Indicated Stall Speed
 

Just wanted to see what you guys think.
As I understand it, Indicated stall speed changes only with load factor, weight, and, power (available).....................

Does indicated stall speed change with altitude?
I cant seem to find any clear answers.
Thanks guys......and gals

No, indicated stall speed should remain the same.

If you have a multi engine rating refer to your Vmc vs Stall speed diagram, should answer the question in graphical terms.

Thank you sir!

Assuming fixed GW, CG, and AOA...

At higher density altitude:

1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.

2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.

Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.


Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.

Good info! Thanks Rick

From what I remember.....indicated stall speed will remain constant, however true airspeed must increase with a increase in altitude.

Folks, I hate to disagree with all you guys, but this issue is pretty important so I think we need to correct this before MORE people get incorrect information.

Stall speed INCREASES with altitude. Whether you look at IAS, CAS, EAS or TAS, they ALL increase with altitude.

Since PFDs display CAS (not IAS), and TAS has no importance whatsoever in the stall margin, we should only really talk about CAS and EAS.

EAS is really the airspeed that we need to be concerned about when relating airspeed to stall margin, and the stall speed in EAS increases with altitude.
A good rule of thumb is the 2kts/5000ft rule, but at cruise altitudes the increase is even greater than this approximation and varies depending on aircraft type.
The CAS stall speed will increase by an even greater degree due to compressibility effects... and since this is what we see on the PFD, we should keep and even greater "buffer".


I urge everyone to review some good aerodynamics or performance books because IMHO there should be no confusion whosoever over such a basic and fundamental aerodynamic principle. High altitude aerodynamics must be one of the most poorly understood areas of flight training and unfortunately it is not taught enough (FAA requires it for a commercial but applicants are seldom quizzed on it....and my experience is that regional airlines do a very poor job at teaching it).
Unfortunately, there are WAY to many books that contain incorrect or in some cases outdated information, so be careful about what you trust.

Long story short -> if you ever find yourself having to deviate from company profile at high altitudes, remember stall speed will be higher than what you would otherwise encounter at sea level. :)

in 2000, didn't spirit airlines stall an md-80 at altitude? what approximate indicated airspeed at fl 330 did they see (just before recovering)?

I was addressing the question in a general aviation context, where IAS is what you normally use and speeds are low enough so that compressibility effects are minimal. If you are a flying a high-altitude turbine airplane with an ADC, you will obviously reference your speed cards.

"AIRSPEED is a term that can be easily confused. The unqualified term airspeed can mean any of the following:
a. Indicated airspeed (IAS) - the airspeed shown by an airspeed indicator in an aircraft. Indicated airspeed is expressed in
knots and is abbreviated KIAS.
b. Calibrated airspeed (CAS) - indicated airspeed corrected for static source error due to location of pickup sensor on
aircraft. Calibrated airspeed is expressed in knots and is abbreviated KCAS. Normally it doesn't differ much from IAS.
c. True airspeed (TAS) - IAS corrected for instrument installation error, compressibility error, and errors due to variations
from standard air density. TAS is expressed in knots and is abbreviated KTAS. TAS is approximately equal to CAS at
sea level but increases relative to CAS as altitude increases. At 35,000 ft, 250 KIAS (or KCAS) is approximately 430
KTAS.
IAS (or CAS) is important in that aircraft dynamics (such as stall speed) responds largely to this quantity. TAS is important
for use in navigation (True airspeed ± windspeed = groundspeed)."


Bottom line IAS/CAS isn't going to change much, or at all for stall speed with altitude unless there is a large static position error or mach related influence.
Light GA piston, nada. Jet at altitude and high mach, some but not huge in most aircraft.

Actually it's Equivalent Airspeed (EAS) that is important in aircraft dynamics.

Equivalent airspeed is simply CAS corrected for compressibility of the air at high speed/low atmospheric pressure.

Again, I hate to disagree but this is simply not true. Stall speed increases with altitude, NO MATTER WHAT. I am not talking simply about a airspeed measurement error (like the fact that at altitude your PFD shows CAS and therefore you are reading a higher value than your plane's EAS, which again is what is related to stall margin)... I am saying that aerodynamically the wing will stall at a higher airspeed, regardless of airspeed measurement errors. To put it differently, it will stall at a lower CLmax.

This change is stall speed is SIGNIFICANT. Like I wrote on my previous post, we're looking at least at a 2kt/5000ft, but likely more depending on wing design. So at 40,000' your stall speed is at least 16kt higher than what it would be at sea-level, but most likely EVEN MORE than that due to additional effects that are wing-specific.

I hate to be disagreeing with everyone, but we've had so many cases of crews flirting with the low-end of the envelope at high altitude in the last 10 yrs (some fatal) that I feel there should be no doubts about the effect that altitude has on stall speed. Like rickair said, if you stick with the HIGH ALTITUDE values on your speed cards you should be safe (not to be confused with landing V'ref speeds....those are calculated for sea level).

I will try to get exact numbers for different airplane types, but the 2kt/5000' rule of thumb should apply to all airplanes.

I'd just like some sort of Works Cited list to back up your claims.

Here's one for ya, its FAA Advisory Circular 61-67B

"j. Altitude and Temperature. Altitude has little or no
effect on an airplane's indicated stall speed. Thinner air at
higher altitudes will result in decreased aircraft performance
and a higher true airspeed for a given indicated airspeed.
Higher than standard temperatures will also contribute to
increased true airspeed. However, the higher true airspeed has
no effect on indicated approach or stall speeds. The
manufacturer's recommended indicated airspeeds should therefore
be maintained during the landing approach, regardless of the
elevation or the density at the airport of landing."

I know that there are plenty of things, especially in aerodynamics, that the FAA has explained incorrectly. However, I'd like to know where your information comes from.

Interesting topic. I am not sure what type of equipment has had all of these accidents that palgia841 claims have happened due to errors in stall speed computation. Most newer jets utilize an Air Data/ Angle of attack system that presents both a minimum speed and a minimum maneuver speed that is valid for all altitudes and mach. We generally don't fly anywhere near stall and I can't really imagine why anyone would do so intentionally.


This is what you are talking about:

At sea level EAS is the same as true airspeed (TAS) and calibrated airspeed (CAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.

Relevant for engineering purposes is the relationship between indicated airspeed and true airspeed (or Mach number) for common altitudes and airspeeds. In engineering it is useful to have a formula that is reasonably accurate and can be used with values provided in International Standard Atmosphere as function of altitude. While for subsonic speeds up to Mach 0.6 the compressibility can be neglected and IAS/CAS can be obtained from TAS using density correction, it must be incorporated above these speeds for accurate results.


Up to mach .6 it isn't a player, and it looks to be small up to .85. I can't generate the numbers, but here is the formula if you would like to.
Equivalent airspeed - Wikipedia, the free encyclopedia

Perhaps Cubdriver or someone with an aero background can provide further input.


In the course of a normal flight weight and desired buffet margin will have a much larger influence on minimum speed than the minor effects of compressibility.

https://www.faa.gov/regulations_poli...cumentID/22881

Shows the AC was canceled back in 2000 (fortunately!).
Keep in mind from the abstract I can tell that AC was clearly meant for GA aircraft operating at very low altitudes. If you're talking sfc-5000'msl and speeds below M0.2 I agree that you can simplify things and just say that stall speed in constant.

I'll dig out some books and find you some sources.
:)

Those books are going to tell you it is a function of Mach, not altitude.:D

Cancelled AC...well played! I wasn't completely buying that simple of an explanation anyway.

I'm very interested in this. Pretty tough concept to get your head around.

I'll have to go find the Riddle AE books around here somewhere...... but....I think you're talking about Reynold's numbers and the effect on C/Lmax , critical AoA when talking of the 2kt/5000ft rule

I don't remember what exact type of airspeed the book refers to (seems to be true-airspeed), but I want to say C/Lmax was a function of Reynolds (at low speed) and mach at over .5 (assuming unaccelated flight). It was really spliting hairs and varied with the particular airfoil. The derivatives were negligible at low mach, if I'm not mistaken.

The professional analysis for the indicated stall speed increasing with altitude is here:

http://www.flightsafety.org/ap/ap_sep91.pdf

It is an interesting argument, but I'm going to have to dig into it some more.

BINGO!
higher altitude -> lower air density + lower air viscosity (because of lower temps) -> lower Reynolds number -> less kinematic energy in the boundary layer to oppose adverse pressure gradients -> earlier flow separation -> lower CLmax and AOAcrit -> higher stall speed

Thats the 2kt/5000ft... in addition to this effect, there is also the effect of local supersonic flow on the wing upper surface that prevents you from truly reaching the sea-level-CLmax equivalent. Even at speeds below Mcrit, at high AOA the pressure distribution over the upper surface causes the boundary layer to accelerate around the leading edge, and at high enough CL (and thus large enough pressure gradients) the flow can be accelerated to locally supersonic values. Of course the magnitude of this effect will depend on wing design, in addition to Mach, altitude and temperature.

As for sources, I remember seeing the 2kt/5000ft in various books, but after looking for the last hour I can only find it in my student manual from the Naval Test Pilot School, although if you ask me, I trust this manual more than certain aero books... I found a PDF version here http://www.usntpsalumni.org/USNTPS_FTM_108.pdf (look for page 3-27)

As for the reduction of CLmax due to Mach you can see a chart here Structural loads analysis for ... - Google Books that shows a significant reduction of CLmax at Mach speeds as low as .2-.4
This book also has a chart on page 263 that shows the slows speed stall speed increasing with increasing altitude. But I hope by now everyone in happy with this concept. If you really wanted, really you could just take any BOB chart for a transport category airplane (buffet onset boundary) and you'll see the same relationship there (look for the slow-speed buffet).

Another worthwhile read is Aircraft Performance - Google Books chapter 2.6

Bottom line is that the only TRUE way to calculate stall speed at altitude for a given wing is through flight testing. And even then, as ryan's EXCELLENT FSI article pointed out, it's not as easy as it seems. There are so many dynamic factors that play in the determination of stall speed that it's not easy to isolate all the variables.

I must have done a poor job at expressing myself. I am not blaming errors in stall speed computation for many of the stall related accidents/incidents of the past years, however I DO believe that knowledge of the simple relationship between stall speed and altitude could have given many of these crews better situational awareness.

You are correct sir. You would think it'd be common sense to stay well above those speeds... But unfortunately common sense is not all that common, as we saw on Oct 14 2004 with the Pinnacle CRJ. In addition, as you mentioned, the low speed cue comes from an AOA probe and a computer that takes several other inputs. As we all know, sensor vanes can and DO fail, as well as the computer itself. In fact, on that ill-fated Pinnacle flight a software error in the stall protection computer caused the slow-speed cue to erroneously be 10kt SLOWER than it should. In other words, the crew hit the shaker 10kts above the top of the slow speed cue.
Although I personally knew the captain of that flight, his actions that day were unjustifiable. I have no idea what he was thinking flying an RJ at 150 KCAS at 41,000- However, we can use the FDR data reported in the accident report to put what we discussed in this thread to practical use. That day his CRJ weighed approx 37,000# when he stalled it at 41,000ft. If we look at the landing flip cards for that weight, the straight-in Vref +5 CLEAN is 160KCAS. That translates in a sea-level 1G stall speed of approx 119KCAS (155/1.3 =119). So IF stall speed was constant with altitude he should have been able to fly all the way down to this speed even at 41,000ft. Unfortunately, at 150KCAS he hit the shaker. I don't recall for sure, but I believe in the CRJ the stick shaker trip point is at an AOA-derived stall margin of about 10kts (someone please correct me if I'm way off). If this is correct, that would but the stall speed at 41,000' (@ 37,000#) at around 140KCAS, which is not far off from Sea-level stall speed of 119KCAS + 2kt/5000' + further reduction in CLmax due to "Mach effects".

Or I may just be all wrong :eek: either way, I'm going to bed :D

Palgia
I'm no aero expert but from what I've read and the little I understand I would say that most of what you're saying makes sense. However, when you start talking in KCAS and KEAS from a pilots perspective unless you're just running the charts you may not be talking in the most practical terms. Depending on the age and design of the aircraft systems there are some A/C out there that display KEAS, some that display KCAS and others that just have KIAS not corrected at all.
The laws of physics remain the same of course like you say but depending on what kind of indication system an A/C has installed the KIAS/altitude relationship could vary significantly I believe.

Got it, if I am flying in an aircraft with defective software and or an inoperative sensor(s) and decide to fly near stall speed at 40,000 feet this info will come in handy.:D

To the OP in this thread, the answer is still no change with altitude for all practical purposes, as far as light GA aircraft go.:)

Having flown a variety of swept wing military aircraft into and out of stalls at altitude many times, I will tell you from a practical standpoint that I never looked at the airspeed but flew the entry and exit based on buffet and performance. Ignorance on my part was bliss in this case.

One thing that I notice missing from this discussion is the effect of c.g. on stall speed. For any given flight conditions, stall speed will always be higher for a forward cg than for an aft cg.

Joe

Maybe I missed this but scanning through it seems like this has been made incredibly complicated when it is fairly easy.

lift = 1/2 (p v2 A Cl) where p = air density. As air density goes down for the given altitude total lift possible will go down. CL is limited by AOA. Lift has to remain equal to weight so put the aircraft at say 50 knots (assume it is at stall) at critical AOA with lift and weight both equaling 3000 pounds.

Now take this aircraft and increase its altitude which lowers its total lift, lets say 2800 just to give it a number. Now we are already at critical AOA, the area of the wing can't change, and this leaves only speed left. We must increase our speed in this situation to regain 3000 pounds of lift to equal weight.


I have never read this airflow separation from thinner air before till it was posted here. This obviously doesn't mean it's not true, but I would love to know more if someone can break it down barney style for me. Thanks.

Shdw,

The question is about indicated airspeed. The formula you are quoting uses true airspeed.

Joe

Joe,

I am talking to those that have been talking about CAS/TAS/EAS and going into airflow separation differences, pressure changes, and decreased resistance in the boundary area merely to describe a simple stall speed increase with altitude (actual speed). If I am not mistaken the question was answered on the first reply and since the topic has veered off.

KIAS doesn't change period, the designers figured that would make sense. Landing at an airport at 10,000 feet would still be approach at 60 in 172, the actual aircrafts speed is however faster. Who cares whether you call that actual speed CAS/EAS/TAS? Sure EAS is the most realistic or "accurate" aircraft speed. In actuality that argument was merely a search for the most accurate aircraft speed and a distraction from the point, actual speed goes up with altitude and indicated does not. This applies across the board for all, with few exceptions as always, other V-speeds.

For us piston guys, EAS is worthless and accounts for at most a knot difference.

~Brian

Edit: PS that whole bit wasn't to you Joe, only the explaining that I was replying to other posts not the original topic. After that is just my opinion on the original topic.

Indicated stall speed does change because CLmax decreases (i.e. critical AoA) with an increase in altitude, thus a higher stall "speed".

Will redo this later I need to do some research on CLmax. You are wrong about indicated airspeed though, the formula is regarding actual not indicated airspeed. That is why the pitot tube is hooked to the static system.

can we find something less exciting to talk about?? i'm blinking flashing warning lights over here... i'm about to erupt

sorry we're boring you

I'm not sure what formula you're talking about. Some engineering textbooks and other aerodynamic books state a little about fluid dynamics (this is after all what we're talking about, when we talk about a "stall" and "lift"). A stall is when a fluid (in this case air and assuming a prior force 'lift' was created) in the boundary layer does not have enough energy to overcome the adverse pressure gradient (or reverse flow). Fluid forces depend on these variables:

(a) mass density
(b) a characteristic area
(c) velocity squared
(d) Reynolds Number (Re) (viscosity)
(e) Mach No (M) (compressibility)

When engineers are windtunnel testing airfoils, they pay special attention to Reynolds numbers because when they vary, they will give different values in CLmax and thus critical AoA. Mach number is not as important to GA aircraft (pertaining to stalls). Reynolds is generally for lowspeed, Mach is for higher.

Reynolds is basically the ratio of inertia to viscosity, especially seen in the boundary layer. Basically it describes part of how much energy is in the boundary layer (and x-amount of energy is needed to overcome the adverse pressure gradient).

Reynolds has a 2D and 3D effect on a stall:
The 2D effect is that a larger Re value means a greater CLmax value, and a higher AoA where it occurs - because it changes the shape of boundary layer (less or more molecules 'stick' to wing).
The 3D effect is not really relevant to what we're talking about and pertains more towards high-aspect ratio design (depends on planform).

Absolute viscosity is a function of temperature and independent (practically speaking) of pressure.



Indicated airspeed measures stagnation pressure (dynamic / static pressure). Pressure can be constant, but if CLmax changes, the required pressure changes as well resulting in a different indicated stall speed.

To break it down simple:

Lower Reynolds = Lower CLmax value = Lower critical Angle of Attack = Higher required pressure = higher indicated stall speed

Most of you may think this is all just stupid - making things too technical blah blah blah

...But stuff like this explains how vortex generators and some other high-lift devices work. As VGs provide turbulent (high energy) flow to the boundary layer delaying seperation, etc.

The formula for lift I used in my original post, read that post through and you will see what I mean I think.

Here so others can see the formula:

Re = (qVL/μ)= VL/v = QL/vA

where:
V is the mean fluid velocity (SI units: m/s)
L is a length of the object that the flow is going through or around (m)
μ is the dynamic viscosity of the fluid (Pa·s or N·s/m²)
ν is the kinematic viscosity (ν = μ / ρ) (m²/s)
is the density of the fluid (kg/m³)
Q is the volumetric flow rate (m³/s)
A is the pipe cross-sectional area (m²)

Now I haven't done the calculations for this, but this is what I meant by going over the top. The change in density is only a small amount of what the Reynolds number is made up of and the Reynolds number is only a small a portion of Clmax. That being said the change of a knot or two indicated that would result from such a change as say 0 to 10,000 feet IMO is not worth the time for pilots and will only confuse the crap out of most of them. The intricacies of fluid dynamics is above and beyond what one needs to consider in when discussing the change of indicated airspeed with altitude IMO.

Edit: Here to see the varying changes in air density and how little their values are, http://en.wikipedia.org/wiki/Air_density. The difference from -25 to +25 (25,000 feet of altitude change at standard lapse rate) is 0.239 difference in density. Compare that to velocity being a part of the formula and in m/s 50 knots would be 84 m/s, the wing of a typical aircraft that would stall around 50 knots would be around 10 meters in length, and the viscosity of air at 0.00001827 Pa·s. Hopefully these numbers make it pretty evident that the change in air density has a very little impact on Clmax.

PS If this boring, technical, stuff didn't exist and wasn't discussed we would still be pushing around carts with square tires.

Just thought some of you might find this interesting and it demonstrates how speed in the Reynolds formula above has a great effect on separation.

http://www.nar-associates.com/techni...ide_screen.pdf

These flight tests in an arrow with and without vortex generators:

With VGs: Vs = 53 and Vso = 50
Without VGs: Vs = 60 and Vso = 53

Even with only a slightly faster speed, only about 10 knots, the difference with and without was 7 where as the slower speed was only 3.

Great article though, good explanation of how VGs work and fairly non technical.

Been looking over thread and want to make sure everyone is on the same page.

Indicated stall speed remains absolutely 100% constant regardless of altitude

The higher we fly our indicated airspeed will represent a higher and higher true airspeed due to the decrease in air density.

Indicated airspeed is measured by molecules being packed into a pitot tube well if we are at a higher altitude the air density is lower (less molecules to enter our pitot tube) so in order to obtain the same indicated airspeed at a higher altitude we must fly at a faster TAS.

In my example lets say we are flying at our stall speed, we are at our critical angle of attack, and are in unaccelerated level flight. If we increase our altitude and all factors (weight, CG, bank angle, ect.) remain constant the decrease in air density will cause our lift to decrease. We compensate for this loss of lift by either increasing AOA, or velocity (TAS) because in my example we are at our critical angle of attack we can only increase or speed, therefore an increase in altitude will increase the true airspeed the airplane stalls.

Even though the true airspeed the airplane stalls increases the indicated speed remains constant due to lower air density at higher altitudes. as said earlier "The higher we fly our indicated airspeed will represent a higher and higher true airspeed due to the decrease in air density." It is an absolute fact that indicated stall speed is the same at sea level as it is at FL600.

Ryan Just to clarify any speed used in an equation whether referred to as speed or velocity refers to True airspeed not indicated airspeed. You are correct........ true stall speed increases with altitude not indicated. indicated stall speed is constant with altitude if other factors (weight, cg, bank, ect) are constant. for an explanation see my previous post.

A few knots of stall speed difference wasn't
 

the problem. Theories aside, if you do something an aircraft is not designed for it will bite you.



Investigation
The investigation into the accident focused mainly on information contained on the flight data recorder and the cockpit voice recorder. This is the official version of events as determined by that investigation.

The two pilots were exploiting the performance of the empty CRJ-200 on the ferry flight. The pilots decided to test the limits of the CRJ, and join the "410 Club," referring to pilots who pushed CRJs to their maximum approved altitude of Flight Level 410 (41,000 feet).

The incident started when the pilots performed several non-standard maneuvers at 15,000 feet, including a pitch-up at 2.3g (23 m/s²) that induced a stall warning. They set the autopilot to climb at 500 ft/min to FL410. This exceeded the manufacturer's recommended climb rate at altitudes above FL380. In the attempt to reach FL410, the plane was pulled up at over 1.2g, and the angle of attack became excessive to maintain climb rate in the thinner upper atmosphere. After reaching FL410, the plane was cruising at 150 knots (280 km/h), barely above stall speed, and had over-stressed the engines.

The anti-stall devices activated while they were at altitude, but the pilots overrode the automatic nose-down that would increase speed to prevent stall. After four overrides, both engines experienced flameout and shut down. The plane then stalled, and the pilots recovered from the stall at FL380 while still having no engines. At that altitude, there were six airports within reach for a forced landing. This led the pilots to pitch nose down in an attempt to restart the engines, which requires a dive sharp enough to attain the required 300 kt for a windmill restart to make the blades in the turbines windmill at 10% N2.

However, those blades and the gears expanded and the metal scraped on each other when the engine overheated.[citation needed] Thus, when the engine cooled, the assembly did not match anymore and the blades could not rotate freely. The crew ended the dive when they had reached 230 kt but the blade rotation rate had not climbed above 2% N2. Since they were too high for an APU start, the ram air turbine (known as an Air Driven Generator on Bombardier products) was deployed to power the aircraft, and the crew donned oxygen masks as the cabin slowly depressurized due to loss of pressurization air from the engines.

The crew glided for several minutes. The crew then tried to restart engines using the APU at 13,000 ft. This was again unsuccessful. They then declared to Air Traffic Control (ATC) that they had a single engine flameout. At this point they had four diversion airports available to them. After continuing unsuccessfully to attempt to restart the engines for over 14 minutes, during which period much altitude was lost, they declared to ATC that they had in fact lost both engines.

Unable to reach the assigned diversion airport, Jefferson City Memorial Airport, they crashed six minutes later outside Jefferson City, Missouri, behind a row of houses (the 600 block of Hutton Lane — two-and-a-half miles short of the airport), and the plane caught fire. Both pilots were killed. There was some damage to houses and a garage, but no one on the ground was hurt.
wiki

Edit: Removed, not worth it, overly technical.

You must be right...the Naval Test Pilot School must be wrong. There was a previous link to the NTPS manual that explains all of this. Remember that CLmax affects indicated stall airspeed, CLmax is affected at altitude by Reynolds, Deceleration rate, Reduced power (for power on stall).

The ill-fated CRJ200 flight (I'll try to find the data) actually stalled at a lower angle of attack than anticipated..resulting in engine core lock.

"The anti-stall devices activated while they were at altitude, but the pilots overrode the automatic nose-down that would increase speed to prevent stall. After four overrides, both engines experienced flameout and shut down. The plane then stalled, and the pilots recovered from the stall at FL380 while still having no engines."



Does anyone actually believe they should have been "surprised" to enter a stall after repeated overrides of the stick pusher/shaker?

I didn't see anything in the NTSB report that mentions an increase of stall speed at altitude as a causal factor. Maybe the stall warning system was smarter than some thought?


3. Conclusions
3.1 Findings
1. The captain and the first officer were properly certificated and qualified under Federal
regulations. No evidence indicated any medical or behavioral conditions that might
have adversely affected their performance during the accident flight. Flight crew
fatigue and hypoxia were not factors in this accident.
2. The accident airplane was properly certified, equipped, and maintained in accordance
with Federal regulations. The recovered components showed no evidence of any
structural or system failures or any engine failures before the time of the upset event.
3. Weather was not a factor in this accident.
4. The accident was not survivable.
5. The pilots’ aggressive pitch-up and yaw maneuvers during the ascent and their
decision to operate the airplane at its maximum operating altitude (41,000 feet) were
made for personal and not operational reasons.
6. The flight crew’s inappropriate use of the vertical speed mode during the climb was a
misuse of automation that allowed the airplane to reach 41,000 feet in a critically low
energy state.
7. The improper airspeed during the climb demonstrated that the pilots did not
understand how airspeed affects airplane performance and did not realize the
importance of conducting the climb according to the published climb capability
charts.
8. The upset event exposed both engines to inlet airflow disruption conditions that led to
engine stalls and a complete loss of engine power.
9. The pilots’ lack of exposure to high altitude stall recovery techniques contributed to
their inappropriate flight control inputs during the upset event.
10. The captain did not take the necessary steps to ensure that the first officer achieved
the 300-knot or greater airspeed required for the windmill engine restart procedure
and then did not demonstrate command authority by taking control of the airplane and
accelerating it to at least 300 knots.

Refer to my earlier post in response to ryan1234, indicated stall speed does in fact change, for the exact reasons ryan had stated. The change is however so minimal that it is not worth mentioning or considering for any pilot, unless they are the first pilot in history that can hold +/- 2 knots anytime he/she wants.

The actual aircrafts stall speed, who cares if you call it TAS/CAS/EAS, as you know changes significantly for the reasons described in my first post on this thread. It all relates to air density, as air density decreases which decreases total lift, AOA will remain at a maximum possible, so the only thing left to increase to keep lift equal to weight is speed, unless of course you make a pit stop and swap wings. Lift = 1/2 (velocity squared * wing area * air density * (2 * pie * AOA))

I agree, it is also worth mentioning that near stall at altitude it doesn't take much hamfisting of control inputs to cause a dramatic increase in stall speed.
This may have been the case here if flight data is considered.

Ive found this discussion very interesting. Making me think about things I haven't thought about since getting an unused aero engineering degree 27 years ago.

Your post is however incorrect. As a former U-2 pilot I can tell you tell you it does change and have been well above FL 600.

The original post didn't define the type of aircraft or the regime of flight the were referencing. For all practical purposes, in an approach phase application, IAS at stall is a constant with variations for density being minimal. If you want to talk about high altitude/performance aero then you bring in other variables.

Bottom line, the atmosphere density combined with the approach speeds to any runway I can think of make your statement true in that application.

Lee