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Guest LahaskaFlyer

Descent formula for arrival at 30 miles?

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Hello all!Im looking for a descent formula that will enable me to arrive at 10,000 feet 30nm away from an airport.I've got the TOD & ROD formulas but not this one and Im horrid at figuring it out!Many thanks :)

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Donny AKA ShalomarFly 2 ROCKS!!!I have never been able to do that either, but I do fine by the three to one descent ratio and extrapolating goal altitudes from my distance from the airport. 10,000 feet thirty miles out is a three to one descent ratio. Often I am within a hundred feet either way of 10,000 thirty miles out, makes my day to nail it exactly which has occurred too (several times, not as often).I try to intercept and stay on a path traveling three nautical miles forward for every 1,000 foot altitude loss. I may exceed that or descend at a lesser ratio if ATC starts me down too close or far from the airport.If you want to be at 10,000 feet 30 miles out, that's 15,000 45 miles out, 20,000 60 miles out, 25,000 75 miles out etc... So this is the formula that works fine for me when I use it in conjunction with goal altitudes, it tells me what VSI would result in a 3 to one descent.1. Take your ground speed, either from GPS or true airspeed minus wind effects. 2. Drop the last digit, you'll be precise enough without it. (To torture yourself, move the decimal point one place to the left:9)3. Divide by half, and that is the VSI in hundreds of feet per minute that at your speed will give you a three to one descent.By crosschecking with your goal atitudes, you can manipulate your descent to arrive at 10,000 feet 30 miles out.If ATC starts me down *way* too soon, I start with a 6 to one. To do that step 4 is divide by half again. If you are in a high performance unpressurised plane, descending at speed is painful for your passengers ears, so real world pilots use a 6 to one till they have slowed enough.Remember, three to one is used on most ILS systems or visual landing aids, so this can help chase the needle less or even to increase situational awareness in an autoland situation.A few examples,90 knots 9 4.5 450 FPM250 knots 25 12.5 1250 FPM300 knots 30 15 1500 FPMWorks at any speed, but really is only useful when compared with goal altitudes and adjusted accordingly. If you are following this formula for an ILS, using true air speed on descent and doing it accurately but are getting higher on the slope, think about a different runway because you have a tailwind. Period.It works for me, never could understand the all-inclusive speed distance VSI formula. The one problem with my method is it does *not* take necessary deceleration into account, So I Fudge and adjust but it still works out.Best regards, Donny:-wave

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Donny - Thank you for taking the time and trouble to type out that nice reply! I am gonna print it out and then play with it for sure. Badly need it for those long-hauls! I need to adjust a lot for GPS-assisted approaches but I think I have the hang of it!I was making the mistake of calculating TOD from present altitude to the airport which of course was all wrong and resulted in me coming in too fast!Now I calculate TOD from present altitude to the OM which works much better but your formula will help even more.Many thanks :)

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Donny AKA ShalomarFly 2 ROCKS!!!Umm, don't forget you are descending to pattern altitude, not airport elevation usually too. For more fudge factor if terrain permits you can start a 3 to one descent from say 25000 feet 80 miles out and that should give you 5 miles to get a visual on the feild or slow down if need be.Glad I could help, it is actually simpler than I probably made it sound, but it my mind when I'm doing it it's easy and practicing it on every descent can help you anticipate the ILS- same descent ratio usually, Three nautical miles forward travel for every 1,000 feet altitude loss. And for TOD, 3 miles for every 1,000 feet you want to descend. ATC has their own ideas sometimes, but it's possible to intercept a three to one usually.Non precision aproach *aproximate* goals *no terrain considerations*- 6 miles from threshold on a straight in 2,000 feet above airport elevation 5 1500 feet 4 1200 feet 3 miles 1,000 2 miles 800 feet 1 mile 300 feet (you should *really, really* have feild in sight by now if no ILS, but your life ain't at stake if you're still going for it!!!) 1/2 mile 200 feet (which is the usual minimum for an ILS in most GA avionics configs!!!)This is just what I do that works for me, and I'm not a math whiz by any means.Best Regards, Donny:-wave

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if you are using a standard star approach you are going to have published step-downs.

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