Back in the news, "Man, we can do it, 41-it," said Cesarz

[TonyC]

Here are the charts from the NTSB report from the PCL CRJ manual:

http://www.ntsb.gov/events/2005/Pinnacle/exhibits/314035.pdf

The first charts look to be the climb thrust settings required for the climb profile with the associated temperature limit in C and F degrees. The last chart is the maximum altitude capability chart (ie max performance) and notice how it is broken down by ISA deviation? By the looks of it anything with a greater than +10 ISA deviation would limit the max altitude. OAT isn't even listed.

All this does is, as I mentioned before, allow the incorporation of another dimension into a single table. The same information could be broken out into 3 tables using OAT °C, or combined on that table in a less-readable format.

For instance, on the line that gives values for a 38,000 # airplane, there are five altitudes given for ISA. The first is a Max 0.70 M climb altitude, and the remaining four are cruise altitudes at various cruise speeds. I could put the OAT °C beside each of the values, and display the same information.

The line for 38,000 lbs might read:

41000(-56.5) | 41000(-56.5) | 41000(-56.5) | 39710(-56.5) | 41000(-56.5)
40850(-51.5) | 41000(-51.5) | 39950(-51.5) | 38190(-51.5) | 41000(-51.5)
40000(-46.5) | 39980(-46.5) | 38580(-46.5) | 36520(-46.5) | 40630(-46.5)


That's right, the same temperature for all those altitudes (on a line), since the temperature is constant from about 36,089' to 80,000.'

Especially considering we fly pressure altitudes at cruise OAT is just about worthless since you don't know your exact true altitude. Do you ask for a current altimeter setting at FL350 just to see what the difference is?

Please tell me where you learned this. I really want to know, because I want to make sure my kids don't go there.

On an ISA day at sea level, what will you set in your altimeter? 29.92 if it's inches of mercury, or 1013 if it's millibars

When you're flying a flight level, you're flying an ISA pressure altitude. Geometric altitude is irrelevant.

ISA deviation gives you a realistic picture of your performance.

Again notice that when it comes to performance that they use ISA deviation for performance. OAT is listed as limit for climb settings and I'm sure that can be tied to engine temperature limitations.

The Max Thrust Settings tables are two dimensional. Two things are needed to determine the thrust settings. It's the same two things that determine the number of molecules available. Temperature, and pressure. The temperature is provided in terms of OAT °C and °F. The pressure is given in terms of pressure altitude.

See there, you can talk to the engines and not even KNOW what the ISA is. AMAZING!

You are right, engines only care about molecules but saying that the OAT is 15 degrees means different things if you are at FL180 vs 5000'. Warm thin air is a lot different than warm dense air and that is why ISA deviation is considered.

No, actually 15°C is 15°C, regardless of the altitude. It means the exact same thing. Now, when you're trying to determine a thrust limit or altitude limit, you need to consider more than just temperature. You also need pressure. I could give you the pressure in pounds per square feet, or kilograms per square cubit, or I could give you the pressure altitude and the answer could be determined. Again, it doesn't matter what units you use.

Let me ask you this: Let's say that at sea level today the temperature is 20°C. That works out to ISA+5. As we climb to FL400, do you think there's some rule that requires the temperatures at all altitudes along the way to maintain the ISA+5 relationship? Would it surprise you to find, say 5°C at 5,000'? (That's ISA-0.1.) Can you predict what the temperature will be at 40,000'? Can you be absolutely sure? When you get there, how will you be able to tell if your prediction was accurate? Why, you'll have to measure the OAT.




You act like there's some sort of magic in an ISA temperature, when there's not. It's just another way of expressing temperature.




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[Nova]

The difference is that ISA deviation indicates the actual variation from the norm/standard. A temperature is meaningless without reference.

ISA +10 equals 25 degrees at sea level but what does 25 degrees equal at 10,000 feet and how will that effect your performance?

You are flying a pressure altitude in cruise, not a true altitude. How can you say that -36.5 C is the standard temperature at FL400 when your true altitude could very well be 41,000? Wouldn't that make your indicated -36.5 C at FL400 actually ISA +2, or a non-standard temperature?

 

[Nova]

Can you predict what the temperature will be at 40,000'? Can you be absolutely sure? When you get there, how will you be able to tell if your prediction was accurate? Why, you'll have to measure the OAT.

The paperwork has the forcasted ISA deviation and the FMS will show me what the actual ISA deviation is at cruise.

 

[johnny taliban]

So when blasting along at FL370 looking at your OAT is a poor indicator of your performance penalty (unless you can do quick math) when the ISA deviation is a better indicator. Any performance chart I've seen is broken down by ISA deviation, not OAT.

Gotta agree with Tony on this one. In fact, I often use my RAT to estimate OAT at what ever altitude I happen to be at. As far a the quick math is concerned, I simply take my negative RAT temp and subtract 10% of my indicated airspeed from that number to ballpark the temp. (ex. RAT reads minus 28C and my indicated airspeed is 230kts. -28-23=-51) Now, how hard is that?

I should confess that the above technique is Old School from my Lear 25 days. The fancy jet I fly now has an FMS that does all this for me!

Johnny

 

[Nova]

The last 10+ pages of this report shows all the cruise performance data for the CRJ. ISA deviation is the calculation variable:

http://www.ntsb.gov/events/2005/Pinnacle/exhibits/322970.pdf

Please show how you use OAT for similar calculations?

 

[TonyC]

The difference is that ISA deviation indicates the actual variation from the norm/standard. A temperature is meaningless without reference.

The reference for the Celsius scale is the freezing point of water. Ditto for Farenheit. The reference for Kelvin is absolute zero, the temperature at which all movement of atoms in a molecule ceases. Take away C and F, and what's the reference for ISA?

ISA +10 equals 25 degrees at sea level but what does 25 degrees equal at 10,000 feet and how will that effect your performance?

25 degrees equals 25 degrees, at any altitude. It's also 25 degrees at all latitudes, in all seasons, and during all hours of the day. Would you like me to convert that to Farenheit or ISA?

You are flying a pressure altitude in cruise, not a true altitude. How can you say that -36.5 C is the standard temperature at FL400 when your true altitude could very well be 41,000? Wouldn't that make your indicated -36.5 C at FL400 actually ISA +2, or a non-standard temperature?

I'm going to hook you up with a couple of websites where you can supplement your education before we discuss this any more. You're making a fool of yourself, and I'm getting that impression of a brick wall imbedded in my forehead.


OK, this one has a nice article about the International Standard Atmosphere (ISA) using relatively small words. Pay particulalar close attention to the very last paragraph on the page.

International Standard Atmosphere (ISA)​

Just in case you have difficulty clicking that link, I'll go ahead and paste the last paragraph here:

Aircraft Performance Data Charts use both pressure and density altitude to determine aircraft's performances. When using these charts, the pilot must ensure the use of the appropriate units. Temperature is often expressed in terms of ISA+ or ISA - (degrees Celsius). For example, in standard atmosphere the temperature at 4000 feet is 7 degrees Celsius. However if the actual temperature at 4000 feet is 12 degrees Celsius, and can be expressed as ISA+5.

Alright, now here's a nifty table of ISA values for different useful altitudes.

International Standard Atmosphere​

Now, to deal with your question. If I'm flying at FL410 (I'm definitely NOT in my 727 ), then I look at that table between 40000 and 45000. (Actually, I could go use one of any number of online calculators after using Google for about 5 seconds, but let's keep it simple for now.) Those altitudes in the leftmost columns ARE PRESSURE ALTITUDES! Since I'm using a table, and there is no line for 41000, I'll have to interpolate. (Please tell me they taught you this skill.) The Temp at 40000 is -56.50ºC, and the temp at 45000 is -56.50ºC. Easy interpolation - - ISA temp is -56.50ºC. SO, your -36.5ºC is actually ISA+20ºC. See how easy that was?


OK, go study those websites. If you run out of things to read, try Googling "International Standard Atmosphere" just like that, with the quotes.


And please, tell us where you learned that stuff. Which school?



Oops - - almost forgot this:

The paperwork has the forcasted ISA deviation and the FMS will show me what the actual ISA deviation is at cruise.

How do you suppose the magic box determines ISA deviation? Is there an ISA probe on the exterior of the airplane skin? Do you turn the ISA probe heat on during preflight? Do you suppose there's a radio signal beamed down from a satellite?

I'll help you out here. Your little FMS gets a pressure altitude signal from an air data computer, and it gets a temperature signal from an external sensor. After corrections for things such as speed are made to convert the temperature to an OAT, that OAT is compared to the ISA temperature for that altitude. One simple subtraction operation, and your magic box can display the temperature as an ISA + or ISA - value.

The magic box on the MD-11 displays OAT ºC. Amazingly, they still fly!







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[TonyC]

The last 10+ pages of this report shows all the cruise performance data for the CRJ. ISA deviation is the calculation variable:

http://www.ntsb.gov/events/2005/Pinnacle/exhibits/322970.pdf

Please show how you use OAT for similar calculations?

I've got several books full of 'em, but I can't shove them through the modem.



Lucky for us.





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[Nova]

25 degrees equals 25 degrees, at any altitude.

Correct, but when dealing with performance 25 degrees at sea level with impact the aircrafts performance different than flying in 25 degree air at 10,000'. How much of a performance hit will you experience flying at 10,000' in 25 degree air? That is where ISA deviation is used. Sure you can interpolate, covert, add/subtract to get the number but what you are doing (or using charts to do) is calculate the density altitude expressed as a deviation from ISA.

As your link above stated:

[size=-1]Aircraft Performance Data Charts use both pressure and density altitude to determine aircraft's performances. When using these charts, the pilot must ensure the use of the appropriate units. Temperature is often expressed in terms of ISA+ or ISA - (degrees Celsius).

[/size]Flying in the Flight Levels half you conversion is done for you as you are flying pressure altitudes and don't have to worry about non-standard pressure. What is left is correcting for non-standard temperature. You seem quite quick with extrapolating the standard temperature at altitude. My question though is how do you apply your standard temperature comparison when you don't know your absolute altitude? Indicating altitude in the Flight Levels is a pressure altitude not absolute. Your linked chart shows -44C as the standard for 30,000' but what altitude are you at when indicating FL300?

Such calculations are quicker below FL180 when you know OAT and the pressure difference but this topic arose when discussing taking a CRJ (or any jet for that matter) above FL340 when their charts indicate numbers for up to ISA +10. OAT isn't the determining factor for your cruise altitude. ISA deviation is the limitation and the associated OAT will vary with the atmospheric pressure up to FL360 since we are flying pressure altitudes up there.

 

[Bandit60]

Our aircraft it certified to 45,000 feet, and we cruise at that almost every single trip we do. Never had one bit of problems, and the aircraft has never acted unusual up there.

The media is insane - but we all know that here!

Incidentally, I found this article before I got on FlightInfo and saw this thread.
http://www.msnbc.msn.com/id/8205660/
(Sobering picture of the cockpit - brings home the human side to this story)

Just to add to this...One of the criteria for the service ceiling is that the airplane can make an emergency decent to a certain altitute(i cant remember the exact altitude or time) in a certain amount of time. That is why some airplanes are not certified to higher altitudes.

Your airplane might ever fly great at altitudes about 45000 but it might be outside certification limitations.

 

[TonyC]

My question though is how do you apply your standard temperature comparison when you don't know your absolute altitude? Indicating altitude in the Flight Levels is a pressure altitude not absolute. Your linked chart shows -44C as the standard for 30,000' but what altitude are you at when indicating FL300?

I answered that question previously, but you seem to be bent on ignoring it. The absolute altitude is irrelevant. When you have 29.92 set on your altimeter, you are reading pressure altitude, and you can enter the table directly. If -44ºC is the standard temperature for 30,000 feet, then it's the standard temperature for FL300. It's just that simple.



One last time - - where did they teach you this stuff?




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[Singlecoil]

I would give it up, Tony. You've done an admirable job trying to teach the basics to Nova, but in his world the temperature at 40,000 feet is somehow different if you happen to be flying over a mountain compared to a valley. Great try, though.

 

[JohnE]

I think the big question is why were they not able to restart the engines? It seemed they were following the checklists. Also, maybe when they flamed out both engines, they could have asked ATC for nearest airport right away, but for some reason people are very afraid of declaring an emergency.

I have seen reports of compressor stalls before (in turbulence, etc), why could they not restart?

 

[TonyC]

I think the big question is why were they not able to restart the engines? It seemed they were following the checklists. Also, maybe when they flamed out both engines, they could have asked ATC for nearest airport right away, but for some reason people are very afraid of declaring an emergency.

I have seen reports of compressor stalls before (in turbulence, etc), why could they not restart?

These questions have been addressed numerous times in several of the more than half-dozen threads that are running about this accident, including this thread. Do a Search for "core lock" and study up. Not only did they put the airplane in an attitude such that the engines flamed out, they held it there long enough to rob the engines of cooling air and made matters worse.


No, it doesn't appear that they were following checklists. Again, this issue has been addressed. The memory items were not accomplished, and the checklist was not complied with. For one thing, they never achieved the airspeed required for a relight.


Yes, they could have asked ATC for nearest airport right away, and they might have lived, even in a glider. The trouble is, they were playing around and didn't want to get caught. They didn't confess the dual engine loss until they had descended through 10,000'. By then, there were NO airfields close enough. They weren't afraid of declaring the emergency - - we'll give them that one. They declared right away. The problem is, they lied about the nature of the emergency. They wanted ATC to think they had only lost one engine.




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[DC8 Flyer]

Here's a good site for those wanting to brush up and learn some new things, I did.

http://fromtheflightdeck.com/Stories/turbofan/

 

[CorpLearDriver]

You mean that a plane doesn't have to actually be flown at the max alititude it is certificated at?

True. I've been to 490 on a test flight, but that's very unusual. The "cabin" is certified and so is the airplane and it has made it there. However, few pilots venture above 430 in a Learjet, save a Learjet 31. And given the outcome if an explosive decompression happened at those altitude, those with the wisdom of experience don't consider it very often.