View Full Version : Jet Standard Atmosphere

23rd Oct 2007, 10:22
Have seen this term somewhere but can't remember where and can't find any info. Is there such a thing or am I imagining it?
Let me down gently.....

23rd Oct 2007, 10:29
Let me try and put this in prospective. Am not aware the atmosphere changes, regardless of the powerplant of the aircraft, or indeed in or not in an aircraft.

23rd Oct 2007, 10:43
ISA: Lapse rate 1.98/1000ft up to 36090ft (average tropopause) then constant value of -56.5 degrees through the stratosphere.

Jet SA: Lapse rate 2/1000ft all the way, ie no tropopause.

24th Oct 2007, 10:50

Thanks, where can I find more information? And why has this been established? When is is this principle applied?

24th Oct 2007, 14:20
I understood that it was just a laid down standard ( kind of like ISA ) to be used for expressing jet engine performance. Usefull when you want to compare parameters - you need the atmosphere to be " standard " which is done by mathematically adjusting data measured in a real ( nonstandard ) atmosphere.

25th Oct 2007, 01:50
There are various "standard atmosphere" definitions used for different purposes. Main aim is to have "something" defined to provide a basis for comparison and data reduction.

At the end of the day there is nothing overly sacrosanct about one definition over another other than the observation that it is useful if the definition bears some resemblance to the actual typical atmosphere that folks are going to be aviating in ...

Nathan Parker
25th Oct 2007, 02:23
Have seen this term somewhere but can't remember where and can't find any info. Is there such a thing or am I imagining it?
Let me down gently.....

I deny that there is any such thing as a "Jet Standard Atmosphere". Google seems to confirm it.:)

There is generally only one standard atmosphere, the US 1976 Standard Atmosphere. The ICAO and ISO Standard Atmospheres just extend this model to higher altitudes.

Brian Abraham
25th Oct 2007, 02:43

At Sea Level on a Standard Day

Temperature 15C (288.15 Kelvin)
Pressure 29.92126 "Hg, 1013.25 mB, 760 mmHg, 14.69595 psi
Air Density 1.225 kg/m^3, .002376892 slugs/ft^3

Standard Lapse Rate

Below the Tropopause (36089.24 ft) 1.9812C/1000 ft
Above the Tropopause -56.5C to an altitude of 20km

25th Oct 2007, 06:15
There is generally only one standard atmosphere

The FT community has used a range of standards over the years .. the ISA is the one in general and common use.


ah .. real numbers instead of this passing fad metrication stuff ..

25th Oct 2007, 09:48
I seem to remember hearing that the Jet Standard Atmosphere was used by high flying military aircraft, ie flying above FL500?
Anybody know if that is correct, or am I talking rubbish?

25th Oct 2007, 11:56
Thanks everybody. It seems, therefore, that JSA is merely one of several models available for purposes of comparison. Further info welcome.

Brian Abraham
25th Oct 2007, 14:04
Jet Standard Atmosphere - no such animal. I'd like to see your providence. ;)

The SR-71 Flight Manual uses the ARDC Model Atmosphere (1956)

A hypothetical vertical distribution of atmospheric temperature, pressure, and density which, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and rocket design, ballistic tables, etc. The air is assumed to be devoid of dust, moisture, and water vapor and to obey the perfect gas law and the hydrostatic equation (the air is static with respect to the earth). Standard atmospheres which have been used are:

(a) The NACA standard atmosphere, also called U.S. standard atmosphere, prepared in 1925, which was supplanted by
(b) The ICAO standard atmosphere, adopted in 1952, which was extended to greater altitudes by
(c) The ARDC model atmosphere, 1956, and
(d) The U.S. extension to the ICAO standard atmosphere, adopted in 1956, which has been revised by
(e) The ARDC model atmosphere, 1959, which incorporated some satellite data which has been supplanted by
(f) The U.S. Standard Atmosphere-1962. Standard updated in 1966 and 1976. The U.S. Standard Atmosphere is the same as the ICAO Standard Atmosphere for altitudes up to 32km. ICAO Standard currently extends to 80km.

Nathan Parker
25th Oct 2007, 14:08
As it happens, the Jet Standard Atmosphere is defined as detailed by skywaytoheaven in post #3.

So you say. Please provide evidence.

25th Oct 2007, 20:15
Can't give you an exact or precise indication as where to find the information you are seeking suffice to say that the twoseperate meteorology guides I used to get my UK ATPL (A) both mention JSA.

Manual 1 states the following:

Jet Standard Atmosphere

The mean sea level values of temperature, pressure and density are identical to those of the ISA, however the temperature lapse rate is assumed to be 2 degrees Celsius per 1000ft with no tropopause. In other words, the temperature in the JSA at 40000ft is -65 degreesCelsius (as compared with -56.5 degreesCelsius in the ISA)

Manual 2 states this:

There is also a Jet Standard Atmosphere which is ISA made easy for pilots. Here the temperature lapse rate is assumed to be 2 degrees Celsius per thousand feetwith no tropopause. You will meet the JSA in Flight Planning;it is not used inmeteorology.

To back up the above claim I encountered the JSA in my Flight Planning examination when presented with CAA publication Data Sheets 33 and 34 (ahh happy memories:hmm:) relating to the 'specimen aircraft' remarkably similar to the VC10 and B747.

Having obtained a FAA ATP the year before my UK ATPL (A), I cannot recall hearing or seeing any reference to JSA in my FAA study notes; maybe a reason youhaven't come across the term is because you don't get examined the way we were in the UK in the early 90's:p

Nathan Parker
25th Oct 2007, 20:43
There is also a Jet Standard Atmosphere which is ISA made easy for pilots.

Thank you.

What appears to be the case is your manuals gave a name to the rule-of-thumb used by aviators everywhere (who use "feet" at least.) The standard lapse rate is about 2C per thousand feet. Yes, the FAA expects us to know that. ;-) But we don't call it anything.

The usage appears to be idiosyncratic, given the non-appearance in any scientific literature I've seen. And lack of Google hits. :)