Why Cirrus is limited to 17500 feet?

I was thinking about over 25000 feet cruise altitude for non-pressurized version of my concept, but I was yesterday Googling about death zone and effects of high altitude to human physiology, and it became quite apparent that it is not healthy to fly at 25000-30000 feet, it is too high altitude for humans to bear even with supplemental oxygen. Even with pressure masks like those on fighter pilots, it might not be very comfortable and safe. It is therefore not a surprise after all, why some non-pressurized GA planes are limited to 17500 feet (like Cirrus SR20 and SR22).

So the need for pressurization comes a lot earlier than I was thinking, and apparently even cruising over 20000 feet would pretty much require it.

Some articles about supplemental oxygen use:
http://www.dr-amy.com/rich/oxygen/

The highest altitude non-pressurized aircraft have been certified usually are 25000 feet according to quick searches to Internet. Columbia 400 (Cessna 400) is non-pressurized and certified to 25000 feet. Flight at that altitude require oxygen mask and it is just above the “death zone” which was mentioned in one Mt. Everest page I was looking yesterday.

According to one UAV report I have (SR22 was compared to a UAV airframe), Cirrus SR22 technical service ceiling is at about 33000 feet. SR20 on the other hand with a lot less excess power does not most likely reach its limit altitude of 17500 feet most likely unless it is very lightly loaded. On our trip to Mojave it barely made it to 11000 feet at gross weight and non-standard atmospheric temperature conditions (it was hotter than on standard atmosphere).

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    • jim
    • January 9th, 2009

    The FAA allows flights up to 18,000 ft with a cannula. Above 18,000 ft an oxygen mask is required.

    The reason Cirrus limits their certified altitude is for liability, not performance.

    • jim
    • January 9th, 2009

    The FAA allows flights up to 18,000 ft with a cannula. Above 18,000 ft an oxygen mask is required.

    The reason Cirrus limits their certified altitude is for liability, not performance.

    • Karoliina Salminen
    • January 9th, 2009

    Indeed, it is the liability, one UAV report was stating the SR22 service ceiling to be actually around 33000 feet.

    I read one article which was stating that during WW2, pilots flew unpressurized fighters up to 40000 feet and suffered from decompression sickness, hypoxia and hypothermia. It seems that it is possible to fly that high without pressurization, but it is not safe, healthy or comfortable way to travel. So pressurization is needed at lower altitudes already which complicates things.

    • Karoliina Salminen
    • January 9th, 2009

    Indeed, it is the liability, one UAV report was stating the SR22 service ceiling to be actually around 33000 feet.

    I read one article which was stating that during WW2, pilots flew unpressurized fighters up to 40000 feet and suffered from decompression sickness, hypoxia and hypothermia. It seems that it is possible to fly that high without pressurization, but it is not safe, healthy or comfortable way to travel. So pressurization is needed at lower altitudes already which complicates things.

    • dodlithr
    • January 10th, 2009

    To fly at 30 000 ft you should maybe switch to jet engine also. They are better at high altitudes and faster.

    This leads to the old conclusion that small propeller planes without pressurized cabins are mostly ment to fly under 15 000 ft and larger jet engines above that.

    Also the speeds are much smaller. There is no point to go much over 300 kts with propellers. I would say that the old Spitfire puts the limits for propeller aircrafts. If you need anything more than you find there, you have to switch to jet engines and pressurized cabins.

    http://en.wikipedia.org/wiki/Supermarine_Spitfire

    Spitfire was ment to be the fastest plane at the time it was designed. It’s model was the speed winning propeller plane of it’s era. At that time there was no jet planes and to go faster the only way was to invent them. The propeller plane had reached it’s limits when WW2 started. And that is still the truth.

    • dodlithr
    • January 10th, 2009

    To fly at 30 000 ft you should maybe switch to jet engine also. They are better at high altitudes and faster.

    This leads to the old conclusion that small propeller planes without pressurized cabins are mostly ment to fly under 15 000 ft and larger jet engines above that.

    Also the speeds are much smaller. There is no point to go much over 300 kts with propellers. I would say that the old Spitfire puts the limits for propeller aircrafts. If you need anything more than you find there, you have to switch to jet engines and pressurized cabins.

    http://en.wikipedia.org/wiki/Supermarine_Spitfire

    Spitfire was ment to be the fastest plane at the time it was designed. It’s model was the speed winning propeller plane of it’s era. At that time there was no jet planes and to go faster the only way was to invent them. The propeller plane had reached it’s limits when WW2 started. And that is still the truth.

    • dodlithr
    • January 10th, 2009

    You should be able to go up to 30 000 ft with the oxygen masks. Why?

    At 30 000 ft the pressure is about 30% of the sea level pressure. And I know tht the astronauts use that pressure for long periods with oxygen without problems. See “Space suit” for more information:

    http://en.wikipedia.org/wiki/Space_suit

    “Above Armstrong’s Line (around 19,000 m/62,000 ft), the atmosphere is so thin that pressurized suits are needed.”

    The ultimate limit is 62 000 ft.

    • dodlithr
    • January 10th, 2009

    You should be able to go up to 30 000 ft with the oxygen masks. Why?

    At 30 000 ft the pressure is about 30% of the sea level pressure. And I know tht the astronauts use that pressure for long periods with oxygen without problems. See “Space suit” for more information:

    http://en.wikipedia.org/wiki/Space_suit

    “Above Armstrong’s Line (around 19,000 m/62,000 ft), the atmosphere is so thin that pressurized suits are needed.”

    The ultimate limit is 62 000 ft.

    • Karoliina Salminen
    • January 10th, 2009

    Thanks, for the comment, I was also thinking that 30000 might not be that hostile with the pressure mask. At least 25000 feet should be still within reasonable limits since aircraft your next guy might buy can reach such altitudes and are certified to such altitudes without pressurization.

    However, about high altitudes, I am particularly interested in them, I consider 30000 ft “medium altitude” rather than high altitude:

    62000 feet is reachable with twin turbo configuration. See my earlier post about Burt Rutan’s raptor. It used two turbos and a Rotax 912. If the UAV was equipped with two similar engines, it could have carried two persons to the high altitude where it now only carried equipment. Designing a plane which can reach this high altitude with Rotax 912 should be feasible. That is in the plans for the step 3. However, the turbos are similar size than the engine itself, not any Rotax 914 stock turbos, these are rather from a truck.

    Jet engines are out of question for various reasons:
    – price of the engine (astronomical)
    – availability of the engines (zero)
    – specific fuel consumption (insane for low bypass ratio small engines)
    – for getting things done, with highly modified Rotax engines (that I know how to work with), doable, with unknown jet engines I may never afford, not doable even in my dreams.

    Jet engine is out of the question, and so is turboprop and it is quite useless to discuss about that since it is not a feasible project at this point financially, technologically or based on the availability of the needed parts, materials and supplies. Surely I will let everyone know if this changes, and it only can change if some magic sponsor like the Santa Claus appears from nowhere and wants to fund some my record flight attempt including the funds needed to build the aircraft including the insanely expensive jet engines. If such sponsor who has vision enough to believe that I can do it, and it is even possible, exists, feel free to send me email (this is not a joke by the way, but I am not very seriously waiting for your mail either right now)!

    I am interested in high altitude rather than extreme speed. I am looking for high efficiency in flight, not extremely fast flight, this means low power, low cost engines, low fuel consumption per kilometer. This is achievable at high altitude. I am also interested in high altitude flight for the sake of the altitude, near space experience at above 60000 ft and about getting there without a gas balloon but with an airplane. I would consider getting there with a self-designed twin engine small aircraft a major success. But that has to belong to model C, not to model A, unfortunately I am not a superhuman that can eat too big piece of the cake at one time. As I am at point zero, I can promise absolutely nothing with current state. I am now concepting, collecting information etc. and it is at least free or almost free (books are not so expensive compared to what it takes to build an airplane, especially when it is not an ordinary airplane but trying to do something normal GA planes don’t do).

    • Karoliina Salminen
    • January 10th, 2009

    Thanks, for the comment, I was also thinking that 30000 might not be that hostile with the pressure mask. At least 25000 feet should be still within reasonable limits since aircraft your next guy might buy can reach such altitudes and are certified to such altitudes without pressurization.

    However, about high altitudes, I am particularly interested in them, I consider 30000 ft “medium altitude” rather than high altitude:

    62000 feet is reachable with twin turbo configuration. See my earlier post about Burt Rutan’s raptor. It used two turbos and a Rotax 912. If the UAV was equipped with two similar engines, it could have carried two persons to the high altitude where it now only carried equipment. Designing a plane which can reach this high altitude with Rotax 912 should be feasible. That is in the plans for the step 3. However, the turbos are similar size than the engine itself, not any Rotax 914 stock turbos, these are rather from a truck.

    Jet engines are out of question for various reasons:
    – price of the engine (astronomical)
    – availability of the engines (zero)
    – specific fuel consumption (insane for low bypass ratio small engines)
    – for getting things done, with highly modified Rotax engines (that I know how to work with), doable, with unknown jet engines I may never afford, not doable even in my dreams.

    Jet engine is out of the question, and so is turboprop and it is quite useless to discuss about that since it is not a feasible project at this point financially, technologically or based on the availability of the needed parts, materials and supplies. Surely I will let everyone know if this changes, and it only can change if some magic sponsor like the Santa Claus appears from nowhere and wants to fund some my record flight attempt including the funds needed to build the aircraft including the insanely expensive jet engines. If such sponsor who has vision enough to believe that I can do it, and it is even possible, exists, feel free to send me email (this is not a joke by the way, but I am not very seriously waiting for your mail either right now)!

    I am interested in high altitude rather than extreme speed. I am looking for high efficiency in flight, not extremely fast flight, this means low power, low cost engines, low fuel consumption per kilometer. This is achievable at high altitude. I am also interested in high altitude flight for the sake of the altitude, near space experience at above 60000 ft and about getting there without a gas balloon but with an airplane. I would consider getting there with a self-designed twin engine small aircraft a major success. But that has to belong to model C, not to model A, unfortunately I am not a superhuman that can eat too big piece of the cake at one time. As I am at point zero, I can promise absolutely nothing with current state. I am now concepting, collecting information etc. and it is at least free or almost free (books are not so expensive compared to what it takes to build an airplane, especially when it is not an ordinary airplane but trying to do something normal GA planes don’t do).

    • dodlithr
    • January 10th, 2009

    I would use pressurized cabin or pressurized suits above 30 000 ft, since you don’t get enaough oxygen without pressurization. The hypoxia is always there behind the corner if you are not cautious.

    Yes, at that height you’ll need bigger turbo chargers to get enough air to the engine(s). And not to forget the freezing temperatures. The plane must have some cabin/suit heating. Maybe best to switch to water cooled engines since it is easier to circulate the hot coolant than try to circulate the hot air from the engine (if there is any left over after leaks).

    • dodlithr
    • January 10th, 2009

    I would use pressurized cabin or pressurized suits above 30 000 ft, since you don’t get enaough oxygen without pressurization. The hypoxia is always there behind the corner if you are not cautious.

    Yes, at that height you’ll need bigger turbo chargers to get enough air to the engine(s). And not to forget the freezing temperatures. The plane must have some cabin/suit heating. Maybe best to switch to water cooled engines since it is easier to circulate the hot coolant than try to circulate the hot air from the engine (if there is any left over after leaks).

    • Karoliina Salminen
    • January 13th, 2009

    Yes, Rotax is half-watercooled engine and the hot water can be circulated.

    Above 30000 feet it may not be comfortable without the pressurized cabin.

    Anyhow, this belongs to step 3 in my plan, so first thing would get the plane to fly with 2x plain Rotax 912ULS without the complicated systems like the pressurization. I’ll think it later.

    • Karoliina Salminen
    • January 13th, 2009

    Yes, Rotax is half-watercooled engine and the hot water can be circulated.

    Above 30000 feet it may not be comfortable without the pressurized cabin.

    Anyhow, this belongs to step 3 in my plan, so first thing would get the plane to fly with 2x plain Rotax 912ULS without the complicated systems like the pressurization. I’ll think it later.

    • Daniel Olai
    • April 17th, 2009

    The Cirrus (SR22) Turbo is certified for FL250 – it is the same plane structurally and technically as the NA SR22, with the addition of Tornado Alley’s twin turbo/IC setup. Turbos are not there to gain speed, but to give engines enough air to breathe (turbonormalisation) – they experience sea-level airpressure from 4 to 25000ft. Makes sense not letting the NA SR’s go too high if youre worried about the engine suffocating…

    Also according to Cirrus, you have to switch to a half-mask when taking the Turbo above 17500ft to make sure you get enough O2 yourser.

    • Daniel Olai
    • April 17th, 2009

    The Cirrus (SR22) Turbo is certified for FL250 – it is the same plane structurally and technically as the NA SR22, with the addition of Tornado Alley’s twin turbo/IC setup. Turbos are not there to gain speed, but to give engines enough air to breathe (turbonormalisation) – they experience sea-level airpressure from 4 to 25000ft. Makes sense not letting the NA SR’s go too high if youre worried about the engine suffocating…

    Also according to Cirrus, you have to switch to a half-mask when taking the Turbo above 17500ft to make sure you get enough O2 yourser.

  1. My feeling is that it has to do with FAA certification/Cirrus design more then anything. Above FL250 you are required to have a high altitude endorsement for flight and that is not something your typical GA pilot/Cirrus Owner is going to possess or want to spend the extra money to get, not to mention performance on the SR22 turbo will get extremely sluggish above this height.

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