True Altitude = 20 000 + ( - 800 ) = 19 200 feet — Page 118, Lesson 113

Let's pick up right where the formula left off, because we've just seen the arithmetic: True Altitude equals 20,000 plus minus 800, giving 19,200 feet. That's the answer we expect, and I want you to really understand why it's less than 20,000. Think about the column of air you're climbing through. It's 10 degrees Celsius colder than ISA. Colder air is denser air. Because it's denser, the pressure reduces more rapidly than ISA as you climb. In other words, there are fewer feet to a hectopascal with denser air. So, as you climb, you pass through the required number of hectopascals needed to give an indication of 20,000 feet in less than 20,000 feet of True Altitude. The altimeter thinks you're at 20,000, but you've actually only climbed 19,200 true feet. Now, that's the theory. But in practice, it's much easier with the Navigation Computer. We carry out this correction using the ALTITUDE window. Let's use the same example. In the ALTITUDE window, you align 20,000 feet with the SAT of minus 35 degrees Celsius. By doing this, you've set up a relationship between the inner and the outer scales. The outer scale now shows the True Altitude corresponding to the Indicated Altitude on the inner scale. So, against 20,000 feet—the Indicated Altitude—on the inner scale, you read 19,200 feet—the True Altitude—on the outer scale. Here's the key point for any exam question: whenever you need to correct Indicated Altitude to True Altitude, the Navigation Computer should always give an accurate answer. There's no need to use the formula. Plus, you can use the ambient temperature directly—no need to calculate ISA deviation. So, True Altitude questions should always be solved by use of the Navigation Computer. Now, let's move on to a different concept: Density Altitude. This one has nothing to do with vertical distance. It's a measure of aircraft performance. Density Altitude is the altitude in the ISA atmosphere which corresponds to the density you are experiencing in your current ambient atmosphere. It affects aircraft performance and engine power. Most manufacturers quote a declared performance in terms of Density Altitude. Most performance graphs already allow for this correction in the entering arguments to the graph, but if yours doesn't, you need to be able to calculate it. As before, there are two methods: the formula and the Navigation Computer. The formula is: Density Altitude equals Pressure Altitude plus (ISA Deviation times 120). Strictly speaking, the conversion factor should be 118.8, not 120, but 120 is close enough. Let's do an example. You're at Nairobi, pressure altitude 5,500 feet. SAT is plus 35 degrees Celsius. What's the Density Altitude? ISA at FL5.5 is plus 4 degrees Celsius, so our SAT of plus 35 is ISA plus 31. Substituting into the equation: Density Altitude equals 5,500 plus (31 times 120). That's 5,500 plus 3,720, giving 9,220 feet. Now, try the same problem on the Navigation Computer. This one is not so obvious, because you start off with the AIRSPEED window. That's the key difference—for Density Altitude, you begin with the AIRSPEED window, not the ALTITUDE window.