buffet?

[henrypottinger]

No not Warren.
Can anyone explain high speed and low speed buffet for airliners, with reference to the 'red army' on the speed tape and aerodynamically.
Many thanks in advance.

[SNS3Guppy]

Henry,

High speed and low speed buffet are terms which simply refer to the airplane aerodynamic response to certain flight conditions. A low speed buffet is nothing more than a stall buffet; it's based on angle of attack and in it's simplest terms, just like any airplane in training, get it slow enough and it will stall. Bank it steep enough and it will stall. It's just a stall buffet.

As an airplane climbs higher and higher at a constant indicated airspeed, true airspeed increases...the airplane goes faster and faster. This is due to air density. Again, on the most simple level, where there's less air resistance the atmosphere is thinner), the airplane goes faster over the ground in still air while indicating the same airspeed in the cockpit. The airplane must move faster through thinner air to produce the same lift. The airplane must move faster to produce the same pressure to produce the same indication in the pitot tubes which tell the airspeed indicator how fast the airplane is going.

Conversely, if the airplane goes faster and faster (higher true airspeed) for the same indicated airspeed, it also holds true that the airplane flies at a slower and slower indicated airspeed for the same true airspeed (other airspeed descriptions fit better, but become more complicated..such as equivilent airspeed, etc...it's easier right now to think in terms of indicated, and true airspeeds showing how fast the instrument says you're going, and how fast you really are). If you hold a given true airspeed and keep climbing, you'll be flying at a lower and lower number on the airspeed indicator as you climb.

While that relationship is going on, another is important. The upper limiting speed of the airplane is the maximum operating speed, known as Vmo at lower altitudes, and Mmo (mach limiting airspeed) at higher altitudes. On the airspeed indicator Mmo is shown as a barber pole (above which on a tape is the "red army" to which you're referring, though there are a number of ways to display information, ranging from angle of attack indications to tapes to round dials...I'm going to use round dials to keep it simple). As you climb higher and higher the mach limit of the airplane decreases. The barber pole striped needle on the indicator may start out at say, 400 knots indicated airspeed at sea level, but at cruising altitude may indicate a much lower number, such as 230 knots.

At altitude, we cruise in relation to mach airspeed rather than knots indicated airspeed. Mach is simply a measurement of the speed of sound in a given air density...that speed decreases as we climb. Much like indicated airspeed. For our purposes, we really don't care how fast sound goes, but it's a good reference number for us because it measures certain functions of air density...most important for us, mach effects...or in other words, what the atmosphere does for us and doesn't do for us at high altitudes and high airspeeds in relationship to our reference speed, the speed of sound.

Hopefully I'm not confusing you. We cruise at .84 Mach all the time. In our airplane, it's the number we use pretty much everywhere at altitude. .84 mach, or in other words 84% of the speed of sound at any given air density, occurs at a much higher indicated airspeed down low than it does up at cruising altitude. .84 is well below any high speed aerodynamic limitations, and keeps us fast enough that we don't stall while flying level or when turning (remember that your indicated stall speed goes up in the turn, so we want to cruise with a high enough airspeed that we can make a standard turn at altitude and still be comfortably above stall margins.

As we climb higher, the stall speed increases. Air is less dense, the airplane must fly at a higher true airspeed, but lower indicated airspeed, to make the same lift, and at a higher angle of attack. A low speed stall or buffet is a function of angle of attack; at some point you reach an angle of attack, the angle at which the wind meets the wing, when the wing will stall...every bit as much in an airliner as on a paper airplane. The speed at which that occurs straight and level will depend on several factors, (weight, altitude, temperature, bank angle, configuration and power setting, among other things), but represents the lower buffet limit. Just think of it as don't-get-too-slow-or-you'll-stall. That speed gets higher and higher as we climb. For a given weight, the stall margins, or space between the airspeed/mach number at which we're cruising now, will shrink with an increase in altitude. We might have 50 knots to play with here at this altitude, for example, but if we climb higher there might be a 25 knot or 10 knot difference between our cruising airspeed and the low speed buffet or stall. This plays part of the role in calculating our cruising altitude, and the altitudes of which we are capable at any given time based on some of the things we've discussed.

The high speed buffet is a little different. That's a result of what are called "mach effects," or simply put, the effects of going too fast. At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible. At higher altitudes at higher mach numbers, especially transonic numbers between about .75 and 1.2 mach, the way the airflow around the airplane affects us changes dramatically. It's the reason we have swept wings and flying surfaces, and the reason we measure our speed in relation to mach (percentage of the speed of sound); it's a measurement of our relationship to actual mach, which is a measurement of how the air is bunching up and compressing around us.

Air compresses to form shock waves and bow waves which change the way pressure is distriubted in front of, across, and behind the airplane. It changes the way the airplane displaces air, and displaced air is how we create lift. Lift is nothing more than a change in pressure around different parts of the airplane, and can be thought of as more pressure underneath than on top. As we move into higher mach numbers, pressure also builds in front of the airplane and forms a wave of compressed air. This wave moves aft and across the wing, and the properties of that air and the way the wing affects it, and the way it affects the wing, change depending on where it is.

Among those effects are things you've probably read about, such as mach tuck, and of course, high speed buffet. You can think of high speed buffet as a warning that we're going too fast. Just like a low speed buffet being produced by air burbling around the wing and striking the wing, tail and other parts of the airplane in a turbulent, random fashion, a high speed buffet is the airplane starting to react to airflow in a different manner. The drag rises sharply, lift can decrease, control effectiveness can be reduced, the airplane may experience a loss of downward force on the horizontal stabilizer or a change in the center of lift on the wing as the shock wave moves aft across it. It may want to speed up even more or begin losing elevator authority or reach it's trim limits if one goes too far. It may shake, or buffet, or it may display nothing really significant, depending on the airplane, it's weight, etc.

We're given upper and lower buffet limits as an operating range in which we must stay. It provides margins in which we can safely operate without going so fast we're into the mach effects, and without going so slow we stall. Sometimes the buffet margins are referred to as stall margins; a high speed tall and a low speed stall. You can also think of the upper buffet margin as a high speed stall in that the airflow changes about the wing (in some cases drastically) to where aircraft control may be difficult or impossible in some airplanes (if flown fast enough), lift is reduced, control forces may become too light (or in some cases too heavy), etc...and the drag rise is so high it saps all the efficiency out of the flight. You think of a low speed stall as a buffet because of some of the same reasons; the changes in pressure about the wing and airplane change drastically, lift is lost, control is affected, etc. Both can produce a buffet (or in some aircraft no buffet, but other warning devices are installed to replicate the buffet for the pilots in the cockpit).

Operationally, at cruise altitude, data is available in form of buffet margins. I use an old fashioned round dial airspeed indicator. I set one of the little plastic bugs in the window of the airspeed indicator to show me where the low speed buffet margin is at cruise, based on data available to us in the cockpit. I use the barber pole or mach limit as my upper margin...though actual buffeting may take place a little before or after that point.

The higher I climb, the closer those numbers are together. You may have heard the term "coffin corner," which is a place where those two numbers come together, or very close together. You can't slow down because you'll stall, and you can't speed up because of mach effects. You're trapped in a very narrow operating range or speeds. Some aircraft like the U-2 operate at such high altitudes that they literally spend most of their flight in the coffin corner...with only a few knots either way before they're either into mach effects of going too fast, or stalling for going too slow. Hopefully that helps.

[roljoe]

Well, nothing to add...excellent..;)

[ChristiaanJ]

SNS3Guppy,
It may be more than he asked for, but I am impressed by your excellent write-up on the subject.
To the extent that I've saved it all!
I hope you won't mind my plagiarizing some of your text sometime (if the same kind of question crops up) in appreciation of the time you put into writing it.

[SNS3Guppy]

I'm flattered. Be my guest.

[411A]

It may be of interest to some that the early models of the B707 and DC8, those with turbojet engines (rather than turbofans) it was indeed possible to climb to such a high altitude that the wing would no longer be efficient, and the mach buffet could be encountered with some regularity.
This was well above the altitude mandated according to the aircraft weight versus cruise mach buffet onset, and led to early jet upsets...until the effect was clearly understood by line crews who had transistioned from four engine piston types.

[CONF iture]

Thank you Guppy for this great reading.

The high speed buffet ...
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.
Does it mean we could, on an airliner, at low altitude, firewall the thrust levers, without major inconvenience or loss of control, just ignoring the overspeed warnings ?
What kind of speed would be reachable ?

[typhoid]

Well to do so would be to ignore the effects of flutter, and would probably be bad news!

[ChristiaanJ]

CONFiture and typhoid,

You've just found my 'coffin corner' of ignorance.
To paraphrase CONFiture, if I firewall the throttles at low level (clean config, obviously) in a typical modern airliner, what's the envelope wall I'll hit? Is it indeed flutter?

Christian

[SNS3Guppy]

Does it mean we could, on an airliner, at low altitude, firewall the thrust levers, without major inconvenience or loss of control, just ignoring the overspeed warnings ?
What kind of speed would be reachable ?


Where Mmo, or the mach operating limit applies at altitude, of course Vmo applies at lower levels. Flutter may be a consideration, but with non-reversible controls, not so likely.

In order to reach a region where the mach effects of compressibility become a factor at low altitudes you'd have to increase your airspeed beyond Vmo...hence the reason at low altitudes we're IAS limited, and at higher altitudes, mach limited. You can push up the power if you like, but you do have aircraft limitations to consider before you go too far.

[ssg]

Well written...

A note: Not all wings do the same at high speeds...some go right through the Mach region without so much of a bump ( Many Falcons) other are notorious for thier issues right after Vmo (Lears).

I like to joke it would be nice to fly a plane with more engine then wing...

[SNS3Guppy]

I've met a lot of pilots who obtained their type in the LR 20 or 30 series, who believe that somewhere right around Mmo the airplane will begin to experience a sensational aileron buzz and all sorts of mach effects...because they experienced this in the simulator.

The real airplane doesn't do that. In fact, the airplane can exceed Mmo by a considerable margin before any such effects occur, and they're not nearly as pronounced as the sim. The effects experienced in the sim are exaggerated to make a point, but they don't replicate the actions of the airplane at those speeds. Even the mach tuck experienced by the airplane is very minor, and amounts to little more than a lessening of stick force below certification minimums...which is the reason for the puller and mach trim above .74 M.

You're quite correct that the high speed habits of an airplane very considerably with the design.

[ICT_SLB]

There is one hopefully rare time when firewalling the throttles IS required at least on some aircraft. I was roped into taking cockpit video of sim trials of windshear encounters on the CRJ-200 (CL65). The finding there was that, as well as following the Windshear guidance (if set), the best method was to firewall everything immediately as the extra thrust just might make the difference.

[SNS3Guppy]

In a windshear encounter, assuming a microburst type situation, the goal is to avoid ground contact. A windshear recovery maneuver involves emergency thrust and pitching as necessary to avoid the ground, right to the stick shaker if necessary. That would be the low speed regime, or the lower buffet margins.

[CONF iture]

I agree ... but in the same time that tail microburst type situation could as suddenly die or even evolve in an head one, and in the confusion of the moment, the speed red upper band limit could be easily well exceeded ... At least it's a simulator déjà vu scenario.

[SNS3Guppy]

No, in a microburst encounter, if you're experiencing a tailwind, you're in the outflow, and decreasing performance. As you fly out of the shear effect and begin to recover, you're doing so from a very slow condition. If the airspeed is increasing from that point and you're approaching Vmo, then you're certainly not flying the airlplane (it's flying you), and you're far out of danger. Time to either convert that extra thrust into a climb rate or get the power pulled back. Exceeding upper speed limitations during a windshear encounter, unless you were right on the edge of Vmo/Mmo at the outset (not particularly likely...and wouldn't require additional thrust if experiencing an excursion above your speed limits)...you're not going to go there when you're pitching for the shaker.

[Mark1234]

Confiture
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.This doesn't mean the air ISN'T compressible - it is. It's the same air, just a little denser. Lighties just don't fly fast enough to enter into compressibility effects. We refer to Vne (Velocity Not to be Exceeded) beyond which lies flutter (which is an interplay of airframe rigidity and aerodynamics), rather than Mmo / Vmo. VNE is a fixed number, and TAS related rather than IAS. (or so I have been taught...)

It's still possible to experience 'Mach effects'/compressibility at low altitude, but less likely / common for reasons phsical (*generally* the speed of sound reduces with height due to the reducing temperature), and more practical (there are speed limits in place below 10,000ft, though they can be cancelled).

Beyond that, I'm into guessing: but I suspect the most speed limiting factor down low is economics! The fuel penalty in operating fast 'n' low in all that draggy air is probably the main reason why civil a/c don't.

What I would like to understand is why airliners are able to operate to a Vmo which (I believe) is variable with altitude, and can typically fly higher TAS at altitude - which suggests that flutter isn't limiting at altitude for them? Wheras I understand flutter to be an airstream speed issue, NOT a dynamic pressure issue. Or are they simply not flutter limited at any speed / alt? In which case, what does limit them low down, other than economics?

Quite frankly, the rationale behind which some limitations apply to IAS, and some TAS escapes me - I know which I have to watch for various, but not WHY!

[411A]

At lower altitudes, where Vmo is limiting with a jet aeroplane, to then exceed Vmo by some margin exposes the aeroplane to increased dynamic pressures, with resultant difficulties, including the possibility of increased control surface flutter.
Even with turbopropeller and large piston designs, this was possible.
For example.
The DC-6B experienced elevator flutter during flight test when the maximum authorized indicated airspeed was reached or just slightly exceeded.
A fix involved additional hinges installed at the outer ends of each elevator.

In my considered opinion, if a pilot should decide, on their own and for no good reason, to exceed Vmo/Mmo deliberately, would/should be grounds for license suspension....if not ready for the rubber room.:}

[Avionero]

Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).

I frankly don´t know how my aircraft behaves beyond Vmo/Mmo, but I´m not too curious to find out about it either. Maybe there´s no buffet or the like, but I don´t care if I overstress the structure with a slight elevator movement :}

[Chris Scott]

Quote from ssg:
I like to joke it would be nice to fly a plane with more engine then wing...


Hope I’m not misunderstanding your remark, but that is precisely what enables you to go into “coffin corner”. As 411A has reminded us, the turbojets of the 1950s and 60s were at their most efficient at high altitude (unlike turbofans); and capable of powering the wings of DC-8s, B707/720s, CV880/990s, and Comets well up into potential trouble.

When later variants of many of these aeroplanes were fitted with by-pass engines and even turbofans − but retained roughly similar wings − the situation changed. Instead of maximum cruise altitude being dictated by the wing, it was thrust that limited it; i.e., you’re at climb thrust and no longer climbing…

In my experience, the two opposing situations were illustrated by the by-pass-powered VC10 and the turbofan-powered B707-320B/C. On the VC10, when looking for a step climb, we merely looked at the low-speed buffet graphs. There was always enough power to get us up to an altitude where 1·35G would cause low-speed buffet at the current weight and cruise Mach; and even higher, where 1·25G would be enough. Up there, the safe range of speed between low-speed buffet and Mmo was interestingly narrow. 1·25G is equivalent to about 35deg bank in smooth air. This leaves little margin for turbulence in turns. Most crews would avoid it unless the ATC alternative was a fuel-costly descent, and reports indicated it was smooth at the higher level.

On the turbofan 707-320B/C, the situation was reversed. Although the graph included an indication of the buffet margins, these were usually 1·4G or more at attainable altitudes. Thrust was invariably what stopped us climbing, particularly when temperatures were above ISA. So, on [U]that type of 707, there was little risk of reaching the coffin corner of the flight envelope.

[SNS3Guppy]

Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).

I don't believe anybody said anything about a TAS limitation. Limitations are described in CAS in some aircraft, however.

I frankly don´t know how my aircraft behaves beyond Vmo/Mmo, but I´m not too curious to find out about it either. Maybe there´s no buffet or the like, but I don´t care if I overstress the structure with a slight elevator movement


The aircraft isn't going to be overstressed by flight up to Vmo/Mmo. Nor are you going to see adverse flight characteristics at that point. A manufacturer isn't putting a limitation at a point beyond which flight characteristics decay or the aircraft is unsafe.

Structural damage can be done at speeds far below Vmo; structural damage can be done, and has been done, below maneuvering speed. A good example of that was the loss of the Airbus A300 vertical stabilizer on American Airlines Flight 587, out of JFK, in November 2001.

You can break your airplane at low speeds, too.

[CONF iture]

Where Mmo, or the mach operating limit applies at altitude, of course Vmo applies at lower levels. Flutter may be a consideration, but with non-reversible controls, not so likely.
...
You can push up the power if you like, but you do have aircraft limitations to consider before you go too far.

...........................................
To paraphrase CONFiture, if I firewall the throttles at low level (clean config, obviously) in a typical modern airliner, what's the envelope wall I'll hit? Is it indeed flutter?

So ... we don’t really know what’s happening … and what we could encounter ?

But the risk looks serious enough that Airbus put in place the High Speed Protection in order to prevent any uncommanded as well as commanded (!) excessive exceeding ...
Their words still remain a bit vague:
There is an increased potential for aircraft control difficulties and structural concerns, due to high air loads

Should we consider that control difficulties would be reached before structural concerns … ?
If so, where would be that limit in term of exceeding VMO + ?

No pressure to answer that question which is obviously over stretched for a day to day operation.
Just my own curiosity.

[SNS3Guppy]

It's not exactly rocket science. Don't exceed Vmo.

[ssg]

Chris,

Yes that's what I meant...flying in an aircraft that has enough engine to get you into Coffin corner, means your generaly flying a faster aircraft, that would be nice.

Designers that build a plane and set a low Vmo for the stupidist pilot meant to fly it means that the rest of us have to fly around 20/30/40+ kts slower then the plane can handle...inefficient...

I don't pretend to be an aeronautical design expert, but I do believe with the advent of computers around the mid 80s, helping with airflow simulations, ect...we have better wings now then a slide rule designed Lear 24. While I have no experience in that aircraft, my informal survey of Lear pilots on this subject state that high speed characteristics vary from nothing to buzzing, to full on tucks, that have killed some guys...who knows...all this happening in the .82+ region...not very fast...

I do know that if you look at an older Lear, they usualy have more VGs, slall fences, and other add on to make the wing better.

Anyway..

A good point of this is the new Premier One. Swept wing, flies right up to FL410 and does .80 right off the bat. Landing? Nice easy ref of 110 kts, no TRs on board, plane stopped in 2500 ft on my first try without really getting on the brakes. Good low and high speed characteristics. The only swept wing corporate jet that the FAA will let a single pilot loose on.

Just a note: If you can get a hold of some of the flight testing summaries of aircraft modells you might notice some things...

The SJ30, and Citation X for example...tend to roll left before they exhibit any wing overstress or flying issues. Others I am told, certain Citation models will start to vibrate uncontrollably, some are limited by thier windshield...so Vmo many times will not be because of tuck, buzz, or your elevator getting washed out but other things....

Bringing this full circle...the idea that you can fly a plane where nothing limits or happens to you, untill you run out of power or wing, again would be flying an aircraft right up it's max. I like that...

[ChristiaanJ]

It's not exactly rocket science. Don't exceed Vmo.It's done routinely, actually, after a major overhaul, precisely to assure that there still is the margin between Vmo, where the "o" stands for "operating", and the limits established during certification, such as Vne.

My stupid question still stands.

Let's take a 737 or an A320. No passengers, little fuel, sea level, level flight.
I fully open the throttles (without exceeding engine limits).
I ignore any overspeed warnings, and override any system limitations.
You with me?

Now, what limits do I run into? Mach buffet? I doubt it, at sea level. Structural failure? Flutter, as has been suggested here? Or do I simply run out of "puff" anyway? Let me add I do not intend to pull significant 'g' in the process, just do a real high-speed run to the real limit.

Stupid question from an ancient aeronautical engineer, who's done mostly stability and control, but is still interested in the subject.

CJ

[Mark1234]

@SN3:

Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).

I did! (It was a question) - Part 1:

I shouldn't have said 'TAS', as it's not quite that: I'm given to understand Vne as marked on the ASI is predicated on standard temp & pressure, and should be factored for altitude - i.e. the IAS will underread at alt due to density, but VNE is a constant *airstream* speed. Or to put it really simply, cruising along at 20,000ft you could hit flutter while the ASI was well short of the redline.

Part 2:

I gather big tin generally doesn't have Vne, but Vmo - am I correct in thinking that varies / is varied with altitude. If so, how does that work - presumably it's driven by factors different to Vne/flutter (not just airstream speed).

[411A]

Let's take a 737 or an A320. No passengers, little fuel, sea level, level flight.
I fully open the throttles (without exceeding engine limits).
I ignore any overspeed warnings, and override any system limitations.
You with me?


Should you find a bird at these low levels and high speeds, you might better perhaps rethink your actions.

In addition, if Vmo was exceeded at low levels with a more classic swept-wing jet aeroplane, the B707, you would rapidly notice increased nose-up trim (mach trimmer action) that would soon run out of stabilizer authority...bad, very bad.

[SNS3Guppy]

Now, what limits do I run into? Mach buffet? I doubt it, at sea level. Structural failure? Flutter, as has been suggested here? Or do I simply run out of "puff" anyway? Let me add I do not intend to pull significant 'g' in the process, just do a real high-speed run to the real limit.


Transport category aircraft certificated under Part 25 do not require a Vne speed, as Part 25 doesn't require establishing Vne. Part 25 requires that up there be no adverse characteristics up to Vmo/Mmo, and above that speed up to the maximum design speeds, no adverse characteristics that are not easily handled by the crew in recovering back to Vmo/Mmo. What will happen if you accelerate through Vmo at low level? Nothing. The specific characteristics that will be encounterd depend, as has been repeatedly stated thus far, on the specific aircraft and configuration.

You want to know what's going to happen if you push the power up and fly to the maximum design speed? Nothing.

Will you get buffeting? Possibly. Depends on the design. Depends on the configuration. Depends on the loading and other factors. Will you run out of trim? No, not up to the design speed, but again, this will really depend on the airplane, how it's configured, loaded, etc.

Does the airplane have the power to reach that speed in level flight? Depends on the airplane and it's loading. It might. Some aircraft do, some don't.

Is exceeding Vmo part of a windshear recovery, as previously suggested? Of course not. It's a ridiculous idea.

Do manufacturers and repair facilities routinely push the airplane to it's design limit speeds to see if the old gal can still do it? No.

While I have no experience in that aircraft, my informal survey of Lear pilots on this subject state that high speed characteristics vary from nothing to buzzing, to full on tucks, that have killed some guys...who knows...all this happening in the .82+ region...not very fast...


Sim warriors. Those characteristics just don't exist in the Lear, especially at .82 Mach. Pete Reynolds, a test pilot for Lear, wrote a good paper on the subject, in which he described taking the airplanes to speeds well beyond those numbers with no adverse characterstics. I've heard the stories too, mostly from pilots who saw it happen in the sim and assumed it happens in the airplane...it doesn't. I've had the airplanes to speeds well above .82, and there are no bad habits lurking there. Even with the earlier wings.

I recently spent time in a LR35A that not only was modified with pylons on the wings and hardpoints, but flying it configured with external stores and gear throughout it's flight envelope, including frequent flights in turns to the shaker with all ranges of AoA explored...and still no bad habits...despite plenty of ways to disrupt airflow over the wing.

The stories of ailerons causing the flight controls to slam from side to side, the terrible dutch roll charactersitics at high altitude following a yaw damper failure, and so on...all untrue. The mach tuck characteristics are so mild that the solution for it is mach trim...slight nose up trim. The only reason it's a concern at all is that the controls become light enough that they don't quite meet the Part 25 requirements for stick pressure...but it's fully manageable, fully controllable...all myth floating around out there. How many of those pilots that you questioned have actually experienced those effects in the airplane...and how many had this first hand knowledge from their sim experience? You're not going to find pilots who experienced those problems in the airplane, because they're sim-isms.

I'm given to understand Vne as marked on the ASI is predicated on standard temp & pressure, and should be factored for altitude - i.e. the IAS will underread at alt due to density, but VNE is a constant *airstream* speed. Or to put it really simply, cruising along at 20,000ft you could hit flutter while the ASI was well short of the redline.


No, not really. Flutter isn't a function of actual speed, it's a function of air density, because it's a function of the atmosphere acting on the control surface or flying surface. The surface has no idea how fast it's actually going. That at a higher density altitude the TAS may be higher has no bearing on fluter issues.

IAS is an imperfect medium through which to consider flight characteristics. Corrected for position error, CAS is often used as a reference in charting and data, but is also inadequate to consider the aerodynamic effects on the airplane, and should be discussed in terms of Equivilent Air Speed, or EAS. To simply our use in the cockpit, and discounting minor error excursions, we stick with Indicated Air Speed (IAS).

If the mass airflow around the wing and it's relevant pressure distributions indeed account for flutter, we can measure it in flight with indicated airspeed. If indicated airspeed drops, we move farther from flutter, or in other words, create a bigger margin. It's air density that determines the speed necessary to produce the same impact force in the pitot tube, and it's the same density that determins flutter. Airspeed decreases, flutter margins increase.

Again, this is simplification, but simply because density altitude has increased does not imply that flutter is achieved at speeds less than Vne. Moreover, whereas Vne is established with a margin already built in, it's just not going to happen unless control balance has been changed or other changes are made to the wing or control system which decrease susceptability to flutter to a lower speed.

Some elements of flutter do occur at lesser speeds, but are addressed by airframe limitations. In the B747 Classic, for example, a limitation exists regarding reserve fuel between FL290 and FL340, with reserve fuel on board, related to speed characteristics. This speed limitation does not exist at other altitudes or when the aircraft weight is outside the parameters of the limitation, or if the additional reserve tanks are not filled. The issue is addressed by the manufacturer.

One would need to differentiate between control flutters or airframe flutters...and we could diverge off on all kinds of tangents. I don't believe the original questions intended that, however, nor are they particularly relevant to the central question of what we experience when accelerating in level flight to a particular design speed. In the range of operating speeds we're going to see, within prescribed limitations for the aircraft we fly, we're not going to encounter adverse characteristics.

The question was asked about accelerating to Vmo on engine thrust...are we going to run into some kind of wall or limiting flight characteristics? No. We're either going to run out of excess thrust as the drag rise rapidly exceeds available thrust and thus stop accelerating, or we're going to hit airspeed limitations and need to reduce the thrust in order to maintain them. This may be complicated by maneuvering and loading the airplane/increasing angle of attack, but then we're back to buffet margins...and even at the upper speed limits at low altitude we're exploring the lower buffet margins when we either operate at critical AoA's...we've simply raised the buffet margin to a higher number. We're not touching upper buffet margins...the mach limits of the airplane aren't usually attainable because they're far above the IAS limitations at lower levels.

[gr8shandini]

I think what's confusing some of the folks here is the fact that Vmo and Mmo are usually independent of one another. Vmo keeps you out of harm's way structurally (either flutter or just plain air loads) and while Mmo protects from adverse Mach effects.

Vmo and Vne are essentially the same thing. One's used on jets, the other on props and helos. Why the difference? I don't know, ask the FAA. It's in IAS because flutter depends (among other things) on dynamic pressure. However, it's not where the bad stuff happens, that's called Vd (for V "dive") and the aircraft has to be demonstrated to be structurally sound out to that speed. Then a series of upset maneuvers are flown to ensure that a pilot flying at Vmo will not exceed Vd due to inattention, disorientation, wind gusts, etc. And despite what was stated earlier, Vmo cannot be legally exceeded for a maintenance test flight (unless it's specifically stated in the flight manual, which I doubt any manufacturer would do).

Mmo is much the same, but it's based on Mach, of course. The aircraft must be demonstrated to be safe out to Md and Mmo is determined with the same upset manuevers. The difference is that handling qualities can also come into play where Mach effects are concerned, so Md may or may not be a structural limit.

And finally, I'm not sure where the concept of an "airstream" speed came from, but that would refer to TAS, and doesn't really come into play unless you're using it to calculate Mach.

I hope that clears up some of the confusion.

[gr8shandini]

Guppy, I guess you beat me to the punch while I was typing. I agree with what you're saying except for one small thing:

"You want to know what's going to happen if you push the power up and fly to the maximum design speed? Nothing."

Probably true, but keep in mind that Vd is demonstrated on a relatively new aircraft with control surfaces rigged to production tolerances (the extremes, to be sure, but in tolerance nonetheless). If one were to try the same on a bird that's been in the fleet for 20 years, there's no guarantee that you wouldn't encounter flutter at a lower speed due to mis-rigging, excessive freeplay, and reduced airframe rigidity due to fatigue.

Not to say that anyone here is foolish enough to try it, but I'd still hate to have the impression that exceeding Vmo is no big deal.

[SNS3Guppy]

Not to say that anyone here is foolish enough to try it, but I'd still hate to have the impression that exceeding Vmo is no big deal.


That's why I previously specifically stated that one shouldn't exceed that speed. I also specifically pointed out that an airplane can be damaged at speeds well below other design speeds, such as maneuvering speed. Whereas pilots are often instructed that the airframe will experience a stall before design limits are exceeded when full control applications are made below that speed (which in turn is well below Vmo, for example)...it's not true. You can break the airplane at speeds well below Vmo or even Va.

So far as control surfaces go, very specific direction is provided for maintenance purposes that spells out tolerances for play in control runs, positioning of controls, control alignment, and control balance.

If the aircraft is properly maintained, flutter should not be an issue.

If the aircraft is improperly maintained, then all other discussions are immaterial. Excess freeplay in control runs or hinges, etc are items which make the aircraft unairworthy. Any discourse on what might be expected of an unairworthy airplane is pointless. It should be expected not to fly until returned to an airworthy condition in fully compliance with applicable approved publications.

[ssg]

Guppy,

You bring up a good point...seems that many pilots seem to want to dramatize stories that they heard...just based on what I know about how planes are tested, seems very illogical that 5 kts over Vmo a Lear would have problems..

In fact...when testing the Premier one, F5 in tow...the number they tried to hit to certifiy for Vmo was 50 kts over...then do some kindvof of a pullup maneuver, with a specified number of Gs. So your right that Vmo testing has to do with recovery back to Vmo, most people don't know that. :)

What I have wondered in old aircraft, that once certified 50 kts over Vmo, ten thousand hours later, would come apart or do something funny at a lower speed. Again, sage advice, don't exceed Vmo.

I have always felt that the numbers aside, the pilots should know the numbers that aren't in the book, that the plane could fly to should emergency situation dicate. I wouldn't recommend it but if you were flying along and someone decided to start dogfighting you, going 50kts past Vmo to get down might not seem such a stretch at that point...

Which brings up a thought..

Does anyboy remember that movie where a plane crashed in the jungle, canibals all around, and the pilot said he couldn't take everyone, because of the reduced thrust(broken plane), and runway requirements. So he came up with the max passenger load...(those were pilots back them) It was a little bit like flight of the Pheonix, except they were in the jungle...it was like a single engine takeoff on a twin or something...so they left the bad guy to get eaten...lol

[slip and turn]

SNS3Guppy, just wanted to add my praise too - that was beautifully clear imparting of knowledge. Thank you. You are a born communicator. If you write books they'll be bestsellers!

I can see we are all queueing up to ask questions here, but ever since I learned a bit about coffin corner I've wondered how big a deal it is in say a 737NG if you drop out one side or the other inadvertently? Maybe you encounter Clear Air Turbulence unexpectedly while you are up there high riding a jetstream with a range of 30 or 40 knots to play with ...

Is the 737NG forgiving, much like most training aircraft don't bite at the onset of stall?

I remember chatting to some old military guys who flew the heavy rear engined stuff and when they talked of 'coffin corner' they knew they had to go there sometimes but they treated it like real bandit country, because if they dropped out tail heavy, they might never escape from a deep stall (?) which I understood to be a taildown plummet due to the mass of the engines.

How much of a deal is dropping out one side or the other thesedays on modern transport aircraft types?

[ChristiaanJ]

slip and turn,
Can't answer all yours, but in brief.

A "deep stall" had nothing to do with the engines, but it happened on T-tailed aircraft, when an excessive pitch-up led to the stalled turbulent flow from the wings totally "blanketing" the elevators, so that the aircraft refused to pitch down even with full down elevator, and just went in nose-up with little forward speed but a very high descent rate, and totally uncontrollable.

The 'hint' about the engines is not totally irrelevant, because most of the T-tails involved in such accidents had rear-mounted engines.

CJ

[gr8shandini]

Although it's more common on T-tails, you don't necessarily need one to have a deep stall, but you do need an aft CG (e.g. F-16). Same description, though, airplaine sits relatively level or nose high and just drops like a rock. It's not an automatic death sentence, though. In the F-16 you can pitch-buck out (if you're good) and on other types, you may have enough lateral or directional control to enter a spin and recover from there.

One more interesting thing about the "coffin corner"; I've heard that the U-2 can actually get so far into it that in a turn you can actually get stall buffet on the inboard wing and Mach buffet on the outboard. Don't know if it's true or not, but it sure sounds like a scary place to be.

[SNS3Guppy]

I can't really address the flight characteristics of a 737NG, but many aircraft lack performance capability to put themselves high enough to be in coffin corner...truly in coffin corner (where any increase in speed will cause a high speed buffet, and any decrease will cause a stall). Margins do decrease as one climbs higher, but no airliner or business aircraft is operating on a razor edge at altitude.

If an encounter with turbulence occurs which causes airspeed excursions such that buffet margins are encountered, then one maintains attitide, accepts altitude and airspeed excursions, and responds to any departures from controlled flight. This doesn't mean one lets control get away, but if one reaches a low speed condition to the point of a stall, then one reacts to the stall with a stall recovery. If one experiences a high speed upset to the point that adverse mach effects are encountered, then one performs a high speed upset recovery. These are standard recoveries that are part of any normal training regime.

Minor excursions require minor corrections in most cases; perhaps just pulling the power back slightly in the event of an overspeed condition, for example. Large excursions, particularly attitude and pronounced airspeed losses or increases, require bigger corrections.

So far as older aircraft...as an airplane ages, it's not granted greater tolerances or more latitude in what's allowed to slide by. When the aircraft rolls out the factory door, it's handed over with maintenance publications spelling out exactly what cable tensions, control rod end play, system limits, clearances, etc, are acceptable. At no time, even 10, 20 or 30 years later, or 80,000 hours later, can the airplane be operated outside those tolerances...so regardless of the hard life and age of the airplane, it's still got to be maintained within the same tolerances that it had the day it was born.

Low speed buffet is tempered by early warning using horns, airspeed indications, angle of attack indicators, stick shakers, and finally pushers...these occur before the critical angle of attack is reached. Some respond to trends in airspeed and angle of attack, others respond only to set A0A in degrees. Each system resets based on aircraft configuration (the stall speed and A0A changes with the deployment of flaps and slats or other leading edge devices, etc...and the aircraft warning systems change to reflect this based on configuration. What all of this has in common is that it warns of what's coming before adverse flight characteristics are encountered.

Deep stall characteristics aren't particular only to T-tailed aircraft. Control authority may be degraded with other horizontal stab configurations, too. Additionally, the use of power to aid in recovery or to slow the decay through increasing angle of attack may have little effect against the drag rise or degraded control authority. This is all really another subject entirely, as it's a condition that's departed well beyond buffet margins. However, allowing most large aircraft, in fact many turbojet airplanes, to degrade well into a stalled condition can put the airplane in a state where recovery may be difficult, very prolonged, or even impossible. The whole idea is to never let the aircraft go that far. The Lear has been used as an example several times now; a protracted stall maneuver in a LR35A may take 10,000' or more for recovery, where as a stall to the shaker or pusher under normal circumstances may result in little or no altitude loss if dealt with properly.

One doesn't simply "drop out" of flight when approaching buffet margins. These are points where adequate warning is given well before adverse conditions occur. An aircraft which is allowed to fly beyond these points by substantial degrees can be brought to a state where adverse flight characteristics can take place.

[slip and turn]

Thanks SNS3Guppy, as I don't plan on getting on any Tu154s or VC10s anytime soon, I can loosen my safety belt a couple of extra notches now in the low-cost cruise, :ok: ... or can I ? deep stall not just T tails you say ... :\

I note what you say about adequate warnings of buffet. But surely the main point about 'coffin corner', if as you say, your aircraft has the performance capability to take you there, and you let it, is that you are potentially boxed in a bit of a corner with potentially no speed range for manoeuvre (as with the U2 example in the extreme) ... frying pan or fire?

I guess what you are saying is that the graph plots of the onset of buffet define the corner, but that after onset there is a further unplotted area of 'forgiveness' in which recovery can be effected without severe upset?

Let me read it again ...

[SNS3Guppy]

I guess what you are saying is that the graph plots of the onset of buffet define the corner, but that after onset there is a further unplotted area of 'forgiveness' in which recovery can be effected without severe upset?


I'm not sure where you're getting that, but no. You seem to be implying that once buffet starts, progressing farther into that region the effects go away. Not the case. Generally as the low or high speed effects begin, any further decay or increase in airspeed will make the effects more pronounced. The exception will be aircraft utilizing artificial means to create the effects, such as a shaker; the aircraft itself may not actually be experiencing any effects at this stage, but rather the shaker may be serving as a warning several knots (or a degree of AoA) in advance of the forthcoming stall.

One may not experience any actually buffeting or stall at the airspeed operating limits, or one may. Where mach or stall effects are encountered, however, you're not going to see them as a brief occurrence that goes away as one progresses beyond the buffet boundaries.

[ChristiaanJ]

SNS3Guppy,
I would think slip and turn is not implying that it goes away, but simply that there is a zone beyond the onset of buffet, where it is mild enough to allow standard recovery procedures, i.e., the onset of buffet does not mean instant loss of control.

[slip and turn]

First, SNS3Guppy, continued thanks for indulging in answering my questions.

What I was getting at (actually still well confused about - be glad I don't fly the things! :p) was your earlier emphasis / correction on my comment about 'dropping out' of the corner.

Actually let me quote you:One doesn't simply "drop out" of flight when approaching buffet margins. These are points where adequate warning is given well before adverse conditions occur. An aircraft which is allowed to fly beyond these points by substantial degrees can be brought to a state where adverse flight characteristics can take place.

I took that (wrongly?) to imply that in many modern aircraft, coffin corner was not the 'bandit country' that it once was or still is on more sensitive types.

But perhaps what you are really saying is to pilots of all types capable of getting up there, is that a healthy degree of respect for these numbers is imperative, because they are the numbers, and can be ignored only at peril of untold severe recovery problems (adverse flight characteristics)?

And yes, thanks ChristiaanJ, or maybe no, not quite. I now read that if you are indeed unfortunate or reckless enough to get into the adverse stuff, which includes the buffet itself, then far from 'going away' you'd better expect things to turn violently and shake you by the throat until you win, or die trying?

Or is the onset of buffet a bit like a narrow second chance zone like the onset of stall in a light trainer - not a place you should ever plan to be, but demonstrably recoverable in standard ways within the slightly broadened speed range that includes low speed and high speed buffet?

Which is it SNS3Guppy? Or have I completely lost myself?

[SNS3Guppy]

Again, what constitutes "buffet" will vary from aircraft to aircraft. I don't think it would be fair or wise to characterize the buffet boundaries as safety zones or to make assumptions regarding what one may encounter beyond, or the ease of recovery. While certainly one may be able to get slow and recover, it may require considerable altitude loss to do so (many thousands of feet, for example). In RVSM airspace with little margin of separation between aircraft, the recovery alone could present a substantial hazard not only for the slow aircraft, but for other traffic as well.

You brought up the example of a light airplane getting slow. You may find in one airplane that as the airplane slows a buffet occurs. You may get vibration from the horizontal stab, you may get rattling of fuselage panels, you may get a clean stall break, you may get a rapid break with an unpredictable wing drop...really depends on the aircraft. An accelerated stall can produce more rapid results with unusual attitudes, etc...again, depending on the airplane, the degree coordination, etc. The buffet margins at altitude can be thought of as an accelerated stall at higher airspeeds and with less ease of recovery, and less physical feel or warning. Where the airplane may stall at 100 knots at a lower altitude, the buffet margin for slow flight may be elevated to 180 knots in cruise.

One should not think of aircraft limitations as points that can be exceeded, and any efforts when beyond those points should only be directed toward recovery. In the case of the early 20 series Lears, the "go fast" switch that many operators installed inhibited mach warnings, allowing the aircraft to be flown at much faster speeds. This was certainly possible in the 20 series airplanes; they had plenty fo thrust, and in times past, was a regular thing for many operators. It's not necessarily a wise thing. Regardless of whether a particular airplane experiences obvious effects when approaching boundaries, one shouldn't make the assumption that any buffer exists with which to pass beyond those boundaries. Nor in normal operations is there any reason to do so.

As we go faster and faster we experience a substantial drag rise, and conversely as we go slower and slower, the same occurs. There's really no reason operationally to go there in revenue operations. Whereas recovery may be possible (or may not, depending on where one has taken the aircraft), it may eat substantial altitude and presen tadditional hazards such as airframe loading or damage which are only indirectly related to the issue of airspeed or the buffet boundary itself.

[ChristiaanJ]

slip and turn,
This topic started off discussing stall buffet and Mach buffet separately.
I meant to say that if you run into one OR the other, you do not suddenly go from no buffet to a buffet that shakes your plane apart...

Stall buffet is a warning you're approaching the stall, slow down a bit more and you DO stall. What happens then depends totally on the aircraft. Some stall docilely, some stall viciously.
Mach buffet is a bit the same thing. Ignore it and continue accelerating, and things will start shaking more, and at some point you may well get control problems.

Now the "coffin corner" is the height/speed combination where both these problems come together. Increase your speed: Mach buffet. Decrease your speed: stall buffet. Something you may be able to handle separately , but not necessarily in combination.

That's why it IS "bandit country", for two reasons.
* When you start trying to get out of it, it's easy to overcontrol, and get yourself really deep in the soft brown matter at the other end.
* Things like clear air turbulence, or the turbulence on the edge of tropical storms, may well suddenly and viciously fling you deep into one of the "forbidden" zones.

CJ

[slip and turn]

Thanks again, you two.

The reason I specifically asked about 737NG is because I fly (down the back) in them rather more than I care to admit and I believe there has been a tendency with my preferred carrier over the last year or two to report in their little captains announcements that we are at 38000 and a few times even higher I think more often than in the past, despite me looking round the cabin and thinkng well hey, we're pretty full!

And whilst sitting high up there nice and pretty and stable in very smooth air, perhaps in pursuit of a jetstream or trying to cruise above one, I have many times felt us go through a patch of slight clear air turbulence or buffet and I've often thought I wonder how near the top corner we actually are, because I know we are well above the weather so that just leaves CAT and other things that I am just not sure about.

It seems to me my chosen carrier has also been achieving regularly shorter airborne times on my preferred routes. Where it used to be 1:20, 1:25 on a particular route, it is rarely so long now - just a feeling - I know that various and delayed approaches at the busy end of the route can affect the airborne times tremendously.

Either the aircrews or the ATCOs or both seem to be getting very good at what they do.

Maybe I hadn't noticed, and load factors and other changed practices have brought the weight down, so 38000 is an obvious place to be.

Maybe I am just plain wrong with my "feelings".

But sometimes right up there, I've felt a little uncomfortable. Maybe its just the cosmic rays that have finally got to me :O

So that's more or less why I asked in case you were wondering.

[gr8shandini]

Ahh, I see. I'm reasonably certain that what you're feeling isn't any sort of buffet. It's one of those things you'll know when you see it. When I worked on C-5, we did a few stall flights. By the time the pusher actually fired, the buffet was so bad that we couldn't even read the airspeed tapes.

[ChristiaanJ]

slip and turn,
You sound like an SLF who knows what he's talking about, and would like to understand more.

In my case you're talking to an ancient...
Concorde flight test support engineer, who's kept an interest (engineer in the sense of university graduate,not maintenace engineer or flight engineer).

SNS3Guppy, do you feel like stating your interest? You definitely know what you're talking about. I just play second fiddle.

CJ

[ChristiaanJ]

Ahh, I see. I'm reasonably certain that what you're feeling isn't any sort of buffet,
I have the impression slip and turn is not talking about what he's felt,but understands about CAT and about the coffin corner, and was wondering how close they were to it....

[airfoilmod]

I think, for me, utilizing A/C examples is instructive. The narrow band of flight between Stall and Stall is exquisitely interesting. Two types come to mind (Essentially the same A/C with different power and wingsets). Both are vicious in the Stall, and virtually unrecoverable. The F-104 and the U-2. Lockheed (Kellly Johnson) designs. Nearly 60 years old and still flying. The U-2 cruises with 2-3 knots separating Vmo and Vne.
Gup? Christiaan?

[ChristiaanJ]

airfoilmod,
Stall-wise, I agree about the huge differences between different a/c.
Some will rumble, then buffet, then just drop their nose and recover almost by themselves.
At the other extreme of the spectrum, some will give no warning whatsoever, and need the full paraphernalia of stick shaker, stick pusher and what have you, to stop them from getting into that domain in the first place.

I'll have to dig out my F-104 flight manual, but IIRC the F-104 didn't stall properly at all. Beyond a certain alpha it just pitched up to such an extent, that it was nearly unrecoverable, unless it happened high enough.

Same applies to most deltas. When mistreated they don't stall in the conventional sense at all, just end up in a very high pitch-up attitude on the back of the drag curve, which tends to end in a hole in the ground....

CJ

[slip and turn]

SNS3Guppy, do you feel like stating your interest? You definitely know what you're talking about.Well a Guppy driver would I imagine be especially alive to exactly how and where all the air goes over/under/around such a bulbous fuselage, with the almost incidental-looking wing, and the modified tail in every conceivable scenario, because I'll be blowed if I could guess what the smoke patterns looked like in the wind tunnel when they took those fat fish off the drawing board without an awful lot of study :O

[airfoilmod]

The Starfighter was best described as being mostly in ballistic, rather than aeronautic flight at any given time. That explains its reluctance to drop its nose at departure (from flight). Why should it? There's nothing holding the tail up, it's dropping faster than the nose, the cg is aft, very aft, when the stubby wings pay off. The dinky H/S isn't any help, and the Rudder is also "useless". At that point the pilot's prayer was, "please, please Spin". If you could spin it, you had a chance at recovery, though slim. The best plan was to exit through the floor of the cockpit. The Pilot ejected Downward in the F-104. A Backward inverted spin is better than Tumbling? I'll just punch out, thank you.

[ChristiaanJ]

airfoilmod,
I have a very soft spot for the F-104, having been involved with it personally in my student days. (Yes, I'm an ancient.)

"Flies like an angel, glides like a brick" describes it very well.

But I think it was only the A model that you puched out through the bottom. The G certainly had a better arrangement.

CJ

[airfoilmod]

Having flown that most beautiful Bird, do you still have the tattoo on the inside of both your eyelids?

"Speed is Life"

[SNS3Guppy]

I don't fly the 737NG, so can't comment based on personal experience with the aircraft. However, no company or crew is going to take you to a position which is unsafe in your travel. You may find that some days the crew is flying at lower levels (say for example, FL330), and another higher altitudes (FL390, for example). The difference will depend on what the aircraft is capable of doing on that particular day based on weight and outside air temperature at altitude, and the decision as to what level to take will also depend on traffic, winds aloft, and turbulence.

Temperatues at altitude can vary depending on the weather systems and airmass movements, from very cold to much warmer than standard, and this in turnd has a large bearing on how high the airplane can climb and where the optimum altitude will be for any given weight.

A turbine engine is fairly inefficient at low altitudes, especially at slower speeds. the most efficient operating RPM for a turbine engine is at the upper RPM speeds, typically about 90-95% of it's maximum operating speed. At slower speeds the engine is "lugging," much like what you get trying to go up a hill in your car in the wrong gear...it's just not efficient. The most efficient state for thrust in the jet comes when the forward speed equals the exhaust gas velocity. When the exhaust gas velocity is faster than the forward speed, you have something akin to the slippage you experience when driving a car or truck in mud or snow; the wheel is spinning fast, but not doing a lot of work in moving the car. Same thing with jet exhaust.

For a given weight and temperature, the airplane will have an optimimum altitude when aerodynamic performance is balanced against engine performance to give the best of both worlds, or as best as can be expected. Different companies and different aircraft use differnt profiles, but flight at the optimum altitude is ideal. On longer flights, often as the trip progresses the aircraft will climb, doing so in "steps" each hour or so. The aircraft is climbed to take advantage of the new optimum altitude based on weight and temperature, and often for wind, as well. This may account for the different altitudes you experience on your journeys over the same route. Some days lower, some days higher. Outside air temperature plays a big role in that, as to other factors.

When you're at the optimum altitude, you have adequate margins between your cruise speed and the buffet. You're not really in "coffin corner." Typical airline aircraft today don't experience horrendous mach tuck performance or other such bad habits...most have been designed out of the airplane. Excursions up to Mmo (the red barber pole on the airspeed indicator, previously discussed), or temporary dips in speed, won't cause large concerns. What's usually required is a slight power increase or decrease, and this is often handled without any input from the pilots at all, by the autothrottles as a function of various aircraft systems or devices.

What you're feeling in the passenger compartment isn't buffeting, but the effects of some turbulence, perhaps most often associated with windshear along the jet stream boundary layers. This occurs where fast moving air of the jet conflicts with slow moving air, much like the eddys you see in a stream where the faster moving water passes slower moving water or rocks. The little whirls on the surface of the water are akin to the same thing that causes the bumps you feel.

You may notice that those bumps are primarily vertical...you're being bounced up and down. Rather than other types of turbulent behavior (such as dutch rolling tendencies or any fishtailing or shaking), the vertical bumps are really more of the wings flexing slightly and damping out the turbulence.

It may be well to remember also that in the event a momentary windshear condition causes a loss of airspeed, the airplane still has mass, momentum, and inertia...it's not going to simply stop flying and fall out of the sky. While airspeed may change briefly and temporarily, the airplane continues to fly just as it did before, and when the shear condition ends, as it must in short order, the aircraft will be right where it was before with no significant changes. The aircraft energy doesn't really change that much, just a temporary change in local atmospheric conditions and pressures about the airplane, and not enough to cause any significant concern.

You should also remember that with power constant and no trim changes, the airplane is stable in cruise and will attempt to return to it's stable, trimmed condition on it's own, even without asistance from the autopilot, auto throttles, and other devices. The airplane is stable, and wants to stay that way.

[CONF iture]

Is exceeding Vmo part of a windshear recovery, as previously suggested? Of course not. It's a ridiculous idea
You might have rush your reading as I didn’t mention VMO but rather "speed red upper band" which identifies not only VMO but also the maximum speed for each flap configuration, at least on a moving speed scale.
To exceed VFE during windshear recovery can be very easy, actually I’ve done that myself during sim exercise, am I below average (?) … that’s not impossible.

But let me go back to my earlier question regarding VMO exceeding at low altitude.
I know that Airbuses have that reputation to be somehow less robust than Boeing products but I’m still amazed that Airbus thinks a few knots exceeding over VMO is serious enough to take over the control from a distracted (?) pilot.

A few years back some Boeings, at pretty level flight or at least far from a dive, hit very precisely some targets. Obviously these birds didn’t experience any kind of control issue.
The Story says these birds were an astonishing 100 knots above VMO !?

[SNS3Guppy]

I don't know what it is to which you're referring...that was six months ago.

Whatever you're referring to, I'm sure you're probably right.

[ft]

For TAS limits, there rarely are any as the required limits are covered by the IAS/Mach limits. However, the onset of flutter is by and large dependant on TAS.

One situation where this can become all too apparent is gliders flying in mountain waves. At the altitudes reached, the flutter TAS can easily be reached below the limiting IAS which, at for the aircraft type more normal altitudes, would normally prevent getting anywhere near flutter onset.

Those long slender wings can harbor a lot of interesting flutter modes, so glider pilots should be aware of their TAS.

As for what happens if you firewall the throttles at low altitude in a jet, I'd say flutter is unlikely to be the first problem you encounter. Your TAS will not be all that high after all. The drag caused by the dynamic pressures encountered will limit your acceleration.

However, the same drag due to dynamic pressure and the same forces which limit your acceleration will have an adverse effect on the structure of your aircraft. To put it another way, you may start shedding antennas, windshield wipers, landing gear doors, control surfaces and aerodynamic surfaces, probably in approximately that order.

In case this was posted earlier in the thread and I missed it, my apologies.

Rgds,
/Fred

[CONF iture]

As for what happens if you firewall the throttles at low altitude in a jet, I'd say flutter is unlikely to be the first problem you encounter. Your TAS will not be all that high after all. The drag caused by the dynamic pressures encountered will limit your acceleration.

However, the same drag due to dynamic pressure and the same forces which limit your acceleration will have an adverse effect on the structure of your aircraft. To put it another way, you may start shedding antennas, windshield wipers, landing gear doors, control surfaces and aerodynamic surfaces, probably in approximately that order.
Thanks for your input Fred.
Some earlier comments mention that flutter could be the first symptom, some say that nothing would happen really except that acceleration would reach a maximum ...
I just don't know the answer to that question and I can see there is no consensus either on this Professional Pilots Forum.
What could be that maximum speed ?
... No clear cut answer !
All I can say is Airbus does not tolerate any VMO exceeding, there must be a reason why ?
Also, and it is just a neutral observation, 9/11 was not possible the way the Story says it happened by using NG Airbuses ...