Why doesn't the FAA print the final fourth digit (0) for altimeter settings in the Pressure Altitude Correction Chart? All altimeter settings are given with four digits. I already know, FAA...........
That is a really good question. I suspect it is to keep the chart small enough to fit easily within the testing supplement. I’ve looked and not found a chart that has more index values. If you know of one, please let me know and I’ll get it posted. Thanks for the comment!
Let me try to dumb this down for myself. Second example problem: 1. The altitude I need for the answer is 1300ft. That falls between 2000ft and 1000ft on the chart. Subtracting those gets us 1000ft. 2. Now we need a magic number between 1300ft and the lower value. So subtract 1300ft from 1000ft. We get 300ft. 3. Now you need the difference between the ground roll distances. (Our ground roll distance must be between the two since it's in between numbers on the chart) So that's 785ft - 715ft = 70ft. 4. Now we have all the numbers we need. The difference in values between the number we need for the answer (1000) The difference between our answer value and the first number (300) And the answer number difference(70) Getting the numbers seems to be the hardest part, now just plug those numbers into the formula. 5. Magic number(300) multiplied by answer number difference(70) = 21000. 21000 divided by our first number(1000) = 21. We have a ground roll between 785 and 715. So it makes sense to add our 21 to the smaller number So the answer is 715 + 21 736!
@monkeybrains911 The answer we are looking for (in this case the takeoff distance) should be the same *relative* distance from the top Resulting Value as the magic number is from the first Index Value. Since the two sets of numbers aren’t the same, we need to calculate a percentage to help us determine where to place the answer. Then multiplying the difference in Resulting values will result in the number to add or subtract from the first one. Let me give a different quick example. Same problem, but let’s say we wanted to find the take off distance for a pressure altitude of 1,500ft. You’ll notice this is exactly halfway between 1000 and 2000. With that in mind, and knowing the answer is the same *relative* distance from the Resulting Values, the answer will be exactly halfway between 715ft and 785ft. Since these two are 70ft apart, halfway would be 35ft from either number or 745ft. Without calculating step 2, you won’t be able to determine the relative distance between the “magic number” and the index values. It is this concept of ratios and percentages that often messes people up when trying to interpolate. That’s why I tried to simplify it down to the five step process. If you use the process, you can just plug in the index values, the resulting values, and the intermediate number, then do the arithmetic to get your answer-no need to set up, then simplify the ratios each time. I hope this helps. Thanks for the questions.
@@ScottKoonCFI And I agree, it's not basic math for some people. Yes, It's a pain to do and I have to do it all the time. Sometimes you can approx it in your head.
