If You Don’t Like the Weather in Texas …
By Bill Gunn
Wait a few minutes, it will change. Texas is big enough to have heat stroke and frost bite both at the same time. Pilots in the Valley may not think about it, but the North and the Panhandle can be COLD in the winter.
Beyond the challenges and advantages of cold weather flight operations, such as ice, frost, cold starts and improved performance, there is a subtle, but important adjustment few pilots think … altimeter error.
Wait a minute, you have your IFR craft checked for static system error and altimeter error every 24 months, right? Absolutely! However, there is another form of altimeter error that has always been there, but is only now being formally recognized in the USA. Transport Canada and those who fly in Canada have known about cold weather altimeter error for years.
So what is it? Well, first think about how an altimeter works. A pressure-sensitive bellows in a sealed case measures the uniform decrease in atmospheric density with increased altitude. Simple, adequate and reliable. However, in temperatures below IO degrees Centigrade (50 Fahrenheit), you need to adjust your height UPWARD by IO to 1680 feet. Why? The sensitive aviation altimeter uses the principle of uniform air density to translate to altitude. Very cold temperatures increase the air density enough to cause the altimeter to “read” a higher altitude than the actual altitude above sea level.
For example, if the correction factor is 180 feet and you are flying an instrument approach to 650 feet MDA or DH, add the correction of 180 to 650. Fly the approach and descend to an INDICATED altitude of (180 + 650 =) 830 feet for an ACTUAL altitude of 650 feet. A new Advisory Circular is being published with simple correction tables. Look for AC 91 -XX, “Altimeter Errors at Cold Temperatures” in the near future.
