A/T: is it really helping you ?
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From: Planet Earth
To repeat the point so you really understand, not to show off: it is possible to trim a 777 sim from manual level flight onto the glide, and fly the ILS hands off to the runway. Autothrottle on, raw data manual ILS. I have done it. The feedback is precise enough to know when pitch is in trim.
It is almost impossible in reality because the atmosphere is not like a simulator.
To get back to the original point: You fly data and think the world is like this sterile unchangeable data. It is not. Pilots are trained to set data as a starting point but continuously adapt to the reality which is not simulatorlike but full of little outside disturbances.
Your original question turns out to be a quest for confirmation that A/T is making our life more difficult. It is the opposite. Because reality is not a simulator using datapoints. Reality is everything from subtle to big sudden changes, where you need to have a hand on the throttle and a hand on the stick and use the feedback provided. Your ’stability problem’ is small compared to the real world disturbances but you don’t seem to understand that as you’ve never been at the controls of a calm wind ILS, let alone a gusty one. Your problem literally disappears into the wind if I may say so. With all due respect, so would your speed if you decided to disconnect the A/T because you think it is easier. The A/T does a fantastic job at it. It doesn’t even need additional wind corrections which makes life over the threshold a lot easier...
If you can’t accept that reality the solution for you is easy: wait for the first A350 that can takeoff with the autopilot and land without a single disconnect. Problem solved.
It is almost impossible in reality because the atmosphere is not like a simulator.
To get back to the original point: You fly data and think the world is like this sterile unchangeable data. It is not. Pilots are trained to set data as a starting point but continuously adapt to the reality which is not simulatorlike but full of little outside disturbances.
Your original question turns out to be a quest for confirmation that A/T is making our life more difficult. It is the opposite. Because reality is not a simulator using datapoints. Reality is everything from subtle to big sudden changes, where you need to have a hand on the throttle and a hand on the stick and use the feedback provided. Your ’stability problem’ is small compared to the real world disturbances but you don’t seem to understand that as you’ve never been at the controls of a calm wind ILS, let alone a gusty one. Your problem literally disappears into the wind if I may say so. With all due respect, so would your speed if you decided to disconnect the A/T because you think it is easier. The A/T does a fantastic job at it. It doesn’t even need additional wind corrections which makes life over the threshold a lot easier...
If you can’t accept that reality the solution for you is easy: wait for the first A350 that can takeoff with the autopilot and land without a single disconnect. Problem solved.
That was demonstrated some time ago
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From: FL390
Especially during flare, there was a risk that the plane would add thrust and cause a long landing if you were a tiny bit late to retard. Also, it means that you cannot really keep your power at 50% for longer if you need to (for example, on the 318, you need to)
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From: Blue sky
AF447 is an excellent example: you can debate a lot about the technical background of it and FBW laws and logics and causes and effects... if the pilots had realised that at high altitude you can’t ”ask” more than 5 nose up, everybody would have been alive today. And hence that is what we focus on, and every tool that allows us to focus on those basics makes our life easier. So yes, A/T on helps a lot because it allows us to ”monitor” speed rather than ”control” it. If we sense the A/T is not doing what it is suppposed to, we will disconnect it.
Which is what I’ve been trying to say. Remarks like ”what if he wasn’t watching..." is a desk people remark who don’t know how to prioritize in a dynamic environment. A pilot flying manual is not allowed to do that. If he needs something, he has a PM. Because the outside world changes every second. Solving your ’minor’ issue you call a stability problem doesn’t make real flying ”sterile”. He still needs to monitor his instruments. Because we also HAVE to monitor CORRECT behaviour. You can’t see something is wrong if you don’t know what is right.
As a company you might want to create awareness, and include a demo in your initial training program, however the problem is resolved by good piloting practice. It falls under the same category of ”awareness” as the limited pitch authority of the elevator compared to THS on a 737. And on a 737 that danger is much bigger than the ”stability” issue with the 777 FBW (something many people claim to be also a stability problem, but it is not). The limited elevator authority in low speed TOGA situations has already caused crashes (RAM Amsterdam), and still no training program explains it. It only explains how to resolve the issue worst case (stall recovery procedure changed).
Last edited by BraceBrace; 18th June 2024 at 08:17.
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CV; if you are flying Airbus FBW and the auto-thrust adds thrust during the flare, it is because you were late retarding the thrust, because you flared incorrectly. We have all made this mistake from time to time and we learn to do it right !
My point about G/S Mini is that many pilots do not understand how it works and how it will correctly react with gusts. Even worse, some pilots want to take it out of circuit and control thrust and speed themselves, while still not understanding what G/S Mini does.
Conventional aircraft have to use a constant IAS strategy because when IAS changes, so does the wing lift and pitch trim. So without FBW, anything other than constant IAS will destabilise the aircraft.
Airbus designed a fantastic aircraft which incorporated fly-by-wire. Now they could look again at speed and stability during finals, and because the FBW will automatically compensate for the pitch power couple and for differing IAS, they could use a system based on energy instead of IAS.
Conventional constant IAS systems add a speed increment to keep the aircraft safe when a headwind gust disappears. The constant IAS system takes energy away during a headwind gust. Airbus adds thrust during a headwind gust which increases its energy.
When the headwind gust disappears, the constant IAS system energy drops to below where it was and the engines need to quickly spool up to regain the lost energy. The Airbus energy, on the other hand, only drops to what it was before the headwind gust occurred so the engines only need to spool down to where they were before. So the Airbus G/S Mini system never loses energy whereas the constant IAS system does.
The constant IAS system is not dangerous because a speed increment is added, but the Airbus system is better and is made possible owing to its FBW.
The real danger occurs when pilots override the Airbus GS Mini strategy without adding a safety margin of increased minimum IAS. So by controlling thrust themselves and ignoring the GS Mini, they actually remove the Airbus safety margins, and this could put them in a precarious situation.
My point about G/S Mini is that many pilots do not understand how it works and how it will correctly react with gusts. Even worse, some pilots want to take it out of circuit and control thrust and speed themselves, while still not understanding what G/S Mini does.
Conventional aircraft have to use a constant IAS strategy because when IAS changes, so does the wing lift and pitch trim. So without FBW, anything other than constant IAS will destabilise the aircraft.
Airbus designed a fantastic aircraft which incorporated fly-by-wire. Now they could look again at speed and stability during finals, and because the FBW will automatically compensate for the pitch power couple and for differing IAS, they could use a system based on energy instead of IAS.
Conventional constant IAS systems add a speed increment to keep the aircraft safe when a headwind gust disappears. The constant IAS system takes energy away during a headwind gust. Airbus adds thrust during a headwind gust which increases its energy.
When the headwind gust disappears, the constant IAS system energy drops to below where it was and the engines need to quickly spool up to regain the lost energy. The Airbus energy, on the other hand, only drops to what it was before the headwind gust occurred so the engines only need to spool down to where they were before. So the Airbus G/S Mini system never loses energy whereas the constant IAS system does.
The constant IAS system is not dangerous because a speed increment is added, but the Airbus system is better and is made possible owing to its FBW.
The real danger occurs when pilots override the Airbus GS Mini strategy without adding a safety margin of increased minimum IAS. So by controlling thrust themselves and ignoring the GS Mini, they actually remove the Airbus safety margins, and this could put them in a precarious situation.
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From: Seat 1A
Originally Posted by Uplinker
Conventional constant IAS systems add a speed increment to keep the aircraft safe when a headwind gust disappears. The constant IAS system takes energy away during a headwind gust. Airbus adds thrust during a headwind gust which increases its energy.
The constant IAS system is not dangerous because a speed increment is added, but the Airbus system is better and is made possible owing to its FBW.
Interesting comments above re throttles in the flare; doesn't sound very user friendly to me.
And for the avoidance of doubt, the ATS in the 717 is excellent and a great reliever of workload and Boeing procedure is to use it at all times.
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From: Blue sky
Just 2 remarks:
1) The idle thrust during the flare works differently. On an Airbus, there is a retard callout and the PF puts the thrust lever to idle. I'm not familiar with this functionality in flight. on a Boeing A/T system, the system is designed to be very aggressive below Vref, it reacts a lot slower when speed increases above Vref. The retard function is automatic, the PF does not have to do anything. Hence the regular speed correction for headwinds and gusts is left out. Works pretty good, only once in 6 years I had to disconnect.
2) Concerning G/S mini
Again, not an Airbus driver, only studied Airbus systems 3 decades ago and kinda "read about it" once in a while. But when I read about a system like "g/s mini", a detail that I think is often overlooked is that it is looked at "independently" from the Airbus and sold as a "safety enhancement". From what I know, I call it a safety necessity.
Airbus pitch control is path related. So if there is a headwind gust, the FBW will pitch the nose down without pilot input. This will not happen on a Boeing, on a Boeing FBW, the aircraft will pitch up and will want to climb. If the headwind gust is gone, on an Airbus, the nose will pitch up as the aircraft "sinks away" from the path. If you want to talk about stability... well I guess you then have to call this unstable behaviour. This is dangerous as the aircraft loses speed and pitches up at the same time. So you need extra protection on the speed side of things.
It's a bit funny to see this discussion surface in a discussion that tackles the "stability" of Boeing FBW. A Boeing FBW that is designed to push the nose down when the speed decreases... (or "unstable" in your words, but I've already understood we don't need to make a distinction between static stability, dynamic stability, control stability,...)
Then again, 3 decades of +FL300 probably degraded my grey cells as well... ;-)
1) The idle thrust during the flare works differently. On an Airbus, there is a retard callout and the PF puts the thrust lever to idle. I'm not familiar with this functionality in flight. on a Boeing A/T system, the system is designed to be very aggressive below Vref, it reacts a lot slower when speed increases above Vref. The retard function is automatic, the PF does not have to do anything. Hence the regular speed correction for headwinds and gusts is left out. Works pretty good, only once in 6 years I had to disconnect.
2) Concerning G/S mini
Again, not an Airbus driver, only studied Airbus systems 3 decades ago and kinda "read about it" once in a while. But when I read about a system like "g/s mini", a detail that I think is often overlooked is that it is looked at "independently" from the Airbus and sold as a "safety enhancement". From what I know, I call it a safety necessity.
Airbus pitch control is path related. So if there is a headwind gust, the FBW will pitch the nose down without pilot input. This will not happen on a Boeing, on a Boeing FBW, the aircraft will pitch up and will want to climb. If the headwind gust is gone, on an Airbus, the nose will pitch up as the aircraft "sinks away" from the path. If you want to talk about stability... well I guess you then have to call this unstable behaviour. This is dangerous as the aircraft loses speed and pitches up at the same time. So you need extra protection on the speed side of things.
It's a bit funny to see this discussion surface in a discussion that tackles the "stability" of Boeing FBW. A Boeing FBW that is designed to push the nose down when the speed decreases... (or "unstable" in your words, but I've already understood we don't need to make a distinction between static stability, dynamic stability, control stability,...)
Then again, 3 decades of +FL300 probably degraded my grey cells as well... ;-)
Last edited by BraceBrace; 18th June 2024 at 15:25.
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Thread Starter
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From: France
To repeat the point so you really understand, not to show off: it is possible to trim a 777 sim from manual level flight onto the glide, and fly the ILS hands off to the runway. Autothrottle on, raw data manual ILS. I have done it. The feedback is precise enough to know when pitch is in trim.
It is almost impossible in reality because the atmosphere is not like a simulator.
To get back to the original point: You fly data and think the world is like this sterile unchangeable data. It is not. Pilots are trained to set data as a starting point but continuously adapt to the reality which is not simulatorlike but full of little outside disturbances.
Your original question turns out to be a quest for confirmation that A/T is making our life more difficult. It is the opposite. Because reality is not a simulator using datapoints. Reality is everything from subtle to big sudden changes, where you need to have a hand on the throttle and a hand on the stick and use the feedback provided. Your ’stability problem’ is small compared to the real world disturbances but you don’t seem to understand that as you’ve never been at the controls of a calm wind ILS, let alone a gusty one. Your problem literally disappears into the wind if I may say so. With all due respect, so would your speed if you decided to disconnect the A/T because you think it is easier. The A/T does a fantastic job at it. It doesn’t even need additional wind corrections which makes life over the threshold a lot easier...
If you can’t accept that reality the solution for you is easy: wait for the first A350 that can takeoff with the autopilot and land without a single disconnect. Problem solved.
It is almost impossible in reality because the atmosphere is not like a simulator.
To get back to the original point: You fly data and think the world is like this sterile unchangeable data. It is not. Pilots are trained to set data as a starting point but continuously adapt to the reality which is not simulatorlike but full of little outside disturbances.
Your original question turns out to be a quest for confirmation that A/T is making our life more difficult. It is the opposite. Because reality is not a simulator using datapoints. Reality is everything from subtle to big sudden changes, where you need to have a hand on the throttle and a hand on the stick and use the feedback provided. Your ’stability problem’ is small compared to the real world disturbances but you don’t seem to understand that as you’ve never been at the controls of a calm wind ILS, let alone a gusty one. Your problem literally disappears into the wind if I may say so. With all due respect, so would your speed if you decided to disconnect the A/T because you think it is easier. The A/T does a fantastic job at it. It doesn’t even need additional wind corrections which makes life over the threshold a lot easier...
If you can’t accept that reality the solution for you is easy: wait for the first A350 that can takeoff with the autopilot and land without a single disconnect. Problem solved.
Trim without an A/T is easier to fly than trim and A/T (Boeing with A/T on)
A/T without trim (airbus) is easier to fly than trim and A/T
No trim and no A/T is also easy to fly for any properly trained pilot. I wouldn't call myself a pilot if I wasn't able to fly a manual flight.
Also not to show off, but I always get above standard marks for the "manual flying" competency. If it can reassure you that it's not a matter of my personnal skills.
My airline requires us to be able to fly approaches in all these conditions. FD or A/T OFF, the other ON, or both OFF.
And I don't have any particular problem doing it. I just think that this is still a design issue.
And the price to pay for this design issue is the type of incident that we just saw.
Also, the proof that this is a design issue is the existence of the "clic clic". Instructors taught us that on this airplane, there is a function that will make the trim speed stick to the A/T speed, which is the only condition that allows an A/T stability. If you trim the airplane within 5 kt of the target speed, and you're not too much out of trim, the trim speed will go to the target speed. I believe it behaves similarly a mouse pointer (like i have on my ipad now writing this text) that sticks to buttons it goes over : a way to improve the finger's precision by sticking to targets.
https://www.flightradar24.com/blog/s...escent-hawaii/
Why is it linked to this problem ? We don't exactly know what happened, but this sentence it's very relevant to the issue here.
" During the go around the first officer “inadvertently pushed forward on the control column while following thrust lever movement commanded by the autothrottle.”"
Followed the thrust lever movement commanded by the A/T. There was an out of trim condition. It was either a trim speed to high, or a force on the stick (which is also an out of trim condition), and the A/T reacted by reducing thrust during a go around, which is not a desirable behaviour.
With a fixed thrust at TOGA, had the F/O pushed, the speed would have increased very quickly, and the normal reaction to that is to pull. Also, without a reaction, it would cause a very conspicuous alarm, the overspeed alarm, which, even if the pilot did not react, would cause the other pilot to react.
To finish with, the stability issue is much larger than real world disturbances. Try to follow an approach in the sim with a voluntary out of trim condition, of just 3 knots, you will see...
Which other disturbance can make you go from normal approach speed to idle or full power in less than 5 seconds ? That order of magnitude is similar to a large windshear.
And I'm not refusing it... I'm however refusing that people would not see what exists.
With all due respect, so would your speed if you decided to disconnect the A/T because you think it is easier.
Just pull the thrust levers back about half-way and Bob's your uncle. Thrust capped at 50%...
Thread Starter
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From: France
It's a bit funny to see this discussion surface in a discussion that tackles the "stability" of Boeing FBW. A Boeing FBW that is designed to push the nose down when the speed decreases... (or "unstable" in your words, but I've already understood we don't need to make a distinction between static stability, dynamic stability, control stability,...)
I'm not talking about the airplane stability, I'm talking about the throttle levers with A/T ON. With A/T ON, the throttle levers are unstable if there is any out of trim condition, even the slightest.
Airplane stability is indeed divided in static and dynamic, but this "stability" is mainly a speed stability. With a positive static margin, a conventional airplane gains static stability. But it's not a path stability.
If you get a headwind gust in a conventional airplane, the plane climbs. The speed is stable, the path is not.*
If you get a headwind gust in an airbus you will keep a constant path, with an increase in speed. Speed is not stable, path is.
I believe that path is more important to a pilot than speed.
Then, once your path is stable, A/T is a positive addition to the airplane without A/T.
On boeing, if you have an out of trim condition, A/T is not a positive addition to the airplane without A/T.
Airbus revolutionized the concept of airplane stability, moving the underlying concept from speed stability to path stability.
That's the most important lesson that I concluded from this 10 seconds maneuver in the flight sim that's much more enlightening than it seems.
*approximating for the fact that the flight control law is indeed a blend of path and pitch. However while at the controls, if you're not told that it's a blend with pitch, you could easily believe it's just path.
This is a whole other topic but it would be interesting to know why they had to blend the pitch in. Maybe it didn't feel natural enough without it ?
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From: UK
Off on a tangent, but that doesn't make sense. On final ie on a fixed approach path, there is only one parameter that is being controlled, that is the IAS, and that means only one energy state. An Airbus cannot "increase it's energy" without increasing speed. I'm not familiar with GS Mini but it sounds similar to the 717's VGust feature, where the FCCs would increase the target speed without changing the IAS bug. In other words, in gusty conditions, the jet would fly faster than bug. This speed would be similar to "bugging up" in the constant-IAS system. If the gust disappears, the Constant airspeed system would only have the speed reduce to the normal bug speed anyway. The advantage of VGust and I assume GS Mini is, of course, that when/if the gusty conditions settle down, the jet would slow down to the no-gust bug speed, thereby improving landing performance.
It's got nothing to do with FBW. The system works perfectly well in the 717 which is as far from FBW as you can get, flight-controls-wise. The pitch-power coupling is a separate issue.
Interesting comments above re throttles in the flare; doesn't sound very user friendly to me.
And for the avoidance of doubt, the ATS in the 717 is excellent and a great reliever of workload and Boeing procedure is to use it at all times.
It's got nothing to do with FBW. The system works perfectly well in the 717 which is as far from FBW as you can get, flight-controls-wise. The pitch-power coupling is a separate issue.
Interesting comments above re throttles in the flare; doesn't sound very user friendly to me.
And for the avoidance of doubt, the ATS in the 717 is excellent and a great reliever of workload and Boeing procedure is to use it at all times.
I don't know anything about the B717, so I cannot comment on that, but the Airbus FBW adjusts for the extra IAS so the pilot doesn't have to and the flight path is not destabilised, hence the Airbus auto-thrust can be used when flying manually, whereas when flying the Boeing B737 manually; it is recommended not to fly with auto-throttle engaged.
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From: Seat 1A
Originally Posted by Uplinker
but the Airbus FBW adjusts for the extra IAS so the pilot doesn't have to and the flight path is not destabilised, hence the Airbus auto-thrust can be used when flying manually, whereas when flying the Boeing B737 manually; it is recommended not to fly with auto-throttle engaged.
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From: Perpetually circling OCK for some reason
747 and 757/767 are the same, for the same reason. AT is used during manual climb and takeoff but is off for manual flight in other phases.
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From: France
When you're engine out without TAC (for example airbus which doesn't have a real TAC) or b777 with tac failure, the instructors at my airline don't recommend at all to disconnect the A/T on the remaining engine.
Even though, there is in this situation, a huge "yaw power" coupling.
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From: Blue sky
I'll reply one last time, because I must have read the original question 20 times... I still don't get it. This is the world upside down.
This is the situation that happens so many times in training or during line checks, where something doesn't really quite works because of a skillset that lacks. However, because the automation was unable to solve it, the crewmember ends up blaming automation for the lack of skillset.
First of all:
The origin of the problem is inadequate trimming. There is nothing extraordinary in trimming the B777, on the contrary, it is dead easy. The FBW is an easy system, that helps out a lot. The problem you describe is not an A/T problem, it is a “trim skill” problem. If you reduce the speed on the A/T, every pilot flying the B777 knows or feels instantly he needs to trim, he can even calculate how long, but he can also feel it. He feels the throttles reducing, he feels the stickforce increasing (or sees moving pitch). You don't need a “number” for that. That's only excessive information that takes away attention from other things. The info is there, in both your hands (tactile or kinestetic feedback, whatever you want to call it).
I'm a training captain, in my company we make trainees do raw data approaches with A/T off. When people have a choice between A/T on or off, the difference is visible in the FDM statistics. Lacking the skill to properly trim the aircraft is one thing, and for trainees I can accept flying full raw data A/T off can be a bit overwhelming. It's just something you need to do once in a while. People who know me know I'll gladly take it to the bar and discuss over a beer, have a laugh and move on with life. No harm done.
However secondly:
If you come back to me during the debrief and state this is an A/T problem that made your life difficult, and on top YOU STATE IT IS GOING TO BE EASIER WITH A/T OFF, and that was the reason why you decided to disconnect the A/T, we are not making it to the bar. You can't trim with the A/T? You certainly won”t be able to do it without.
This is the typical pilot excuse blaming automation as the cause for his inability (VNAV is another one of those examples, but let's not go down that rabit hole). When people disconnect the A/T on gusty days, I strap my seatbelt. Because you take one vital parameter, and you take over control as opposed to “monitor”. You feel the engines move, your hands are on the throttle. It's instant, it's immediate feedback without looking at numbers. Now you want to do it yourself? You have the wise people who decide to use A/T on gusty windy days as it is easier. Or you have the “I don't trust” or “I doesn't work properly” people who disconnect and decide they can do a better job without. You should see it from the jumpseat. Very interesting. I don't know why so many people keep holding on to the fact they have better speed control than the A/T.
So 1) stop blaming the A/T for what you can't achieve, or use the excuse “I glanced away”. Because if you're not capable of trimming the aircraft on the approach, you probably couldn't in the departure, you couldn't do it anywhere.
And 2) people who state it would be easier without the automation are usually the ones who feed the FDM statistics
I will leave it at that and stay out of the thread, nothing against you, I prefer beers at the bar. I'll take A/T off approaches any time weather permitting up to a certain level, don't worry, but the A/T is a consistent reliable system. No man's/woman's brain is. There is always a lapse, a miss, ... and a chase to get back on whatever parameter we ended up missing/chasing.
Last edited by BraceBrace; 19th June 2024 at 13:41.
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From: USA
A pilot flying manual is not allowed to do that.
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From: Blue sky
A badly trimmed aircraft is not an unstable aircraft.
You can aileron trim the heck out of a 737 autopilot and it won't move at all the aircraft as the autopilot will counteract. But it ain't going to be staying there when you disconnect the autopilot because the aircraft is actually out-of-trim. It is perfectly stable though, it will violently look for it's new balance.
It's on this basis many people claim the 737 is an unstable aircraft. It is not. It is a stable aircraft with a control issue. Before anyone jumps on that 737 bandwagon again (we already did I noticed). Pitch-power couplings don't make an aircraft unstable, it does shift the balance. We already learn this ie on a Cessna with something called P-factor. Increasing thrust does not make your aircraft unstable, but the balance changes (albeit in a different axis on the Cessna).
And that's why the eyes need to stay on the attitude. Because "control" of balance is our responsability and we call it trimming. And you have a PM to act as a back-up primary back-up. You do realise the PM has to do deviation callouts? Because yes, eyes and brains of the PF wander around sometimes... (and autopilots disconnect... or get bad inputs...)
Keep it simple people, stop mixing around to complicate... I’m off to the bar now...
You can aileron trim the heck out of a 737 autopilot and it won't move at all the aircraft as the autopilot will counteract. But it ain't going to be staying there when you disconnect the autopilot because the aircraft is actually out-of-trim. It is perfectly stable though, it will violently look for it's new balance.
It's on this basis many people claim the 737 is an unstable aircraft. It is not. It is a stable aircraft with a control issue. Before anyone jumps on that 737 bandwagon again (we already did I noticed). Pitch-power couplings don't make an aircraft unstable, it does shift the balance. We already learn this ie on a Cessna with something called P-factor. Increasing thrust does not make your aircraft unstable, but the balance changes (albeit in a different axis on the Cessna).
And that's why the eyes need to stay on the attitude. Because "control" of balance is our responsability and we call it trimming. And you have a PM to act as a back-up primary back-up. You do realise the PM has to do deviation callouts? Because yes, eyes and brains of the PF wander around sometimes... (and autopilots disconnect... or get bad inputs...)
Keep it simple people, stop mixing around to complicate... I’m off to the bar now...
Last edited by BraceBrace; 19th June 2024 at 19:37.
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From: big green wheely bin
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As has been explained thoroughly already, and demonstrated in the sim, yes it is, in this circumstance. I don't have anything new to add but sometimes different phrasing works so I'll try:
A statically stable aircraft returns to its setpoint after a disturbance. (A merely out-of-trim aircraft, as you're insinuating this is, has a setpoint different from where you are, and will seek that. But once reached, it will return there after disturbances.)
A plane with a trim speed different from autothrottle speed, does not return to its original flight path or attitude, but diverges all the way until throttle is full or idle. This is because there are two setpoints, one each in a different parameter: The autothrottle holds the autothrottle speed, and the flight path holds the trim speed. If they're the same, then no problem. But if they're different, they interact in a way that makes the flight path unstable:
Let's say autothrottle speed is 150 and trim speed is 160. The flight path (seeking trim speed) will go down to seek 160. But this causes the autothrottle speed to exceed 150, and in seeking it, it retards the power. This brings the speed lower than 160, which is good for the autothrottle speed but bad for the trim speed, which causes the pitch to go further down to seek 160, which causes the autothrottle to further retard to seek 150, etc. It is in a runaway feedback loop. Or, put another way, the pitch is stable on speed but unstable on flight path, because the autothrottles are continuously pulling the speed rug out from underneath us.
I have not heard this claim before.
If you yell at the hydraulic pump harder that it's not supposed to fail, maybe it won't?
A statically stable aircraft returns to its setpoint after a disturbance. (A merely out-of-trim aircraft, as you're insinuating this is, has a setpoint different from where you are, and will seek that. But once reached, it will return there after disturbances.)
A plane with a trim speed different from autothrottle speed, does not return to its original flight path or attitude, but diverges all the way until throttle is full or idle. This is because there are two setpoints, one each in a different parameter: The autothrottle holds the autothrottle speed, and the flight path holds the trim speed. If they're the same, then no problem. But if they're different, they interact in a way that makes the flight path unstable:
Let's say autothrottle speed is 150 and trim speed is 160. The flight path (seeking trim speed) will go down to seek 160. But this causes the autothrottle speed to exceed 150, and in seeking it, it retards the power. This brings the speed lower than 160, which is good for the autothrottle speed but bad for the trim speed, which causes the pitch to go further down to seek 160, which causes the autothrottle to further retard to seek 150, etc. It is in a runaway feedback loop. Or, put another way, the pitch is stable on speed but unstable on flight path, because the autothrottles are continuously pulling the speed rug out from underneath us.
It's on this basis many people claim the 737 is an unstable aircraft
And that's why the eyes need to stay on the attitude. And you have a PM to act as a back-up primary back-up. You do realise the PM has to do deviation callouts? Because yes, eyes and brains of the PF wander around sometimes... (and autopilots disconnect... or get bad inputs...)
Last edited by Vessbot; 19th June 2024 at 20:09.
Joined: Feb 2003
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From: Blue sky
How about this:
* Push AP A CMD, confirm CMD is active on FMA.
* Wait hands under your pants, then put hands on controls again
* Push AP A CMD, confirm FD is active on FMA.
Is that better? Either you restore the in-trim situation, or if you can't, you can hope the AP can do it (within limits). It's called aircraft control.
Remarks like your last one are pretty ridiculous. Using the possibility of fail or lapses as an excuse not to be responsible. The responsibilities of PF and PM are clearly defined in the FCOM. There is a difference between responsibility, failure and blame. You start with a responsibility, you might fail somewhere along the line, and worst case be called in the office. Aviation has thankfully moved away from the blame in many companies and revised to learning from mistakes. But you will not leave that office with less responsibility in the role of PF or PM.
Joined: Sep 2016
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