propjunkie

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.

propjunkie

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.

jungle

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

shdw

Edit: Removed, not worth it, overly technical.

ryan1234

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.

jungle

"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.

shdw

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))

jungle

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.

LeeFXDWG

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

jungle

Exactly correct, and I don't think it is a simple matter to quantify those changes without specific aircraft test data. Simple rules of thumb aren't going to accurately depict the outcome.