buffet?
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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.
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.
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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
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
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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
So that's more or less why I asked in case you were wondering.
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
So that's more or less why I asked in case you were wondering.
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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.
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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
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
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Originally Posted by gr8shandini
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....
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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?
Gup? Christiaan?
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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
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
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SNS3Guppy, do you feel like stating your interest? You definitely know what you're talking about.
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As described to me
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.
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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
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
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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.
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.
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Originally Posted by SNS3Guppy
Is exceeding Vmo part of a windshear recovery, as previously suggested? Of course not. It's a ridiculous idea
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 !?
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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
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
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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.
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.
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 ...
