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B-737 Speed Trim System

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B-737 Speed Trim System

Old 12th Nov 2018, 05:56
  #41 (permalink)  
 
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Has any one in this forum have access to B737 MAX AMM (Pages from AMM Chap 34-20-00) and if you can post the same system info for B737 MAX redundancy management of AOA signals.

I just cannot believe that one faulty AOA sensor can make the aircraft trim nose down. There has to be more protection in the system design for this not to happen. I think the FAA AD more or less confirms a poor design in B737 MAX.
In the B777 which I am familiar with, each of the two ADIRUs (Air Data Inertial reference unit) receive both AOA inputs (There are two AOA sensors on most aircraft, same config on B737 also). This is compared with 'Calculated AOA' and a mid value is used. This is the redundancy built in the system on B777. Also each of the AOA sensor has two outputs, feed into two different computational channels. See the redundancy. There are actually 4 signals from two AOA sensors.
The full text from the B777 AMM is as below.
AOA Redundancy Management
The AOA redundancy management logic uses a modified midvalue selection.
The modified mid-value selection chooses the mid-value of these three AOA values:
* Left corrected AOA
* Right corrected AOA
* Calculated AOA.
The AOA redundancy management logic receives inputs from the inertial and air data systems to calculate the calculated AOA.


So I am a bit curious about the B737 MAX AOA signal Logic.
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Old 12th Nov 2018, 06:27
  #42 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post


Take off today. 76 tons. So not low gross weight. I let the STS do itís job (it was not supposed to do anything due to high gross weight?). It was trimming way longer than the claimed 10 seconds band it should trim after take off. I had to really push forward on the controls after it had finished trimming aft. So much that if I had let go, the nose would have pitched up really fast.
I had to trim 5-6 turns forward before the aircraft was stable. Thatís a lot!
So, we have a system that brings the aircraft out of trim, and trims when it is not supposed to trim.
Not super easy. Dangerous. I donít think about this when I fly manually since the first thing I do is to trim forward in order to cancel out the STS.
This system is plain stupid. Try it, and see for yourself.
Never flown 73s, but always thought it would have less behind the scene stuff...wasn't it suppose to be hands-on & no computer interference machine?
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Old 15th Nov 2018, 17:54
  #43 (permalink)  
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I thank you all for your contributions to this discussion. I think I get what you are trying to explain, but I am a simple soul and by my logic, any system that works agains my control inputs when I fly manually is making my work harder.
And the STS works when the aircraft is heavy or light, GC fwd or aft, derated or full thrust. That is not how the STS is explained. I have flown the NG since they made it. I have never bothered with the STS before, I simply cancel it with manual trim. I was just when I saw the system discussed that I started to check what it did.
So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens.
For those of you saying ęjust let the STS do it’s jobĽ. No. You can’t because it will bring the aircraft well out of trim.

It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall!
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Old 15th Nov 2018, 23:15
  #44 (permalink)  
 
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It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall!
No it won't, as the speed reduces, the nose will drop, helped by the STS. The STS ensures the aircraft is speed-stable.
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Old 16th Nov 2018, 12:31
  #45 (permalink)  
 
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Originally Posted by RetiredBA/BY View Post
Just an interested bystander as itís a long time since I flew the 737, 200 and the very first 300s. I canít remember the 73s or the 75 and 76 having a speed trim system as described here. Found them all delightfull aircraft to fly manually throughout the speed range.

So, may I ask if it so vital, to meet the arbitrary speed stability requirements of the FAA that another system , with potential for malfunction with potentially serious consequences is really needed. Do we really NEED such a system and since it IS installed is there an instant cutout switch.

When were the values of 3 pounds per 10 knots decided on and why are these exact values so important. Are current models of the 73 significantly more speed unstable than the earlier versions ?

I still remember so clearly the problems caused by runaway tailplanes on my first jets, the Canberra and Valiant. Of course I know the flight control electronics, electrics, were nowhere near so advanced or capable as on present jets.

Is the FAA using, or requiring Boeing to use a sledgehammer to crack a nut?

Just curious!
Just read this good article by Bjorn Ferhrm which explains why the MAX requires this MCAS. The aircraft is more unstable than the NG with the larger engines.
https://leehamnews.com/2018/11/14/bo...to-the-pilots/
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Old 16th Nov 2018, 12:59
  #46 (permalink)  
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Originally Posted by Capn Bloggs View Post
No it won't, as the speed reduces, the nose will drop, helped by the STS. The STS ensures the aircraft is speed-stable.
Forgive me, I have only been flying for 30 years and nearly 20 on the NG, but all my departures in this period have been about raising the landing gear, accelerate and retract the flaps before I head off for far away places.
It has never been about flying at 150 kts in a ęspeed stableĽ condition.
I admit I donít know how far towards a stall the STS will bring the aircraft, but with the amount of back pressure it creates, I think it will get pretty close before anything happens. If it tries to trim the aircraft back to the ęstable speedĽ of 150 kts in clean config, things will get interesting.
By the way, which aircraft type do you fly, Bloggs?
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Old 16th Nov 2018, 13:26
  #47 (permalink)  
 
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Forgive me, I have only been flying for 30 years and nearly 20 on the NG
So, after 20 years of 737NG experience, you suddenly have a problem with STS because you read something on PPRuNe.

So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens.
At the risk of repeating myself, who told you to fly the aircraft like that? That is not how it is intended to be flown. It is designed with speed stability, which means that an intentional speed change requires trim, like any non-FBW aircraft. So TRIM FFS. Like youíve been doing for 20 years.

For those of you saying ęjust let the STS do itís jobĽ. No. You canít because it will bring the aircraft well out of trim.
No-one is saying that. Theyíre just saying donít expect it to trim to what you want. Thatís not what itís there for. Trimming is your job. Boeing designs aircraft for pilots.

So either be a pilot or go fly an Airbus. Believe me, youíll have a lot more ďwhy is it doing thisĒ questions on an Airbus.
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Old 16th Nov 2018, 14:02
  #48 (permalink)  
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Originally Posted by Derfred View Post



So, after 20 years of 737NG experience, you suddenly have a problem with STS because you read something on PPRuNe.



At the risk of repeating myself, who told you to fly the aircraft like that? That is not how it is intended to be flown. It is designed with speed stability, which means that an intentional speed change requires trim, like any non-FBW aircraft. So TRIM FFS. Like youíve been doing for 20 years.



No-one is saying that. Theyíre just saying donít expect it to trim to what you want. Thatís not what itís there for. Trimming is your job. Boeing designs aircraft for pilots.

So either be a pilot or go fly an Airbus. Believe me, youíll have a lot more ďwhy is it doing thisĒ questions on an Airbus.
Why so agressive? I have used the trim to cancel out STS for nearly 20 years, without it I would need to trim much less.
I have just not bothered to see what it really does before now. What is wrong with that?
Keep defending a system that works continously against the pilot. Never had any other aircraft do that before.
I have never flown an Airbus. Why bring it into this discussion? And why are you implying that pilots who fly Airbus aircraft are not pilots?
Do you work for Boeing?
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Old 16th Nov 2018, 14:07
  #49 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post
Keep defending a system that works continously against the pilot. Never had any other aircraft do that before.
By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.
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Old 16th Nov 2018, 15:40
  #50 (permalink)  
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Originally Posted by Vessbot View Post
By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.
And in three sentences, you have explained the STS perfectly. Thanks!

But I still donít like it.
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Old 16th Nov 2018, 17:34
  #51 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post


And in three sentences, you have explained the STS perfectly. Thanks!

But I still donít like it.
Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.
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Old 16th Nov 2018, 20:27
  #52 (permalink)  
 
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Originally Posted by Vessbot View Post
Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.

Iíve been fly the MAX for 16 months now and itís a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt....

The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000í to go) is back up at 1500fpm

Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season.

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Old 16th Nov 2018, 21:41
  #53 (permalink)  
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Originally Posted by Vessbot View Post
Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.
I have no clue as to pounds pr knots, but I am familiar with trim down when speed increases and trim up when speed decreases. I am not familiar with the trim up when speed increases STS action. You have explained that weird behaviour with just a few words.
So, I will just continue to trim against it, just as as before.
Have a nice weekend!
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Old 16th Nov 2018, 21:46
  #54 (permalink)  
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Originally Posted by EIFFS View Post



Iíve been fly the MAX for 16 months now and itís a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt....

The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000í to go) is back up at 1500fpm

Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season.

NG -800 has a bad rep when it comes to landing mishaps. I really hope your predicitons about the Max does not come through.
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Old 17th Nov 2018, 01:30
  #55 (permalink)  
 
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According to boeing how is the Lior air accident prevented?
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Old 17th Nov 2018, 09:10
  #56 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post


NG -800 has a bad rep when it comes to landing mishaps. I really hope your predicitons about the Max does not come through.
i


There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another ďsub systemĒ new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing.

The Landing​ Attitude Modifier (LAM) system performs two​ functions. The first​ ​
LAM function applies​ when the flaps are in​ the 30 or 40 position. To maintain ​
acceptable nose landing gear contact margin, LAM symmetrically deploys flight ​
spoilers on approach to reduce lift and force the airplane to use a higher angle of ​
attack. The amount of spoiler deflection depends on the approach speed. ​
Deflection begins at approximately 10​ knots above VREF.​
The second LAM function applies​ when flaps are positions 15 through​ 30​ and​ the ​
thrust levers are near idle. This function also symmetrically​ deploys flight ​
spoilers, in order to generate additional drag

The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets)

Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many donít ) and youíve correctly completed a landing distance calculation ( again many donít) the NG SFP & 737-800 (MAX) model have excellent stopping capability.

As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO
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Old 17th Nov 2018, 09:54
  #57 (permalink)  
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Originally Posted by EIFFS View Post
i


There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another ďsub systemĒ new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing.

The Landing​ Attitude Modifier (LAM) system performs two​ functions. The first​ ​
LAM function applies​ when the flaps are in​ the 30 or 40 position. To maintain ​
acceptable nose landing gear contact margin, LAM symmetrically deploys flight ​
spoilers on approach to reduce lift and force the airplane to use a higher angle of ​
attack. The amount of spoiler deflection depends on the approach speed. ​
Deflection begins at approximately 10​ knots above VREF.​
The second LAM function applies​ when flaps are positions 15 through​ 30​ and​ the ​
thrust levers are near idle. This function also symmetrically​ deploys flight ​
spoilers, in order to generate additional drag

The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets)

Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many donít ) and youíve correctly completed a landing distance calculation ( again many donít) the NG SFP & 737-800 (MAX) model have excellent stopping capability.

As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO
Thanks for your explanation on the Max. We have max 15 kts addon on our NGs, this was just changed. We operate SFP NGs and regular ones. There is a marked difference between the two models when it comes to landing distance.
-800 and FL 40 = pitch nearly 0 degrees. I see this as one of the reasons why pilots have a tendency to go above the glide when they get close to the runway.
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Old 23rd Nov 2018, 21:52
  #58 (permalink)  
 
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Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.

I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two.

Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing.

FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention.

However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail.

The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability.
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Old 24th Nov 2018, 00:55
  #59 (permalink)  
 
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Vessbot's analysis is excellent. The only other thing he should added is that it works fine. I've only ever had one significant failure in several thousand hours on the type. Dont over think it. Runaway Stab. Checklist works fine if you have a problem and all else fails put the little feet of the PFD aircraft symbol on the horizon and set 80% N1 then work the problem. Fly the [email protected]#king aircraft. Not that hard.
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Old 27th Nov 2018, 22:54
  #60 (permalink)  
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Originally Posted by LEOCh View Post
Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.

I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two.

Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing.

FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention.

However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail.

The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability.
But STS works primarily in low speed situations, itís not active during normal cruise:
  • STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68
How does this pound pr knot certification requirement work on FBW aircraft where the aircraft is kept in a trimmed state at all times? Or do 777 drivers trim against a STS as well when they fly manually?

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