Topographical Chart — Page 139, Lesson 140

I want to walk you through the effect of temperature on your altimeter readings when you're flying with the standard pressure setting of 1013.25 hPa set in the sub-scale window. This is a critical concept for flight planning and monitoring, because the temperature of the air you're flying through directly affects how your altimeter interprets pressure, and that can mean the difference between clearing an obstacle safely or not. Let's start with a concrete example. Imagine you're flying at Flight Level 85, which is a pressure altitude of 8,500 feet with 1013.25 hPa set. The standard temperature at that level, according to the International Standard Atmosphere, or ISA, is minus 2 degrees Celsius. Now, if the Corrected Outside Air Temperature — we call it COAT — is actually minus 15 degrees Celsius, your aircraft is flying in air that is colder and denser than the standard atmosphere. In a column of colder air, pressure decreases more rapidly as you go up. Because your altimeter is calibrated to the standard pressure lapse rate, it will interpret that faster pressure drop as being higher than you actually are. In other words, the altimeter will over read. Conversely, when the COAT is warmer than standard, the air is less dense, pressure decreases more slowly, and the altimeter will under read. So here's the rule when 1013.25 hPa is set: lower COAT means the altimeter over reads; higher COAT means the altimeter under reads. Now, how do we correct for this? The "ALTITUDE" window on the reverse side of the CRP5 flight computer is the tool we use. Let me walk you through the CRP5 procedure using our FL85 example. You set the Flight Level, which is 85, against the COAT, which is minus 15 degrees Celsius, in the "ALTITUDE" window. Then you read off the True Altitude on the outer scale, in feet, against the Flight Level on the inner scale. In this case, you get 8,100 feet. So your altimeter is reading 8,500 feet, but your true altitude is only 8,100 feet — an over read of 400 feet. Why does this matter operationally? Suppose you chose FL85 to provide a 1,000-foot clearance above an obstacle that sits at 7,500 feet above mean sea level, within the stated limits on your planned sector. Because your true altitude is only 8,100 feet, your actual clearance above that 7,500-foot obstacle is only 600 feet — not the 1,000 feet you planned for. That's a significant safety margin reduction. A safer level, to accord with the ICAO VFR Semi-circular Cruise Levels, would be FL105. Let's look at the opposite case. Consider an aircraft flying at FL75, where the ISA standard temperature is 0 degrees Celsius, and the COAT is plus 16 degrees Celsius. Using the CRP5 again: set Flight Level 75 against COAT plus 16 in the "ALTITUDE" window. Read off the True Altitude on the outer scale against Flight Level 75 on the inner scale. You get 7,925 feet. So the altimeter reads 7,500 feet, but your true altitude is 7,925 feet — an under read of 425 feet. In this instance, FL75 was chosen to provide a 1,000-foot clearance above an obstacle of 6,500 feet AMSL. Because you're actually higher than your altimeter says, your actual clearance is 1,425 feet — more than planned, but still important to know. There's a classic aviation adage that sums up the relationship between temperature and pressure: "High to low, mind how you go." It reminds you that when you go from a high pressure or high temperature area to a low pressure or low temperature area, your true altitude is lower than indicated, so you need to be careful. Finally, let me show you how this applies in a typical flight planning problem. Example 9 in your material gives this scenario: an aircraft is airborne from an airfield with an elevation of 800 feet, on a track of 090 degrees magnetic, and the QNH is 996 hPa. Part (a) asks: what VFR Flight Level must the aircraft maintain to clear an obstacle at 4,400 feet AMSL by 1,000 feet? Part (b) asks: what height is climbed to this Flight Level? The answers are on page 141 of your book, and I encourage you to work through that problem using the CRP5 and the temperature correction principles we've just covered.