Sunday, March 04, 2007


I apologize for not writing any blog entries for the last couple of weeks. I had to recover from the Women in Aviation conference (and thanks to some behind the scenes difficulties -- am still recovering...) and had to go on the road for a few days with work. I have been sorting and counting all of the merchandise which is now on the Girls With Wings online store: If I do say so myself, I think it's a great interface and much easier to navigate than the old eBay store. Please let me know what you think by writing me at

I am also studying for my Beechjet 400A/Hawker 400XP transition class starting this week. My employer is getting rid of the Citation 560s I'm currently flying. I enjoyed that airplane and I'm sorry to see it go. But studying a new airplane really gets those synapses firing again in my dusty brain...
I am memorizing the emergency procedures and limitations for the aircraft, and one of them has to do with minimum and maximum operating temperatures, which are -65 degrees Celcius and ISA + 35. What's ISA and why don't they just say a temperature in degrees?
...International Standard Atmosphere (ISA)
The term ISA (pronounced as eyes-zha) is the abbreviation for International Standard Atmosphere. ISA was established by the International Civil Aviation Organization (ICAO) as a uniform reference for temperature and pressure. The properties of the Earth's atmosphere are constantly changing. The barometric pressure, temperature and the amount of humidity in the air are subject to annual, seasonal and diurnal variations. The pressure, temperature and humidity are also subject to altitude changes over the same geographical location. A uniform reference became a necessity not only for operational reasons but also essential for aircraft design.

The standard atmosphere was derived from the average conditions for all latitudes, seasons and altitudes. The properties of a standard day are related to sea level at latitude 45 degrees with absolutely dry air. The standard temperature is 15 degrees Celsius (59 degrees Fahrenheit) and a standard temperature lapse of 2 degrees Celsius (3.5 degrees Fahrenheit) per 1000 feet. The standard barometric pressure is 1013.25 hectoPascal (milibars) or 29.92 when expressed in inches of mercury.

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.

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