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

I want to walk you through two important altitude corrections that we handle with the Navigation Computer: converting Indicated Altitude to True Altitude, and then calculating Density Altitude from Pressure Altitude and temperature. Let's start with the True Altitude example. We have a situation where the Indicated Altitude is 20,000 feet, but the air is 10°C colder than ISA. The calculation gives us True Altitude = 20,000 + (-800) = 19,200 feet. That minus 800 is the correction, and the result makes sense physically. Why does a colder column of air give a lower True Altitude? Think about it this way: the column of air is 10°C colder than ISA, which means it is denser air. Because the air is denser, the pressure reduces more rapidly than ISA as you climb through the atmosphere. In other words, with denser air, there are fewer feet per hectopascal. So, as you climb, you will 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. Your pressure altimeter thinks you're at 20,000 feet because it senses that pressure, but you are actually physically lower — at 19,200 feet true altitude. Now, the Navigation Computer makes this much easier. We carry out the correction using the ALTITUDE window. Let me describe how it works using the same example. In the ALTITUDE window, you align 20,000 feet with the SAT of -35°C. SAT stands for Static Air Temperature — that's the actual ambient temperature outside the aircraft. By doing this, you have set up a relationship between the inner and the outer scales on the computer. The outer scale now shows the True Altitude corresponding to the Indicated Altitude on the inner scale. So, against 20,000 feet — that's the Indicated Altitude — on the inner scale, you read 19,200 feet — the True Altitude — on the outer scale. Here is the key point for your exam technique: in any question which requires you to correct Indicated Altitude to True Altitude, the Navigation Computer should always give an accurate answer and there is no need to use the formula. In addition, you can use the ambient temperature directly — there is no need to calculate ISA deviation first. True Altitude questions should, therefore, always be solved by use of the Navigation Computer. Now let's move on to a different concept: Density Altitude. This is calculated from Pressure Altitude and temperature. First, understand what Density Altitude actually is. It has nothing to do with vertical distance. Density Altitude is a measure of aircraft performance. Specifically, Density Altitude is the altitude in the ISA atmosphere which corresponds to the density which 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 does not, you need to be able to calculate it. As before, there are two methods of solving the problem — the formula and the use of the Navigation Computer. The formula is as follows: Density Altitude = Pressure Altitude + (ISA Deviation × 120) Now, strictly speaking, the conversion factor should be 118.8, not 120, but 120 is close enough for practical purposes. Let me work through an example. You are at Nairobi. Pressure altitude is 5,500 feet. SAT is +35°C. What is Density Altitude? First, we need the ISA temperature at that pressure altitude. At FL55 — that's Flight Level 55, or 5,500 feet — ISA is +4°C. So our SAT of +35°C is ISA +31°C. That is our ISA Deviation: plus 31 degrees. Now substitute into the equation: Density Altitude = 5,500 + ( +31 × 120 ) Density Altitude = 5,500 + 3,720 = 9,220 feet. So the Density Altitude is 9,220 feet. That means the air density your aircraft is experiencing corresponds to what you would find at 9,220 feet in the standard ISA atmosphere — even though your pressure altitude is only 5,500 feet. That hot air makes the performance much worse. Now, try the same problem on the Navigation Computer. This one is not so obvious because you start off with the AIRSPEED window, not the ALTITUDE window. That is the key difference: for True Altitude you use the ALTITUDE window, but for Density Altitude you begin with the AIRSPEED window. We will work through that computer method in a moment.