The Navigation Computer - TAS and Altitude Conversions — Page 118, Lesson 115

I want to walk you through the Density Altitude calculation on the Navigation Computer, and then we'll work through the practice tables. Let's start with the worked example. Set 5500 feet against +35°C on the computer. Now look in the DENSITY ALTITUDE window. Read off the answer of 9200 feet. That's the procedure — you align pressure altitude against outside air temperature, and the density altitude appears in that dedicated window. Now, a practical warning. The divisions on the Navigation Computer DENSITY ALTITUDE window are rather small. It can sometimes be difficult to separate closely spaced ATPL examination answer options. With Density Altitude calculations, unlike True Altitude, it may sometimes be necessary to use the formula rather than the Navigation Computer to get a sufficiently accurate answer. So for exam precision, you might need to fall back on the mathematical formula when the scale is too coarse to distinguish between close answer choices. Let's look at the figures that illustrate this. shows the COAT against the Index Arrow — that's the Compressed Outside Air Temperature scale aligned with the index. shows Mach Number on the inner scale against TAS on the outer scale. shows FL370 against -47°C. shows 280 KCAS against 500 KTAS. shows the COMP CORR window — that's the compressibility correction window. shows the index moved to the left, which has moved the inner scale to the left, altering the relationship of CAS to TAS, correcting it. shows TAS corrected for compressibility — calculation of TAS from Mach Number. shows the location of the Mach No. Index. Now, the book gives you practice tables to complete. The first table asks you to find TAS given CAS, Pressure Altitude, and SAT in degrees Celsius. For example, 140 knots CAS at 10,000 feet pressure altitude with SAT -10°C gives a TAS of 162 knots. 175 knots CAS at 8,000 feet with +5°C gives 200 knots TAS. 220 knots at 15,000 feet with -22°C gives 273 knots. 300 knots at 30,000 feet with -40°C gives 472 knots. 180 knots at 12,500 feet with -25°C gives 212 knots. 165 knots at 9,000 feet with +7°C gives 193 knots. 163 knots at 11,000 feet with +10°C gives 199 knots. 295 knots at 31,000 feet with -53°C gives 460 knots. 160 knots at 12,000 feet with -10°C gives 192 knots. 260 knots at 35,000 feet with -45°C gives 450 knots. The second table is Mach Number to TAS. Mach 0.82 at SAT -50°C gives 477 knots TAS. Mach 0.66 at -60°C gives 375 knots. Mach 0.71 at -38°C gives 425 knots. Mach 0.77 at -60°C gives 438 knots. Mach 0.92 at -11°C gives 580 knots. Mach 0.71 at -47°C gives 415 knots. The third table is True Altitude from Indicated Altitude and SAT. 10,000 feet indicated at -20°C gives 9,450 feet true. 18,000 feet at -30°C gives 17,400 feet. 6,000 feet at +10°C gives 6,150 feet. 27,000 feet at -50°C gives 25,600 feet. 21,500 feet at -35°C gives 20,800 feet. 8,000 feet at +10°C gives 8,350 feet. The fourth table is Density Altitude from Pressure Altitude and SAT, with both Navigation Computer solution and Formula solution. At 10,000 feet pressure altitude with +15°C, the computer gives 12,500 feet and the formula gives 12,400 feet. At 6,000 feet with +20°C, computer gives 8,200 feet, formula gives 8,040 feet. At 27,000 feet with -40°C, computer gives 26,300 feet, formula gives 26,880 feet. At 23,000 feet with -22°C, computer gives 23,600 feet, formula gives 24,080 feet. At 25,000 feet with -45°C, computer gives 23,100 feet, formula gives 23,800 feet. At 29,000 feet with -38°C, computer gives 28,900 feet, formula gives 29,600 feet. Notice the small differences between the two methods — that's exactly why the book warns you that the computer's small divisions can make it hard to separate close answer options, and why the formula may give you a more precise result for exam purposes. These are the book's practice questions — let's try them one at a time.