Rule of thumb help
hello everyone, Just wondering if anyone knows a quick rule of thumb to solve this kind of problems:
Descending at X fpm with X ground speed what distance over the ground would you cover descending from X to Y altitude? And distance Vs altitude on 3° glide with a visual vertical guidance during the approach. many thanks |
1. If you divide difference in altitude by rate of descent, you get time in minutes. You then multiply this by ground speed, dividing by 60 (since ground speed is in knots = NM/h).
Track distance = (Altitude difference / Rate of Descent) x (Ground Speed / 60) For example, you have to descent 10000ft with ROD 2000ft/min and you have ground speed of 300 knots. Dividing 10000 by 2000 will get you 5 minutes. If you divide 300 knots by 60, you get 5 NM/min. If you multiply 5x5, you get 25 NM - very simple indeed. 2. The rule of thumb for 3° glide slope is very simple, you multiply difference in altitude (let's say from present altitude to runway elevation) with 3 and you get distance. Distance [NM] = Altitude Difference [ft] x 3 So if you are 4000ft above the runway, you should be 12 NM from the threshold. If you're closer, you're coming high and if you're further away, you're shallowing :) |
I don't know if this will help as it's not a particular rule of thumb but...
Nothing wrong with drawing out the triangles and converting the units. I would guess that for your first example, where there are 5 variables, of which there are presumably four given and one unknown, it's unlikely that there is a rule of thumb. They normally only work when certain things remain constant, not when everything is variable, so for these I reckon you'll just have to work them out. Your second example you are much more likely to get a rule of thumb for, as the 3 degrees component stays constant. At 3 degrees, for every 2 miles there is a 545ft drop. At 5 degrees, for every 2 miles there is a 10416ft drop. At 10 degrees, for every 2 miles there is a 1836ft drop. These were worked out by trigonometry, using tan a = opposite/adjacent with the adjacent being the distance (in feet, 1mile is 5208ft), and the opposite being the height difference. Conclusion - trigonometry is probably the area to be looking at. Any good for you? Edit - FS wrote at the same time that I was, and is making a lot more sense than I am! |
Hi Flystone,
Thank you so much for the answers. Very nice indeed |
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