In this lesson, we will look at the 1. of air for an aircraft cabin air 2. system. And we will examine the methods used to maintain the air at a 3. temperature. We will begin, however, with a short review of the makeup of the atmosphere, to explain the need for air conditioning and 4. __________ systems. Modern jet airliners cruise at high 5. __________up to 43,000 feet. The advantages of high altitude flying, a greater fuel efficiency, higher speed. And on most occasions, the aircraft will be above any bad weather. The disadvantages are that there is 6. __________ 7. __________ to breathe and the air is extremely cold in a standard atmosphere, the temperature drops at a rate of 1.98 degrees per thousand feet up to the tropopause, which is at approximately 36,000 feet or eleven kilometers. This means that on a standard day, at 31,000 feet, the outside air temperature will be a 8. _______ minus forty six degrees Celsius. The atmosphere is made up of 78% 9. __________ and 21% oxygen, with the other 1% consisting of various trace 10. __________.