jun 2002
Location: VVTS (Saigon)
Posts: 5,202
The autotrim works perfectly fine in the airbus. You have a wheel that moves and has graduations on it so you can easily see where the trim is.
Except on the A-380. No wheel, just a switch on the central console.
India Four Two is offline  
Old 12th Nov 2018, 05:56
  #10 (permalink)  
 
Join Date: Nov 2010
Location: Denver
Age: 52
Posts: 36
Originally Posted by Vessbot View Post
That may be so, but trim isn't some extraneous feature that they can decide to include or leave out of the C* control law... the lack of it is a fundamentally inherent behavior. I can't imagine what a C* control with trim would even... be.
If they can program the thing to artificially add nose down in the flare so I have to pull the nose up for a more "natural feel", I am pretty sure they can add manual trim. While they are at it, they really should consider having direct roll in the flare instead of roll rate, so you can actually use cross control for a cross wind landing.
hans brinker is offline  
Old 12th Nov 2018, 06:04
  #11 (permalink)  
 
Join Date: Jun 2009
Location: Mountain View
Posts: 110
The OP was quite lengthy but what I came away with was "is trim primary or secondary?". By definition, and aircraft certification, trim is secondary. I learned to fly when airplanes were controlled by cables and later by hydraulics. I now fly FBW boeing and stick with my training. Trim should NEVER be a primary means of flight control. By definitions trim is a secondary flight control function. Maybe current flight training should focus more on actual flight skill rather than system management skills.
WrldWide is offline  
Old 12th Nov 2018, 09:56
  #12 (permalink)  
 
Join Date: Sep 2011
Location: FL390
Posts: 325
Beginning a climb or descent from level. I know that the thrust change is gonna try to do the opposite thing with the nose relative to what I intend, so (in beginning a climb) a long-ish blip of nose-up trim as I increase thrust kills the nose-down tendency before it even starts, starts the nose going up smoothly, and then I manage slight deviations with the elevator until it settles into the new attitude, whereupon I do some final clean-up trimming.
Does increasing power in an aircraft with fuselage mounted engines cause a pitch down moment?
Fursty Ferret is offline  
Old 12th Nov 2018, 19:33
  #13 (permalink)  
Thread Starter
 
Join Date: Sep 2016
Location: USA
Posts: 458
Originally Posted by WrldWide View Post
The OP was quite lengthy but what I came away with was "is trim primary or secondary?". By definition, and aircraft certification, trim is secondary. I learned to fly when airplanes were controlled by cables and later by hydraulics. I now fly FBW boeing and stick with my training. Trim should NEVER be a primary means of flight control. By definitions trim is a secondary flight control function. Maybe current flight training should focus more on actual flight skill rather than system management skills.
"Primary" and "secondary" are abstract labels that don't do much more than simplify and lay out the basic scheme for new students. But I was trying to foster a discussion where we talk about actual flying technique. I.e., what do you do, not what you call something. That's why I laid out examples. Your post addressed none of that, it might as well have said "the elevator is Ozzy Osborne and the trim is Axl Rose," and would have carried the same amount of meaningful content.

Originally Posted by Fursty Ferret View Post
Does increasing power in an aircraft with fuselage mounted engines cause a pitch down moment?
The CRJ does; I don't know how strong the tendency is compared to other types. I'd be curious to learn.

Originally Posted by TangoAlphad View Post
Anticipating a trim requirement and flying it on the trim are two very different things.
​​​​​​​
That's why I tend to stay away from abstractions. A phrase is said, to mean one thing, and can be understood to mean a whole lot more than the speaker intended.
Vessbot is offline  
Old 12th Nov 2018, 20:09
  #14 (permalink)  
 
Join Date: Feb 2009
Location: Seattle
Posts: 379
Vessbot - thanks for bringing this topic out of the LA 610 discussion for a deeper dive here. One of the key elements to this topic is the definition of "trim". I see two related, but distinct meanings: (1) move the horizontal tail or what ever surface, tab, etc. is used to allow repositioning of the primary pitch control surface (most often an elevator), and (2) relieve pitch control input forces. On an unaugmented airplane where elevator is controlled by the pilots pitch control input (no Hal involved) these two definitions of "trim" are one and the same. On an airplane that has augmented pitch control they are not.

An example of an airplane with no augmentation of the elevator control is the 737. Making manual "trim" inputs on this model drives the horizontal stabilizer directly. There are functions on the 737 that drive the stabilizer directly (STS and MCAS with autopilot disconnected, and stabilizer offload with autopilot engaged) that can cloud the picture, but in general manual pitch trim on a 737 both moves the stabilizer directly and impact the level of column force that the flight crew must maintain.

An example of an airplane with augmented elevator control is the 777. The 777 elevator responses to the pilot's pitch control input through both a proportional, feed forward path and through feedback paths that take into account pilot maneuver demand and airplane response. Such a system can be designed to reject some pitching moment disturbances such as those resulting from thrust and configuration changes and preserve or even generate others such as providing the desired level of positive speed stability. On 777 this pitch augmentation system is given the name C*U. With C*U a separate, automated function monitors elevator position and automatically commands stabilizer motion to maintain the desired long term, steady state position of the elevator. Steady column forces that the pilot must maintain with C*U are related to maneuvers and changes in speed. With C*U, the pilot's manual pitch trim input does not drive the stabilizer. Pilot manual pitch trim input manages a reference speed that serves as the trim point for hands off flight. In this way, 777 has separate portions of the C*U control law that address the two different meanings of "trim".
FCeng84 is offline  
Old 13th Nov 2018, 21:38
  #15 (permalink)  
Thread Starter
 
Join Date: Sep 2016
Location: USA
Posts: 458
Originally Posted by FCeng84 View Post
An example of an airplane with augmented elevator control is the 777.
Yeah, ever since I learned about this I've been wanting to fly a C*U airplane. It's like a normal airplane, but better in every way. Always holds the speed, takes out any pitch couples, I bet they damp out phugoids too...
Vessbot is offline  
Old 13th Nov 2018, 22:45
  #16 (permalink)  
 
Join Date: Jun 2001
Location: at the edge of the alps
Posts: 339
On the Q400 "anticipatory" trimming was helpful during acceleration from V2+10 to cruise climb speed. Would have been quite some pitch force to push and then trim away.

Optimum technique varies between A/C using the stabilizer for trimming and A/C that just change the force required to move the elevator (by trim tabs like the F50 or by changing the "neutral force" position of the spring-loaded control column like the Q400).
Alpine Flyer is offline  
Old 14th Nov 2018, 00:07
  #17 (permalink)  
 
Join Date: Feb 2009
Location: Seattle
Posts: 379
Vessbot - curious you should mention phugoid damping. It turns out that the higher the phugoid damping you provide the more aggressively the airplane pitches in response to a speed change. Pilots tend not to want the airplane to quickly deviate from the flight path they have set every time they adjust speed up or down 10 or 20 knots. They seem to want the speed stability / speed awareness to come in more gradually which means keep the augmented phugoid frequency low and with relatively low damping. It may be a matter of giving them what they have been used to on an unaugmented airplane with "classically good speed stability characteristics".
FCeng84 is offline  
Old 14th Nov 2018, 00:20
  #18 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
Originally Posted by Vessbot View Post
Yeah, ever since I learned about this I've been wanting to fly a C*U airplane. It's like a normal airplane, but better in every way. Always holds the speed, takes out any pitch couples, I bet they damp out phugoids too...
CSeries (A220 now!) is C*U.
The easiest way to describe it is it feels like a conventional aeroplane.

We use a preemptive trim technique when accelerating or decelerating so rather than holding an increasing force on the sidestick and then trimming it all out at the new speed we nibble away at the trim as the speed changes. If done correctly then no force is required on the stick at all. It may appear to be flying on trim but really it's flying on zero stick force enabled by trim inputs.
FE Hoppy is offline  
Old 14th Nov 2018, 03:29
  #19 (permalink)  
 
Join Date: Feb 2009
Location: Seattle
Posts: 379
FE Hoppy - Is the trim technique you describe for the A220 included in type training for that model? If so, how is it described to the pilots during that training? Are there any particular displays that the flight crew are trained to pay attention to with this trim technique so that they know they are giving it the right amount of trim input?
FCeng84 is offline  
Old 14th Nov 2018, 19:22
  #20 (permalink)  
 
Join Date: Dec 2003
Location: Tring, UK
Posts: 1,409
Yeah, ever since I learned about this I've been wanting to fly a C*U airplane. It's like a normal airplane, but better in every way. Always holds the speed, takes out any pitch couples, I bet they damp out phugoids too...
The 777 is mostly nice to fly - it even removes the need to coordinate the elevator in a turn up to moderate AoB. Where I find it less so sometimes is on the approach where natural speed fluctuations can lead to needing to push/pull a lot more than in other aircraft Iíve flown, as the stick forces track variations away from the FBW trim speed. I have a few theories, one of which is that airspeed is sampled in a couple of locations near the nose, driving the electronic Ďfeelí, but the aircraft itself responds to actual airflow over the wings and control surfaces. This can be noticeably different in turbulence and/or near the ground.

In terms of trimming technique, the 777 is a bit of an outlier as a little blip on the trim will move the trim reference to the current airspeed (as long as you are near it), so the temptation is to blip away constantly to remove trim forces, which I try to resist. I donít feel the need to lead with the trim, which I sometimes did on things like the 737 when changing the configuration or the power setting significantly as you knew you would be out-of-trim shortly after starting, so anticipated the change to keep the forces low.
FullWings is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.