Obviously I am not an expert and I do not understand every nook and cranny of what is being told here, but I deeply enjoy reading this thread and I also enjoy understanding what I can understand. Thanks a lot, everyone, for taking the time and the care to also write for lay persons to understand.

Just remember a “derate” is like putting a different thrust engine rating on the plane, “Reduced” is the engine’s thrust rating reduced due to conditions, but full rated thrust is there.

GalaxyFlyer wrote:

Just remember a “derate” is like putting a different thrust engine rating on the plane, “Reduced” is the engine’s thrust rating reduced due to conditions, but full rated thrust is there.

A key point. And an important one to bring up during the departure briefing. If you're using a derate you specifically cannot use full thrust at low speed because that could exceed the control authority of the rudder with an engine out.* If you're using reduced thrust (AKA flex AKA assumed temp) you can always go TOGA.


* On the 'bus (and I assume Boeing also) there is an exception. Even with a derate, in case of windshear, we go TOGA, the logic being that getting out of the windshear trumps the very small risk of an engine failure at that exact moment. Besides, if we do get an engine failure right then, we can pull the thrust back. Once above F speed, (flaps maneuvering speed) we can go TOGA at any time.

Starlionblue wrote:

GalaxyFlyer wrote:

Just remember a “derate” is like putting a different thrust engine rating on the plane, “Reduced” is the engine’s thrust rating reduced due to conditions, but full rated thrust is there.

A key point. And an important one to bring up during the departure briefing. If you're using a derate you specifically cannot use full thrust at low speed because that could exceed the control authority of the rudder with an engine out.* If you're using reduced thrust (AKA flex AKA assumed temp) you can always go TOGA.


* On the 'bus (and I assume Boeing also) there is an exception. Even with a derate, in case of windshear, we go TOGA, the logic being that getting out of the windshear trumps the very small risk of an engine failure at that exact moment. Besides, if we do get an engine failure right then, we can pull the thrust back. Once above F speed, (flaps maneuvering speed) we can go TOGA at any time.

These days you'd think the computers would be smart enough to compute a VMCa for each thrust setting and dynamically adjust the thrust ceiling upward with speed as the rudder authority goes up. Are there types that do that?

Starlionblue wrote:

* On the 'bus (and I assume Boeing also) there is an exception. Even with a derate, in case of windshear, we go TOGA, the logic being that getting out of the windshear trumps the very small risk of an engine failure at that exact moment. Besides, if we do get an engine failure right then, we can pull the thrust back. Once above F speed, (flaps maneuvering speed) we can go TOGA at any time.

Your assumption is correct. The 737 FCOM specifies that, with a derate takeoff, the thrust levers should not be advanced beyond the displayed N1 limit "except in an emergency" and windshear would most certainly qualify. An assumed temp thrust reduction, on the other hand, is "not considered a limitation" and the crew is permitted to manually apply full thrust (i.e. the displayed N1 limit) at any time.

Can't speak for the 'bus but on the 737 (and I would presume other Boeings) a derate will actually move the N1 (or EPR) limit cursors while an assumed temp reduction will not, therefore when TOGA is activated the autothrottle will not command the displayed N1 limit when an assumed temp is active. The cursors continue to display the actual N1 limit (full or derated) so we have a reference if we want to manually advance the thrust levers.

Fun fact: The 777-200 LR GE90-115 was down rated to 110,000 lbs of thrust and designated the GE90-110. Exact same engine, just an artificially imposed limitation

This limitation was imposed because testing revealed that 115,000 lbs of thrust moved so much air that flutter was induced on the elevator on this short (compared to the 300) airplane. Apparently, the smaller amount of air moved by the engine at the 110, 000 thrust rating did not cause flutter.

Here's where things get a little bit interesting. On a hot day at a high altitude airport (the classic hot and high situation) the 110000 rating could be bumped up to 115 because the engine was not capable of producing anywhere near full power due to the less dense air, and thus flutter was not a concern. All just a big math game played in the software.

SteelChair wrote:

Fun fact: The 777-200 LR GE90-115 was down rated to 110,000 lbs of thrust and designated the GE90-110. Exact same engine, just an artificially imposed limitation

This limitation was imposed because testing revealed that 115,000 lbs of thrust moved so much air that flutter was induced on the elevator on this short (compared to the 300) airplane. Apparently, the smaller amount of air moved by the engine at the 110, 000 thrust rating did not cause flutter.

Here's where things get a little bit interesting. On a hot day at a high altitude airport (the classic hot and high situation) the 110000 rating could be bumped up to 115 because the engine was not capable of producing anywhere near full power due to the less dense air, and thus flutter was not a concern. All just a big math game played in the software.

That is very interesting, a fascinating detail

Max Q wrote:

SteelChair wrote:

Fun fact: The 777-200 LR GE90-115 was down rated to 110,000 lbs of thrust and designated the GE90-110. Exact same engine, just an artificially imposed limitation

This limitation was imposed because testing revealed that 115,000 lbs of thrust moved so much air that flutter was induced on the elevator on this short (compared to the 300) airplane. Apparently, the smaller amount of air moved by the engine at the 110, 000 thrust rating did not cause flutter.

Here's where things get a little bit interesting. On a hot day at a high altitude airport (the classic hot and high situation) the 110000 rating could be bumped up to 115 because the engine was not capable of producing anywhere near full power due to the less dense air, and thus flutter was not a concern. All just a big math game played in the software.

That is very interesting, a fascinating detail

Almost as good as Concorde engine #4

SteelChair wrote:

Fun fact: The 777-200 LR GE90-115 was down rated to 110,000 lbs of thrust and designated the GE90-110. Exact same engine, just an artificially imposed limitation

This limitation was imposed because testing revealed that 115,000 lbs of thrust moved so much air that flutter was induced on the elevator on this short (compared to the 300) airplane. Apparently, the smaller amount of air moved by the engine at the 110, 000 thrust rating did not cause flutter.

Here's where things get a little bit interesting. On a hot day at a high altitude airport (the classic hot and high situation) the 110000 rating could be bumped up to 115 because the engine was not capable of producing anywhere near full power due to the less dense air, and thus flutter was not a concern. All just a big math game played in the software.

This is false.

The high thrust operations does not cause flutter, flutter is an engineering term that is related to exciting the natural structural frequencies through airflow causing divergence.

The horizontal stabiliser simply vibrates in the slipstream, there is no divergence due to exciting structural modes.

As for hot and high, the thrust is not increased. The sea level flat rating is simply maintained to a higher altitude, this is a feature on a number of engine types., not unique to the GE90.

zeke wrote:

SteelChair wrote:

Fun fact: The 777-200 LR GE90-115 was down rated to 110,000 lbs of thrust and designated the GE90-110. Exact same engine, just an artificially imposed limitation

This limitation was imposed because testing revealed that 115,000 lbs of thrust moved so much air that flutter was induced on the elevator on this short (compared to the 300) airplane. Apparently, the smaller amount of air moved by the engine at the 110, 000 thrust rating did not cause flutter.

Here's where things get a little bit interesting. On a hot day at a high altitude airport (the classic hot and high situation) the 110000 rating could be bumped up to 115 because the engine was not capable of producing anywhere near full power due to the less dense air, and thus flutter was not a concern. All just a big math game played in the software.

This is false.

The high thrust operations does not cause flutter, flutter is an engineering term that is related to exciting the natural structural frequencies through airflow causing divergence.

The horizontal stabiliser simply vibrates in the slipstream, there is no divergence due to exciting structural modes.

As for hot and high, the thrust is not increased. The sea level flat rating is simply maintained to a higher altitude, this is a feature on a number of engine types., not unique to the GE90.

Perhaps I explained it poorly, or in a manner not to your satisfaction, but I didn't make any of this up. It came from a mutti-billion dollar multi-national company to another multi-national multi-billion dollar company. Perhaps you, as an anonymous internet poster, should contact them and tell them that they are wrong. Please let us know how that goes.

It’s false. Flutter is the interaction between the airflow over an aero structure and how it excites the inherent structural modes are excited.

The aero structure does not know the difference between air coming from an engine or from free stream flow.

The 77F/77L can have either a General Electric GE90-110B1 or GE90-115B.

zeke wrote:

It’s false. Flutter is the interaction between the airflow over an aero structure and how it excites the inherent structural modes are excited.

The aero structure does not know the difference between air coming from an engine or from free stream flow.

The 77F/77L can have either a General Electric GE90-110B1 or GE90-115B.

Do you know if the -115 option was available when the LR came out? Or was it only offered later, after more testing? I'm telling you I'm not making this stuff up. I was trained in it.

Does it not follow logically that the exhaust of the 115 would be stronger than a 110? I can tell you the reason I remember. Its because I didn't believe that the 5,000 lb/th difference would be that impactful.

zeke wrote:

It’s false. Flutter is the interaction between the airflow over an aero structure and how it excites the inherent structural modes are excited.

The aero structure does not know the difference between air coming from an engine or from free stream flow.

The 77F/77L can have either a General Electric GE90-110B1 or GE90-115B.

What is the difference between flutter and vibration in the wind from your perspective?

It’s probably like the C-5 during air refueling—the horizontal stab was in the engine wake efflux creating vibrations that eventually showed up in the tie box cracking. Flutter is a harmonic, aerodynamic vibration isn’t.

GalaxyFlyer wrote:

Flutter is a harmonic, aerodynamic vibration isn’t.

Wut?

kalvado wrote:

GalaxyFlyer wrote:

Flutter is a harmonic, aerodynamic vibration isn’t.

Wut?

Flutter is divergent because of the structure’s harmonics. Vibration is just the impingement of the jet exhaust in the current.

https://youtu.be/qpJBvQXQC2M

kalvado wrote:

What is the difference between flutter and vibration in the wind from your perspective?

Flutter is a specific engineering, it means an unstable, self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure.

It can occur in buildings, ships, submarines, pumps, aircraft, spacecraft. The Tacoma bridge is one of the most well known one in civil engineering, https://youtu.be/XggxeuFDaDU

This is an emailed of flutter in a glider
https://youtu.be/kQI3AWpTWhM

If you look at this B2 RC aircraft video at 9:30 you can see the flutter mode in a high speed dive
https://youtu.be/rRqHw4ru084

As for vibrating the tailplane with thrust, this occurs with a lot of aircraft, both military and civil and stops as the airflow over the surface is enough to dampen it out.

There are specific requirements in the FARs for flutter, it is often one of the limiting factors for maximum speed/Mach.

zeke wrote:

kalvado wrote:

What is the difference between flutter and vibration in the wind from your perspective?

Flutter is a specific engineering, it means an unstable, self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure.

It can occur in buildings, ships, submarines, pumps, aircraft, spacecraft. The Tacoma bridge is one of the most well known one in civil engineering, https://youtu.be/XggxeuFDaDU

This is an emailed of flutter in a glider
https://youtu.be/kQI3AWpTWhM

If you look at this B2 RC aircraft video at 9:30 you can see the flutter mode in a high speed dive
https://youtu.be/rRqHw4ru084

As for vibrating the tailplane with thrust, this occurs with a lot of aircraft, both military and civil and stops as the airflow over the surface is enough to dampen it out.

There are specific requirements in the FARs for flutter, it is often one of the limiting factors for maximum speed/Mach.

Oh, I didn't know that physics of military and civilian aircraft is different! Although someone told me that cosine of an angle can be as high as 2.7 during wartime... Are you say flutter still affects both types?
Still, let me repeat the question - what is the difference between vibration and flutter? Or, rather, under what conditions one becomes the other? Terms you may find useful for your answer: restoring force, dampening, Q-factor, lift as a function of flow velocity.

Excessively immature.

I am very familiar with the difference between vibration and flutter, I have provided the definition of flutter.

Many many moons ago I used to do ground vibration testing using Brüel & Kjær shakers and accelerometers connected to a HP FFT system doing modal analysis on a HP-UX workstation.

We would excite the structure with shakers to calculate the modes and natural frequencies. Have done that for aircraft, military applications, rudders, and spacecraft.

zeke wrote:

Excessively immature.

I am very familiar with the difference between vibration and flutter, I have provided the definition of flutter.

Many many moons ago I used to do ground vibration testing using Brüel & Kjær shakers and accelerometers connected to a HP FFT system doing modal analysis on a HP-UX workstation.

We would excite the structure with shakers to calculate the modes and natural frequencies. Have done that for aircraft, military applications, rudders, and spacecraft.

Zeke, with all my respect to you as a pilot, you don't know what you are talking about here. Although, as a passenger, I find it encouraging that pilots don't have to worry about things like that any more!

So, flutter is an aeroelastic phenomenon. You are thinking about extreme examples of destruction, while phenomenon is still there on the aircraft you fly. It is kept under control, so the only place where it somewhat affects pilots is hours/cycles limit of the aircraft.
So, what is aeroelasticity? Simple approach to vibration is that there are 2 energies, T and V in the lagrangian, and energy flows from one to the other with some dissipation. There may be external force - constant excitation in you shaker example. Interaction with air is considered a small correction if at all.
Now, once airflow velocity goes up, so is interaction of structure with airflow (as v^2), and at some point it is no longer a small correction. So, it is and aeroelastic problem now. Flutter does not have to be destructive on one hand, it cannot be eliminated on the other. Good news it can be controlled

Ref:
https://www.kimerius.com/app/download/5 ... lutter.pdf
http://www.aerodynamics4students.com/aeroelasticity/

Simple, almost incorrect example which gives you the flavor though:
Imagine an airfoil in the flow, which vibrates so that AoA is changing. Without airflow, you have a deflection, a corresponding restoring elastic force, and airfoil oscillates around some central position. Frequency, amplitude, all that.
Now, turn on airflow - and lift. Lift changes linearly with deflection; the more deflection is, the more difference between equilibrium lift and actual lift at defection. This difference cancels out restoring force - so now you have a different, likely lower frequency, and higher amplitude.
Now, load bearing structure still endures that higher deflection, and higher forces. Which can mean more metal fatigue, reduced service life etc etc.
It is not a catastrophic event now - but still a problem. Well, if a (test) pilot didn't get a message from manufacturer or maintenance and increased the airspeed to the point where aerodynamic force exceeds structural force...
This is NOT an actual description of the flutter, but should give a good flavor of the type of problem.

This is drivel, the jet exhaust over the tailplane as cited above is not flutter. Lots of aircraft types experience similar.

Your posts are just red herrings, and do not address the claim at all.

I was very familiar with flutter, in air, in water, and in space. The engineering principles have not changed. I got out of that line of work as it’s boring as xxx xxxx.

zeke wrote:

This is drivel, the jet exhaust over the tailplane as cited above is not flutter. Lots of aircraft types experience similar.

Your posts are just red herrings, and do not address the claim at all.

I was very familiar with flutter, in air, in water, and in space. The engineering principles have not changed. I got out of that line of work as it’s boring as xxx xxxx.

Well, your definition of flutter please. Other than that it is a problem affecting both civilian and military airplanes, of course. and without referencing "good working understanding of non-applicable equations". Some references may be helpful, though.

You really don't care what is the reason for the flow - engine exhaust, ram air, or geese farting onto oncoming plane. The way I read that "stabilizer in engine exhaust" story is "vibrations due to flutter amplification became unacceptably high in some part of envelope". Totally plausible from my perspective

kalvado wrote:

zeke wrote:

This is drivel, the jet exhaust over the tailplane as cited above is not flutter. Lots of aircraft types experience similar.

Your posts are just red herrings, and do not address the claim at all.

I was very familiar with flutter, in air, in water, and in space. The engineering principles have not changed. I got out of that line of work as it’s boring as xxx xxxx.

Well, your definition of flutter please. Other than that it is a problem affecting both civilian and military airplanes, of course. and without referencing "good working understanding of non-applicable equations". Some references may be helpful, though.

You really don't care what is the reason for the flow - engine exhaust, ram air, or geese farting onto oncoming plane. The way I read that "stabilizer in engine exhaust" story is "vibrations due to flutter amplification became unacceptably high in some part of envelope". Totally plausible from my perspective

Thank you for providing in your last paragraph a more precise definition of what I attempted to describe.

BTW geese farting....hilarious.

SteelChair wrote:

zeke wrote:

It’s false. Flutter is the interaction between the airflow over an aero structure and how it excites the inherent structural modes are excited.

The aero structure does not know the difference between air coming from an engine or from free stream flow.

The 77F/77L can have either a General Electric GE90-110B1 or GE90-115B.

Do you know if the -115 option was available when the LR came out? Or was it only offered later, after more testing? I'm telling you I'm not making this stuff up. I was trained in it.

Does it not follow logically that the exhaust of the 115 would be stronger than a 110? I can tell you the reason I remember. Its because I didn't believe that the 5,000 lb/th difference would be that impactful.

-115 was only offered later. It was not initially offered on the -200LR.

kalvado wrote:

Well, your definition of flutter please. Other than that it is a problem affecting both civilian and military airplanes, of course. and without referencing "good working understanding of non-applicable equations". Some references may be helpful, though.

You really don't care what is the reason for the flow - engine exhaust, ram air, or geese farting onto oncoming plane. The way I read that "stabilizer in engine exhaust" story is "vibrations due to flutter amplification became unacceptably high in some part of envelope". Totally plausible from my perspective

The definition I presented is correct, and unless you are going to model a simplified 2D system the equations are rather useless. FEA and GVT are required for realistic systems ( at stated in design requirements).

I have done flutter testing on objects ranging from large container ship rudders, small light aircraft, modified airliners for military applications, carriage and stores release on military aircraft, and satellites.

The flutter modes on commercial airliners are well understood, from memory there are a couple of NACA reports or similar that have published these.

I have not seen or read of a jet thrust induced flutter mode on any aircraft, the flutter mode normally solved for under slung engines is a swirl mode of the engine in relation to the wing.

I am aware of thrust induced excessive ground loads resulting in cracks and reduced fatigue life.

I am not aware of any maximum speed reduction on the LR/F to suggest a flutter issue from the 77W.

zeke wrote:

kalvado wrote:

Well, your definition of flutter please. Other than that it is a problem affecting both civilian and military airplanes, of course. and without referencing "good working understanding of non-applicable equations". Some references may be helpful, though.

You really don't care what is the reason for the flow - engine exhaust, ram air, or geese farting onto oncoming plane. The way I read that "stabilizer in engine exhaust" story is "vibrations due to flutter amplification became unacceptably high in some part of envelope". Totally plausible from my perspective

The definition I presented is correct, and unless you are going to model a simplified 2D system the equations are rather useless. FEA and GVT are required for realistic systems ( at stated in design requirements).

I have done flutter testing on objects ranging from large container ship rudders, small light aircraft, modified airliners for military applications, carriage and stores release on military aircraft, and satellites.

The flutter modes on commercial airliners are well understood, from memory there are a couple of NACA reports or similar that have published these.

I have not seen or read of a jet thrust induced flutter mode on any aircraft, the flutter mode normally solved for under slung engines is a swirl mode of the engine in relation to the wing.

I am aware of thrust induced excessive ground loads resulting in cracks and reduced fatigue life.

I am not aware of any maximum speed reduction on the LR/F to suggest a flutter issue from the 77W.

OK, lets try again.
>Flutter is a specific engineering, it means an unstable, self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure.
This is a copy-paste of a textbook, showing no understanding of underlying physics. You will have hard time explaining why spinning windmill or wind howls are not flutter based on that definition.

There are certainly propulsion-induced flutter effects as airflow interaction doesn't care about the source of airflow. three top links from Google:

EFFECT OF THRUST ON BENDING-TORSION FLUTTER OF WING
https://dhodges.gatech.edu/wp-content/u ... lutter.pdf

The Aeroelastic Impact of Engine Thrust and Gyroscopics on Aircraft Flutter Instabilities:
https://core.ac.uk/download/pdf/31022155.pdf

Airplane Propeller-Induced Flutter
https://peimpact.com/airplane-propeller ... d-flutter/

Simulations are usually as good as the understanding of underlying physics. That seems to be a modern plague - simulations are the king, understanding it is on a back burner. Look no further than wingroot issues of 787...

kalvado wrote:

This is a copy-paste of a textbook, showing no understanding of underlying physics.

Correct, it is a textbook definition of flutter. I don’t understand how you can claim it is incorrect.

kalvado wrote:

There are certainly propulsion-induced flutter effects as airflow interaction doesn't care about the source of airflow. three top links from

Sure there are flutter modes associated with propulsion, however the claim made above was the GE90-115 had to be derated to the GE90-110 as flutter was induced on the ELEVATOR on the 77L.

This is clearly false, the 77L was certified with both the GE90-110B1 AND GE90-115B engines on 02.02.06. Refer to the TCDS. https://www.easa.europa.eu/en/downloads/7521/en

zeke wrote:

kalvado wrote:

There are certainly propulsion-induced flutter effects as airflow interaction doesn't care about the source of airflow. three top links from

Sure there are flutter modes associated with propulsion, however the claim made above was the GE90-115 had to be derated to the GE90-110 as flutter was induced on the ELEVATOR on the 77L.

This is clearly false, the 77L was certified with both the GE90-110B1 AND GE90-115B engines on 02.02.06. Refer to the TCDS. https://www.easa.europa.eu/en/downloads/7521/en

A very good point. Although there may still be more to the story like (PURE SPECULATION) structure of stabilizer had to be modified from the previous iteration of 772 to accommodate 115 thrust. Rumors tend to smear such details.

zeke wrote:

kalvado wrote:

This is a copy-paste of a textbook, showing no understanding of underlying physics.

Correct, it is a textbook definition of flutter. I don’t understand how you can claim it is incorrect.

If I had a penny every time I see a poorly written textbook...
This is a formally correct definition, but it is not a good one. It doesn't provide any insight into the aeroelastic part of the process. For one, it doesn't allow you to distinguish flutter from less hazardous vibration. If you add "flutter is when the wing falls off", that will be an incorrect part of it. Such a definition would be good as a first line of a chapter on the topic, expanding "energy extracted from the airstream" part, and very weak without that chapter.

kalvado wrote:

A very good point.

I made the exact same point 4 days ago, “The 77F/77L can have either a General Electric GE90-110B1 or GE90-115B.”

kalvado wrote:

If I had a penny every time I see a poorly written textbook...

If the design regulations had a definition of flutter I would have quoted that, it doesn’t . A textbook definition is what anyone in industry would revert to when interpreting the regulations.

kalvado wrote:

For one, it doesn't allow you to distinguish flutter from less hazardous vibration. If you add "flutter is when the wing falls off", that will be an incorrect part of it.

I didn’t make any statements about wings falling off due to flutter.

I am however aware of accidents that were a result of structural failure of the empennage from ground loads caused by the slipstream. That is fatigue failure, not a flutter failure which is my original point.

zeke wrote:

kalvado wrote:

If I had a penny every time I see a poorly written textbook...

If the design regulations had a definition of flutter I would have quoted that, it doesn’t . A textbook definition is what anyone in industry would revert to when interpreting the regulations.

kalvado wrote:

For one, it doesn't allow you to distinguish flutter from less hazardous vibration. If you add "flutter is when the wing falls off", that will be an incorrect part of it.

I didn’t make any statements about wings falling off due to flutter.

I am however aware of accidents that were a result of structural failure of the empennage from ground loads caused by the slipstream. That is fatigue failure, not a flutter failure which is my original point.

This is a good attitude at equipment operator and technician level, it becomes (became!) disastrous at designer or engineer level.