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

We're moving into the practice phase now. This excerpt is the question and answer section for the TAS and altitude conversions we've been working through. Let me walk you through what's here. We have four tables to complete, and they test everything we've covered: TAS from CAS, TAS from Mach number, True Altitude from Indicated Altitude, and Density Altitude. The first table gives you CAS, Pressure Altitude, and SAT, and you find TAS. For example, 140 knots CAS at 10,000 feet pressure altitude with a SAT of minus 10 — the answer is 162 knots TAS. And you can see the full set of answers there: 175 CAS at 8000 feet gives 200 TAS, 220 at 15,000 gives 273, 300 at 30,000 gives 472, and so on down to 260 CAS at 35,000 feet with minus 45 SAT giving 450 knots TAS. The second table works from Mach Number and SAT to find TAS. Mach 0.82 at minus 50 gives 477 knots. Mach 0.66 at minus 60 gives 375. And notice the third row — the Mach number is missing, but you have 425 knots TAS at minus 38 SAT, so the Mach number is 0.71. Same idea for the others: Mach 0.77 at minus 60 gives 438, Mach 0.92 at minus 11 gives 580, and Mach 0.71 at minus 47 gives 415. The third table is True Altitude from Indicated Altitude and SAT. 10,000 feet indicated at minus 20 gives 9,450 feet true. 18,000 at minus 30 gives 17,400. 6,000 at plus 10 gives 6,150. 27,000 at minus 50 gives 25,600. 21,500 at minus 35 gives 20,800. And 8,000 at plus 10 gives 8,350. Now the fourth table is the important one, because it has two columns of answers — the Navigation Computer solution and the Formula solution. This is exactly what I warned you about earlier. The Density Altitude window on the computer has small divisions, so for exam accuracy you may need the formula. Look at the first row: 10,000 feet pressure altitude at plus 15 SAT. The computer gives 12,500 feet, but the formula gives 12,400. That's a 100-foot difference. Second row: 6,000 at plus 20 — computer says 8,200, formula says 8,040. That's 160 feet apart. Third row: 27,000 at minus 40 — computer 26,300, formula 26,880. Fourth: 23,000 at minus 22 — computer 23,600, formula 24,080. Fifth: 25,000 at minus 45 — computer 23,100, formula 23,800. Sixth: 29,000 at minus 38 — computer 28,900, formula 29,600. So you can see the pattern — the differences range from about 100 to 700 feet, and they grow as the altitude and temperature deviation increase. That's why the book says for Density Altitude, unlike True Altitude, you may sometimes need the formula to separate closely spaced exam answer options. These are the book's practice questions — let's try them one at a time.