B777 Speed Trim
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B777 Speed Trim
Can any one who flies the 777 enlighten me as to how the speed trim works?
I fly the airbus and many years ago the 737, so it's a new concept to me.
The way I have read it is that in the Airbus if you only increase thrust you will fly straight and level but will speed up. The aircraft will compensate trim automatically for speed changes.
In the Boeing if you only increase thrust, the aircraft will pitch up to maintain the speed it was trimmed at.
So the Airbus is flying straight and level but faster, and the Boeing is now climbing but at the same speed.
I'm assuming speed trim is used for the Boeing because the flight control system uses the stab as well as the elevator to give the pilot what he wants when he makes a pitch control input, but I can't get my head around how it works.
Is it as simple as set pitch and power and then trim out the force?
Thanks in advance
I fly the airbus and many years ago the 737, so it's a new concept to me.
The way I have read it is that in the Airbus if you only increase thrust you will fly straight and level but will speed up. The aircraft will compensate trim automatically for speed changes.
In the Boeing if you only increase thrust, the aircraft will pitch up to maintain the speed it was trimmed at.
So the Airbus is flying straight and level but faster, and the Boeing is now climbing but at the same speed.
I'm assuming speed trim is used for the Boeing because the flight control system uses the stab as well as the elevator to give the pilot what he wants when he makes a pitch control input, but I can't get my head around how it works.
Is it as simple as set pitch and power and then trim out the force?
Thanks in advance
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From FCOM Vol 2. Systems Description
Primary Pitch Trim Control
Primary pitch trim is controlled by the dual pitch trim switches on each control
wheel. Both switches must be moved to command trim changes. The primary
pitch trim switches are inhibited when the autopilot is engaged. Pitch trim does not
move the control column.
In the normal mode, primary pitch trim operates differently on the ground than it
does in flight. On the ground, the stabilizer is directly positioned when the pilot
uses the pitch trim switches. In flight, the pitch trim switches do not position the
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
The trim reference speed is the speed at which the airplane would eventually
stabilize if there were no control column inputs. Once the control column forces
are trimmed to zero, the airplane maintains a constant speed with no column
inputs. Thrust changes result in a relatively constant indicated airspeed climb or
descent, with no trim inputs needed unless airspeed changes.
When pilot trim inputs are made, the PFCs automatically move the elevators to
achieve the trim change, then move the stabilizer to streamline the elevator.
Stabilizer motion may also automatically occur to streamline the stabilizer and
elevator for thrust and configuration changes.
Primary Pitch Trim Control
Primary pitch trim is controlled by the dual pitch trim switches on each control
wheel. Both switches must be moved to command trim changes. The primary
pitch trim switches are inhibited when the autopilot is engaged. Pitch trim does not
move the control column.
In the normal mode, primary pitch trim operates differently on the ground than it
does in flight. On the ground, the stabilizer is directly positioned when the pilot
uses the pitch trim switches. In flight, the pitch trim switches do not position the
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
The trim reference speed is the speed at which the airplane would eventually
stabilize if there were no control column inputs. Once the control column forces
are trimmed to zero, the airplane maintains a constant speed with no column
inputs. Thrust changes result in a relatively constant indicated airspeed climb or
descent, with no trim inputs needed unless airspeed changes.
When pilot trim inputs are made, the PFCs automatically move the elevators to
achieve the trim change, then move the stabilizer to streamline the elevator.
Stabilizer motion may also automatically occur to streamline the stabilizer and
elevator for thrust and configuration changes.
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Having thought about it for a bit, could you say that the 777 acts like a non FBW aircraft eg. 737 for pitch/power coupling?
In other words, when you apply power in the 737 while straight and level & in trim, the pitch/power coupling of the under slung engines makes the nose rise, so if your flying straight and level you push the column forward and trim the force out.
In the 777 the aircraft behaves the same but for a different reasons (in that it raises the nose to maintain the trimmed speed) so you push forward as before to maintain level flight and trim that force out.
I'm just trying to get my head around the concept as I've been flying the Airbus for 16 years and will shortly be flying the 777.
In other words, when you apply power in the 737 while straight and level & in trim, the pitch/power coupling of the under slung engines makes the nose rise, so if your flying straight and level you push the column forward and trim the force out.
In the 777 the aircraft behaves the same but for a different reasons (in that it raises the nose to maintain the trimmed speed) so you push forward as before to maintain level flight and trim that force out.
I'm just trying to get my head around the concept as I've been flying the Airbus for 16 years and will shortly be flying the 777.
Having thought about it for a bit, could you say that the 777 acts like a non FBW aircraft eg. 737 for pitch/power coupling?
Think back to what used to be taught by some
as "straight and level two"( power + attitude + trim and all that) on a single piston/jet, the triple from a handling POV really is that straightforward, assuming the FBW is in normal mode.
Last edited by wiggy; 28th Dec 2014 at 09:39.
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FLY BY WIRE
in the Airbus if you only increase thrust you will fly straight and level but will speed up. The aircraft will compensate trim automatically for speed changes.
Airbus doesn't fly straight and level nor does it maintain pitch. Stick free it maintains 1g. With thrust increase it will enter into shallow climb trying to do that While with thrust reduction you will see pitch increasing.
in the Airbus if you only increase thrust you will fly straight and level but will speed up. The aircraft will compensate trim automatically for speed changes.
Airbus doesn't fly straight and level nor does it maintain pitch. Stick free it maintains 1g. With thrust increase it will enter into shallow climb trying to do that While with thrust reduction you will see pitch increasing.
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I've only been on the 777 for 5 years or so, with that in mind I think a detailed explanation misses the point. It's how it works in practise that is important, and of course how your SOP's fit into that.
Where I work we keep the A/T on the whole time, and I'm going to assume it's the same where you are.
The thing flies like a big C172, go faster and it wants to pitch up...so trim it out, slow down and it wants to pitch down...again with the AP disconnected..trim it out.
There, that explains "speed trim" or more correctly "trim for your current speed"
In practise I find It all rather intuative and it's one of the reasons I love the Jet, the cockpit seating on the other hand......
Where I work we keep the A/T on the whole time, and I'm going to assume it's the same where you are.
The thing flies like a big C172, go faster and it wants to pitch up...so trim it out, slow down and it wants to pitch down...again with the AP disconnected..trim it out.
There, that explains "speed trim" or more correctly "trim for your current speed"
In practise I find It all rather intuative and it's one of the reasons I love the Jet, the cockpit seating on the other hand......
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Or, to turn it on its head:
With the AP disconnected, if you climb at 250kts, then fly level at 250kts, then desend at 250kts, you shouldn't need to trim as you trim for speed changes, not pitch.
With the AP disconnected, if you climb at 250kts, then fly level at 250kts, then desend at 250kts, you shouldn't need to trim as you trim for speed changes, not pitch.
From B777Boeings
One interesting point that takes some getting used to is that it does not matter which way you select the pitch trim switches in the B777 for a speed change. Inflight all the trim switches do is tell the PFC of a need to change trim reference speed whether you trim forward or back makes no difference.
In flight, the pitch trim switches do not position the
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
In a conventional airplane you feel the pitch power coupling as soon as the power comes in. In a 777 this doesn't happen. In fact if you are going to climb at constant speed you don't trim at all you just pull back. If you are going to accelerate then you need to trim. About one click per 10 knots works. You just have to get it close to in trim as the plane makes the fine adjustment so there is no 1 or 2 clicks back and forth to get it perfect. If you understand this the plane is very easy te fly. Maybe even easier than an Airbus.
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B777 Speed Trim
Pitch trim switches on B777 cause the trim reference speed to ramp up or down (depending on direction of switch activation) at a rate of 10 knots per second. Trim reference speed is limited to the range of top of speed tape amber band (on the low end) to Vmo/Mmo (on the upper end). Reference speed limits form the first layer of B777 underspeed and overspeed protection functions. B787 pitch trim behaves identically.
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Late Comment to @haughtney1, But...
Sir,
I will not rant nor rave and my note is another question, not really an opinion...
Oh, yes, I get the SOPs and the 'usual practices' at you line and almost every other. Still, after 5+ years on the 777, with a TC, or not, left seat or right, I would hope that you understand the 777's logic, inside-out, including the 'why is it doing that,' stuff, almost as well as the software engineers do.
The idea that you never turn-off the A/T is perfectly OK if that is SOP, yet someplace I have to wonder, can you cope with one or two or three automation failures, recognize them for what are - or are not and drive that very smart airplane without them?
Again, sir, this is not a skills challenge to you or any other 777 driver (or one who drives the A330, ,a different thinking beast) but, was your training sufficient to thoroughly understand how the beast thinks - and command otherwise, if necessary. In your opinion, do the majority of 777 drivers understand the 777's operating software to a degree sufficient to hand fly it to safe landing if multiple, otherwise automated control functions are compromised? (In the case of the A330 and A320 series, I guess I'd ask the question twice.)
Again, sir, not a criticism! Do any of today's large airplane pilots have the operating systems knowledge to hand-direct, (stick and rudder if I must) the flight of transport jet built after ~~2000?
No flame here and no criticism, but questions to arise. I would like to hear from you and yes, PM is just fine.
On the bright side, a few professionals comment in this space and many monitor. I commend those who continue to learn. Transport-class flying is a challenging profession, or at least it should be.
Best wishes...
I will not rant nor rave and my note is another question, not really an opinion...
Oh, yes, I get the SOPs and the 'usual practices' at you line and almost every other. Still, after 5+ years on the 777, with a TC, or not, left seat or right, I would hope that you understand the 777's logic, inside-out, including the 'why is it doing that,' stuff, almost as well as the software engineers do.
The idea that you never turn-off the A/T is perfectly OK if that is SOP, yet someplace I have to wonder, can you cope with one or two or three automation failures, recognize them for what are - or are not and drive that very smart airplane without them?
Again, sir, this is not a skills challenge to you or any other 777 driver (or one who drives the A330, ,a different thinking beast) but, was your training sufficient to thoroughly understand how the beast thinks - and command otherwise, if necessary. In your opinion, do the majority of 777 drivers understand the 777's operating software to a degree sufficient to hand fly it to safe landing if multiple, otherwise automated control functions are compromised? (In the case of the A330 and A320 series, I guess I'd ask the question twice.)
Again, sir, not a criticism! Do any of today's large airplane pilots have the operating systems knowledge to hand-direct, (stick and rudder if I must) the flight of transport jet built after ~~2000?
No flame here and no criticism, but questions to arise. I would like to hear from you and yes, PM is just fine.
On the bright side, a few professionals comment in this space and many monitor. I commend those who continue to learn. Transport-class flying is a challenging profession, or at least it should be.
Best wishes...
Anybody who can't turn off all the automated features of any aircraft he is flying and safely land the plane has no business anywhere the cockpit of the plane. On every part 121 check ride I have ever seen you have to do it with an engine out.
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No Fly, let me answer your questions as best I can.
With respect to understanding the system as well as or almost as well as the software engineers, if anyone that has done a Boeing 777 TR in the last 10 years does have this level of knowledge....I'd be surprised. Put simply, the level of in depth knowledge provided has been significantly reduced, why? I guess because Boeing have decided that EICAS is there to solve and manage any abnormals, similarly the MEL provides relief and operational guidance for all those pesky unservicabilities (including A/throttles etc)
As to how the beast thinks? Well again put simply, I use the automation when required or compelled too and in such a way as to allow me to think for the aeroplane (which is how Boeing designed it to be).
With respect to understanding vs manipulation skill and hand flying, like I said, the aircraft flys just like a big C172....if you can understand those basic fundamentals, you can understand flying the 777.
I have an immense amount of confidence in the overall design philosophy as well as the flight control and system redundancy, it means that on the day when it all goes wrong I know that pitch and power will = performance.
With respect to understanding the system as well as or almost as well as the software engineers, if anyone that has done a Boeing 777 TR in the last 10 years does have this level of knowledge....I'd be surprised. Put simply, the level of in depth knowledge provided has been significantly reduced, why? I guess because Boeing have decided that EICAS is there to solve and manage any abnormals, similarly the MEL provides relief and operational guidance for all those pesky unservicabilities (including A/throttles etc)
As to how the beast thinks? Well again put simply, I use the automation when required or compelled too and in such a way as to allow me to think for the aeroplane (which is how Boeing designed it to be).
With respect to understanding vs manipulation skill and hand flying, like I said, the aircraft flys just like a big C172....if you can understand those basic fundamentals, you can understand flying the 777.
I have an immense amount of confidence in the overall design philosophy as well as the flight control and system redundancy, it means that on the day when it all goes wrong I know that pitch and power will = performance.
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Manual Flight should not compromise safety
Rick777 - when you refer above to turning off all of the automation am I correct to take that as autopilot, flight director, and autothrottles? Or are you also referring to selecting the lowest level of augmentation available (i.e., Direct Mode via the overhead PFS Disconnect switch on the B777)?
All - Disengaging the autopilot, flight director, and autothrottles should not result in comprimised safety. Without the autopilot altitude deviation during cruise will likely be greater and without the autothrottle speed control will not be as tight - those are given. The flight crew, however should be able to stay within expected ATC margins of assigned altitudes and speeds and should be able to adequately perform all phases of flight needed to safely arrive at their destination. Achieving this goal may require more experience flying manually at a wider range of flight conditions than is customary with today's routines.
All - Disengaging the autopilot, flight director, and autothrottles should not result in comprimised safety. Without the autopilot altitude deviation during cruise will likely be greater and without the autothrottle speed control will not be as tight - those are given. The flight crew, however should be able to stay within expected ATC margins of assigned altitudes and speeds and should be able to adequately perform all phases of flight needed to safely arrive at their destination. Achieving this goal may require more experience flying manually at a wider range of flight conditions than is customary with today's routines.
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Dear c100driver, not sure you actually fly the 777. But if you do, next time try taking the airplane from flaps up to about 300-320kts and see if you can trim it out by running the trim switches back instead of forward. You might be in for a small surprise!
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Calmcavok
Or, to turn it on its head:
With the AP disconnected, if you climb at 250kts, then fly level at 250kts, then desend at 250kts, you shouldn't need to trim as you trim for speed changes, not pitch.
Or, to turn it on its head:
With the AP disconnected, if you climb at 250kts, then fly level at 250kts, then desend at 250kts, you shouldn't need to trim as you trim for speed changes, not pitch.
From climb to level-off, the thrust reduction on the under-slung engines creates an exaggerated nose down moment, requiring nose up trim to maintain level flight and airspeed.
Likewise, a big thrust increase pushes the nose way up, requiring nose down trim to maintain speed in the climb.
