A/T: is it really helping you ?
A/T: is it really helping you ?
Hello,
What follows is more oriented towards engineering, airplane flying qualities, rather than learning how to fly.
Any qualified pilot is able to deal an A/T especially if the A/T is available on their type. But does the A/T make the job easier, or harder ?
This topic is more related to knowing if airbus or boeing airplanes are easier to fly.
We're going to talk about airplanes longitudinal aerodynamic modes.
As you may know, you have two main modes in longitudinal aerodynamics. The phugoid and the AOA oscillation (or whatever its name in english)
The latter is less know since it's more difficult to observe. It's highly damped.
I'm no expert at certification requirements but quite obviously, an airplane with a diverging AOA oscillation would not be flyable by a human hence would not be certified.
The following aims at showing how the A/T can modify the aerodynamic modes of an airplane. The phugoid.
Let's suppose you have a completely conventional airplane. A piper PA28.
You have a trim, you do not have an A/T. Suppose you're in stabilized flight. You trim down the airplane. You do not touch the controls. What happens ?
The airplane keeps its power, accelerates, descends, goes over the new equilibrium trim speed (let's call that trim reference speed or TRS), then climbs back.. It's a phugoid. Eventually it will stabilize in a slow descent, with a descent angle that gives just enough "gravity thrust" to help the current thrust reach the new TRS speed.
Now let's suppose you have an A/T engaged.
You have a speed bug. At first you stabilize the airplane in level flight. Perfectly trimmed. So TRS = speed bug. Let's create a gap between the TRS and the speed bug. You can trim down, but since not many planes show the TRS, let's reduce the speed bug, this will allow us to make a small reduction that we can measure. Let's say a few knots. 5 knots. How much time have you got until you're in deep trouble ?
At first, almost nothing happens, and the instructor in the sim wonders why you asked to do this weird exercise.
There is only a 5kt difference so the A/T slowly reduces thrust, the speed reduces slowly too, it reduces even more slowly because the airplane is trimmed to keep its current speed at TRS. But the A/T continues to see a speed difference and continues to reduce thrust. Soon, the airplane behavior becomes exponential.
In a time that I could not evaluate in all configurations, and that is quicker if the speed difference is higher, but in any case much quicker than you would imagine, your airplane is stabilized in a descent with idle thrust. If you were on approach, in less than 30 seconds you end up with idle thrust and terrain warning to pull up.
The same goes for the other way around. If you increased the speed bug, the airplane would soon enter a climb with max power.
You have a hidden speed bug, the trim reference speed, and it must be in sync with the A/T, displayed speed bug.
Imagine they're in sync but you encounter a wiind gust, your speed reduces. The A/T will increase thrust, the trim will pitch down, both work to stabilize the airplane.
But if you're just 1kt off, the airplane will continuously want to enter an exponential dive to idle descent or climb to max power.
On an airbus, the hidden speed bug moves automatically with your speed. So you can have a difference between speed bug and current speed, but it won't cause any problem of a similar nature. The airbus is purely stable.
The boeing FBW have a similar fly by wire law, except that the introduction of the trim speed changes everything.
However, this issue is entirely manageable, of course. It's called flying the plane. It's just a bit more difficult, you cannot spend extended periods of time looking somewhere else if your plane is not in perfect trim.
Hence, you can find it easier to fly a trimmable airplane without the A/T.
Does anybody on a trimmable airplane like to fly approaches without A/T ? I hear most pilots at my airline almost always keep it on.
What follows is more oriented towards engineering, airplane flying qualities, rather than learning how to fly.
Any qualified pilot is able to deal an A/T especially if the A/T is available on their type. But does the A/T make the job easier, or harder ?
This topic is more related to knowing if airbus or boeing airplanes are easier to fly.
We're going to talk about airplanes longitudinal aerodynamic modes.
As you may know, you have two main modes in longitudinal aerodynamics. The phugoid and the AOA oscillation (or whatever its name in english)
The latter is less know since it's more difficult to observe. It's highly damped.
I'm no expert at certification requirements but quite obviously, an airplane with a diverging AOA oscillation would not be flyable by a human hence would not be certified.
The following aims at showing how the A/T can modify the aerodynamic modes of an airplane. The phugoid.
Let's suppose you have a completely conventional airplane. A piper PA28.
You have a trim, you do not have an A/T. Suppose you're in stabilized flight. You trim down the airplane. You do not touch the controls. What happens ?
The airplane keeps its power, accelerates, descends, goes over the new equilibrium trim speed (let's call that trim reference speed or TRS), then climbs back.. It's a phugoid. Eventually it will stabilize in a slow descent, with a descent angle that gives just enough "gravity thrust" to help the current thrust reach the new TRS speed.
Now let's suppose you have an A/T engaged.
You have a speed bug. At first you stabilize the airplane in level flight. Perfectly trimmed. So TRS = speed bug. Let's create a gap between the TRS and the speed bug. You can trim down, but since not many planes show the TRS, let's reduce the speed bug, this will allow us to make a small reduction that we can measure. Let's say a few knots. 5 knots. How much time have you got until you're in deep trouble ?
At first, almost nothing happens, and the instructor in the sim wonders why you asked to do this weird exercise.
There is only a 5kt difference so the A/T slowly reduces thrust, the speed reduces slowly too, it reduces even more slowly because the airplane is trimmed to keep its current speed at TRS. But the A/T continues to see a speed difference and continues to reduce thrust. Soon, the airplane behavior becomes exponential.
In a time that I could not evaluate in all configurations, and that is quicker if the speed difference is higher, but in any case much quicker than you would imagine, your airplane is stabilized in a descent with idle thrust. If you were on approach, in less than 30 seconds you end up with idle thrust and terrain warning to pull up.
The same goes for the other way around. If you increased the speed bug, the airplane would soon enter a climb with max power.
You have a hidden speed bug, the trim reference speed, and it must be in sync with the A/T, displayed speed bug.
Imagine they're in sync but you encounter a wiind gust, your speed reduces. The A/T will increase thrust, the trim will pitch down, both work to stabilize the airplane.
But if you're just 1kt off, the airplane will continuously want to enter an exponential dive to idle descent or climb to max power.
On an airbus, the hidden speed bug moves automatically with your speed. So you can have a difference between speed bug and current speed, but it won't cause any problem of a similar nature. The airbus is purely stable.
The boeing FBW have a similar fly by wire law, except that the introduction of the trim speed changes everything.
However, this issue is entirely manageable, of course. It's called flying the plane. It's just a bit more difficult, you cannot spend extended periods of time looking somewhere else if your plane is not in perfect trim.
Hence, you can find it easier to fly a trimmable airplane without the A/T.
Does anybody on a trimmable airplane like to fly approaches without A/T ? I hear most pilots at my airline almost always keep it on.
On a pa28 you won't have a phugoid, you'll have several sinuidoal ups and downs that will eventually stabilise towards whatever trim you put.
Not to mention airbus that will stabilise by itself so yeah, you're basically inducing everything to prove your "theory" right which is not.
Not to mention airbus that will stabilise by itself so yeah, you're basically inducing everything to prove your "theory" right which is not.
Originally Posted by Cividasku
If you were on approach, in less than 30 seconds you end up with idle thrust and terrain warning to pull up.
Imagine they're in sync but you encounter a wiind gust, your speed reduces. The A/T will increase thrust, the trim will pitch down, both work to stabilize the airplane.
But if you're just 1kt off, the airplane will continuously want to enter an exponential dive to idle descent or climb to max power.
But if you're just 1kt off, the airplane will continuously want to enter an exponential dive to idle descent or climb to max power.
My last jet, the 717, was a classic example of this. Slack glidepath control, allowing the nose to wander, resulted in the ATS staying up when it should have ben coming back, and staying back when it should have been coming up. However, with tight slope control eg if you get hit on the face with a gust and you balloon, you stay on the slope with a push (ie don't let the nose come up) and the instant you start to apply forward pressure the ATS says OK! and pulls the power off. The speed then comes back to the TRS/bug speed. But all the while, I have actively stayed on the desired approach slope.
I always used the ATS. It was a fantastic system in the 717.
Jets should/can't be flown like Pipers on final. You don't pull the power off to make the jet down down by relying on the secondary effects of controls. You push the nose down, then adjust the power if you need to (or let the ATS do it for you).
You're obviously an FO: "Always remember and forever take heed, right hand for glidepath and left hand for speed!". Or, if you have an ATS, it can do the left-hand bit for you.
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However, this issue is entirely manageable, of course. It's called flying the plane. It's just a bit more difficult, you cannot spend extended periods of time looking somewhere else if your plane is not in perfect trim.
TBH this is a feature on the 777/787 not a bug (C*U). The aircraft (through the yoke, thrust levers, PFD and/or view out of the window) is telling you that you are not at the speed you trimmed for and to pay a bit more attention. If you are on approach near the ground and it becomes nose heavy you know before you scan the speed that it has dropped, plus the AT will help you by increasing thrust to compensate. It’s behaving dynamically much as a non-FBW aircraft would under these circumstances, so allowing it to feel more natural, save the lack of pitch/power couple. Control the path with the elevator, control the speed with power (AT does this) and keep it trimmed.
When you fly the actual aeroplane, it makes more sense.
If you are on approach near the ground and it becomes nose heavy you know before you scan the speed that it has dropped, plus the AT will help you by increasing thrust to compensate. It’s behaving dynamically much as a non-FBW aircraft would under these circumstances, so allowing it to feel more natural, save the lack of pitch/power couple. Control the path with the elevator, control the speed with power (AT does this) and keep it trimmed.
It behaves as a non-FBW aircraft with A/T would, but not the same as a PA28.
If you have your TRS above your speed bug (that is, being nose heavy) the A/T won't help, the A/T will progressively retard the levers and crash the airplane if you do nothing.
Without an A/T, if the trim speed is above the desired speed, and the power is adequate for the desired speed, the aircraft will descend slightly faster than desired. It won't retard the levers since it can't.
And your flight path will be much closer than desired.
The A/T will only help if the airplane is perfectly in trim.
We usually don't do this type of maneuver during sim sessions, they're not line pilot maneuvers, they're engineers or test pilots maneuvers. Doing it just once was extremely enlightening.
You're making the assumption that AT only controls speed. In a Boeing at least, the AFDS is a very complex system with a multitude of modes, many of which rarely get used.
For most of the descent the thrust should be at idle, with the speed controlled by elevator. In the final stages of the approach other modes may come into play such as ALT HOLD, V/S, FLCH, SPD PATH (both geometric and approach) etc... but that all happens as part of an integrated system with AP or FD. In a Boeing, when you turn off the AP you turn off the AT as well and, you know... fly the airplane.
For most of the descent the thrust should be at idle, with the speed controlled by elevator. In the final stages of the approach other modes may come into play such as ALT HOLD, V/S, FLCH, SPD PATH (both geometric and approach) etc... but that all happens as part of an integrated system with AP or FD. In a Boeing, when you turn off the AP you turn off the AT as well and, you know... fly the airplane.
If you have your TRS above your speed bug (that is, being nose heavy) the A/T won't help, the A/T will progressively retard the levers and crash the airplane if you do nothing.
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We usually don't do this type of maneuver during sim sessions, they're not line pilot maneuvers, they're engineers or test pilots maneuvers. Doing it just once was extremely enlightening.
...
We usually don't do this type of maneuver during sim sessions, they're not line pilot maneuvers, they're engineers or test pilots maneuvers. Doing it just once was extremely enlightening.
Pilots are trained on the basis of target attitudes and power. The change of aircraft attitude will tell the pilot something is off, and you will naturally want to bring the nose up. It's a very natural reaction. Any pilot will ask questions if on a glide the nose passes through the horizon. He will raise the nose which is a natural "request" for pitch up trim, which reduces the TRS. At least it should be for every pilot. That's why I hope you're not a pilot...
The Airbus FBW on the other hand is another peace of unnatural response, as it is tries to maintain a path. So with speed reduction on the A/T, the nose will rise, and many starting pilots will feel this is unnatural pitch behaviour of the aircraft.
You're making the assumption that AT only controls speed. In a Boeing at least, the AFDS is a very complex system with a multitude of modes, many of which rarely get used.
For most of the descent the thrust should be at idle, with the speed controlled by elevator. In the final stages of the approach other modes may come into play such as ALT HOLD, V/S, FLCH, SPD PATH (both geometric and approach) etc... but that all happens as part of an integrated system with AP or FD. In a Boeing, when you turn off the AP you turn off the AT as well and, you know... fly the airplane.
For most of the descent the thrust should be at idle, with the speed controlled by elevator. In the final stages of the approach other modes may come into play such as ALT HOLD, V/S, FLCH, SPD PATH (both geometric and approach) etc... but that all happens as part of an integrated system with AP or FD. In a Boeing, when you turn off the AP you turn off the AT as well and, you know... fly the airplane.
At my airline, most of the time the plane in manually flown with the A/T still engaged.
If I understand correctly, the A/T will reduce thrust, however the pitch of the aircraft will move down as well as the pitch channel is chasing a higher reference speed.
Pilots are trained on the basis of target attitudes and power. The change of aircraft attitude will tell the pilot something is off, and you will naturally want to bring the nose up. It's a very natural reaction. Any pilot will ask questions if on a glide the nose passes through the horizon. He will raise the nose which is a natural "request" for pitch up trim, which reduces the TRS. At least it should be for every pilot. That's why I hope you're not a pilot...
Pilots are trained on the basis of target attitudes and power. The change of aircraft attitude will tell the pilot something is off, and you will naturally want to bring the nose up. It's a very natural reaction. Any pilot will ask questions if on a glide the nose passes through the horizon. He will raise the nose which is a natural "request" for pitch up trim, which reduces the TRS. At least it should be for every pilot. That's why I hope you're not a pilot...
But being able to do so doesn't say if it easy or easier than another airplane.
Your last sentence shows that you didn't understand the goal of my post.
Depending on the airplane characteristics, it can be easier or trickier to fly. The manufacturer usually tries to make the airplane as easy as possible. At least in my opinion they should.
If you can look at something else for an extended period of time, and the airplane hasn't changed it's trajectory, it's easy to fly. It means that if you pay reasonable attention you will achieve a very precise flight path.
If the airplane has an instability that can develop exponentially with time, it means both that if you look at something else for an "extended" period of time (it can be as short as 10 seconds) the airplane can already be in a very upset situation, and that you need to pay close attention to the airplane to correct this unstability everytime it appears.
Airplane manufacturers during flight test have to demonstrate the aircraft behavior with the controls released, to observe the different modes of the airplane. Dutch roll, spiral, phugoid, aoa oscillation... There are requirements about the stability of the airplane's behavior.
What guarantee do you have that the trim reference speed is exactly at the speed bug ? As little as 5 kt can create an instability. What can guarantee that if you want to trim for a speed bug at say 150kt, you're not going to oscillate the trim speed between 151 and 149kt each time you attempt to match closely the speed bug ?
Apparently some of the instructors at my airline have a supplementary document, not included in the manuals, that describes more precisely the trim system on the B777. Apparently it will say that the trim should match the speed bug when given a order consistent with doing so.
That's exactly the kind of information that I'm looking for.
At my airline, most of the time the plane in manually flown with the A/T still engaged.
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Your last sentence shows that you didn't understand the goal of my post.
Depending on the airplane characteristics, it can be easier or trickier to fly. The manufacturer usually tries to make the airplane as easy as possible. At least in my opinion they should.
At my airline, most of the time the plane in manually flown with the A/T still engaged.
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Your last sentence shows that you didn't understand the goal of my post.
Depending on the airplane characteristics, it can be easier or trickier to fly. The manufacturer usually tries to make the airplane as easy as possible. At least in my opinion they should.
2) yes I do understand your question, but it is not a question of "easy" or not. It is a question of "natural" or not, because with natural behaviour there will be natural pilot response. I also don't understand your questioning when talking about "easyness" and then only consider 1 single item in stability. You open the door of stability, there is a lot more to discuss. The reality for the pilot is that it is a natural behaviour. The nose will drop in a "seems natural direction" way, in a "seems natural slow/fast response", and hence we will react and support it (if you want to include stability, then all it a "perceived stability"?). There is also a reason why there is a PF and a PM. The PF is not supposed to look anywhere else for 10 seconds, especially not when he's reducing the speed. Why would he look anywhere else? This is basic piloting skills. Hence the question if you're a pilot or an engineer.
I'm a pilot (trainer) and an engineer in aviation. It's interesting for the instructors to have all the background information, it's even more interesting to be able to show it. But it doesn't change anything. When you go into the simulator and demo the behaviour, a behaviour that is considered "natural" whatever is causing it, it requires a natural response of the pilot. So yes, A/T ON is easier on a Boeing FBW as it reduces workload and reduces the chance of overstimulation in case of problems.
As far as the answer to your "hard/easy" question: it is more than easy enough as it doesn't require thinking. It is a trained skill that doesn't need adaptation. It's simply more relax.
Last edited by BraceBrace; 20th May 2024 at 11:59.
1) we are talking about fly-by-wire systems. Boeing advises the use of A/T with A/P off on FBW aircraft. The statement "A/T OFF" is incorrect and applies to none FBW Boeing. On a FBW Boeing you are allowed to fly A/T off, but this is mainly done for proficiency and preferably in good weather conditions and otherwise standard operations (ie standard ILS approach into known airport).
I don't understand what you typed here. Which statement is incorrect ?
You mean having the A/T OFF should not be done, or a statement that I made regarding what happens with A/T OFF is incorrect ? If so, which one ?
In some fatal accidents, pilots have been seen not to look at their displays for "extended" period of times.
Eastern 401 is the most prominent example. Much more recently, prime air/atlas air 3591, one was disoriented, the other one wasn't properly monitoring.
Another very good example is Turkish 1951. The A/T retarded and it wasn't noticed until too late. All of these are proof that pilots are fallible at maintaining a perfect constant monitoring of flight parameters.
Hence, a stable aircraft is a better idea than one that you have to keep a very close eye on.
In this debate you could simplify or caricature my position as one of the following :
- advocating for airbus law without the speed in the FBW law
- advocating for an easier trim. Display the trim reference speed (I'm told that the A220 displays it, which I couldn't verify), or have a way to synchronise the trim with the speed, or reduce the stick force to make the unstable behavior slower...
Then, easy or easier is subjective.
Imagine you're flying the airplane on final, you know the power setting for this phase of flight. Usually around 55% on most jets. Imagine you perfectly matched your speed bug and your TRS.
Your plane should be stable, right ? Still not !
If you pitch down ever so slightly, the A/T will reduce the thrust to keep the speed. You will lose energy. When you notice the glide error (be it small or large), the thrust will increase to help you climb back to the glide, then decrease again. All of it is destabilizing.
If you flew A/T OFF, with the correct power setting, if you pitch down slightly, now your airplane will accelerate. Hence you now have a second cue on your PFD that you're low : the glide and the IAS. You can just pitch up, you will trade speed for altitude, and you will be back on the glide at the correct speed, without thrust variation.
Some people will find the first method easier, because they focus on one cue and don't have to worry about knowing power settings. Other ones will prefer the second method.
This is morphing into jets should be flown like PA28s.
No unfortunately it doesn't.
It behaves as a non-FBW aircraft with A/T would, but not the same as a PA28.
If you have your TRS above your speed bug (that is, being nose heavy) the A/T won't help, the A/T will progressively retard the levers and crash the airplane if you do nothing.
Without an A/T, if the trim speed is above the desired speed, and the power is adequate for the desired speed, the aircraft will descend slightly faster than desired. It won't retard the levers since it can't.
And your flight path will be much closer than desired.
The A/T will only help if the airplane is perfectly in trim.
It behaves as a non-FBW aircraft with A/T would, but not the same as a PA28.
If you have your TRS above your speed bug (that is, being nose heavy) the A/T won't help, the A/T will progressively retard the levers and crash the airplane if you do nothing.
Without an A/T, if the trim speed is above the desired speed, and the power is adequate for the desired speed, the aircraft will descend slightly faster than desired. It won't retard the levers since it can't.
And your flight path will be much closer than desired.
The A/T will only help if the airplane is perfectly in trim.
It’s akin to saying “I landed in a crosswind with my feet not on the rudder and we went off the side of the runway!” Not optimal but not surprising.
Boeing would say: ”Fly the Airplane”.
I really wonder why you would think I don't intend flying the plane.
With the same reasoning, we would not train for stall, approach to stall, or GPWS maneuvers in the sim.
This maneuver (being out of trim with or without A/T) is not a way of flying but a way of observing the airplane behavior.
Much like the manufacturer will make maneuvers without control inputs to observe dutch roll, phugoid, spiral...
It's a way of understanding the airplane behavior, and I think it could be used to demonstrate and prove how important being in perfect trim is.
On the long term, thrust will control flight path and trim will control speed.
With the A/T ON the flight path loses all of its long term stability, and that's worth noting, in my opinion.
When we tried that in the sim, the instructor had a similar reaction to the ones here. "Just fly the plane". My sim partner however, who was previously an engineer, was amazed with the speed at which events unfolded.
By the way, it's not easy to predict that the A/T will retard all the way to idle. A different speed tracking law, for example proportional, could not retard to idle, or increase to TOGA, for a small speed difference. This experiment hence proves that there is an integral component in the A/T speed tracking law. That is, the longer the plane senses a difference in speed, the bigger the thrust variation.
With the same reasoning, we would not train for stall, approach to stall, or GPWS maneuvers in the sim.
This maneuver (being out of trim with or without A/T) is not a way of flying but a way of observing the airplane behavior.
Much like the manufacturer will make maneuvers without control inputs to observe dutch roll, phugoid, spiral...
It's a way of understanding the airplane behavior, and I think it could be used to demonstrate and prove how important being in perfect trim is.
On the long term, thrust will control flight path and trim will control speed.
With the A/T ON the flight path loses all of its long term stability, and that's worth noting, in my opinion.
When we tried that in the sim, the instructor had a similar reaction to the ones here. "Just fly the plane". My sim partner however, who was previously an engineer, was amazed with the speed at which events unfolded.
By the way, it's not easy to predict that the A/T will retard all the way to idle. A different speed tracking law, for example proportional, could not retard to idle, or increase to TOGA, for a small speed difference. This experiment hence proves that there is an integral component in the A/T speed tracking law. That is, the longer the plane senses a difference in speed, the bigger the thrust variation.
This maneuver (being out of trim with or without A/T) is not a way of flying but a way of observing the airplane behavior.
Much like the manufacturer will make maneuvers without control inputs to observe dutch roll, phugoid, spiral...
It's a way of understanding the airplane behavior, and I think it could be used to demonstrate and prove how important being in perfect trim is.
Much like the manufacturer will make maneuvers without control inputs to observe dutch roll, phugoid, spiral...
It's a way of understanding the airplane behavior, and I think it could be used to demonstrate and prove how important being in perfect trim is.
Reality is that you will very rarely have this situation as "identifyable" as you will be flying through changing disturbances all the time and it becomes hard to tell what causes what when the changes in pitch _initially_ are small. From a pilots point of view there is no point in trying to make a distinction. A pilot will make a correction.
You ask the question if A/T ON is really helping us by pointing out a "problem". I'm saying the issue isn't making our life worse, and A/T ON has a lot more advantages than the subtle disadvantage you are trying to focus on here, a disadvantage that usually ends up "unidentifyable" in the "noise" of other natural inputs from a changing atmosphere outside of the world. If we reduce speed we know how to trim at the same time. My 777 time has been a while, but I think the value is 2 sec trim per 10kts speed change? How easy do you need?
Your conclusion here of "how important being in perfect trim is" has always been true. It's a bit strange to then compare to a system that trims itself completely. The reality is that many pilots transferring to Airbus fly-by-wire need more "thinking" as some aircraft reactions are contradictory to what is experienced as "natural". On a Boeing fly-by-wire, the reaction is natural.
To explain the "incorrect statement": on any non-fly-by-wire Boeing, Boeing will always recommend to fly with A/T OFF, when the A/P is off because of the pitch-power coupling (leaving the A/T ON will create unexpected pitch changes with power changes). However, on fly-by-wire Boeing, Boeing will recommend to fly A/T ON with A/P OFF as well, as the fly-by-wire will correct for pitch-power coupling (and pitch changes due to config changes).
Last edited by BraceBrace; 20th May 2024 at 20:56.
This is what the phugoid is.
If we reduce speed we know how to trim at the same time. My 777 time has been a while, but I think the value is 2 sec trim per 10kts speed change? How easy do you need?
To explain the "incorrect statement": on any non-fly-by-wire Boeing, Boeing will always recommend to fly with A/T OFF, when the A/P is off because of the pitch-power coupling (leaving the A/T ON will create unexpected pitch changes with power changes). However, on fly-by-wire Boeing, Boeing will recommend to fly A/T ON with A/P OFF as well, as the fly-by-wire will correct for pitch-power coupling (and pitch changes due to config changes).
To explain the "incorrect statement": on any non-fly-by-wire Boeing, Boeing will always recommend to fly with A/T OFF, when the A/P is off because of the pitch-power coupling (leaving the A/T ON will create unexpected pitch changes with power changes). However, on fly-by-wire Boeing, Boeing will recommend to fly A/T ON with A/P OFF as well, as the fly-by-wire will correct for pitch-power coupling (and pitch changes due to config changes).
So, if you had a speed trend of 50kt up, you would need to trim down continuously, and if you had a speed trend of 5kt up, you have to trim one second every ten seconds ?
Interesting but it's not something that you can count on.
As for the other part I quoted, I never said boeing recommended something or something else. I doubt they would recommend looking at the airplane without touching the controls, anyway.
777/787 has a blip trim system that will set the speed term in the C*U law to your current speed if within 5 kts, so you just need to flick the trim switches every few seconds and you'll stay in trim.
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