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#1
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How to calculate TOC and TOD?
Hi everybody,
just a simple question: does anybody of you know a simple way of calculating the Top of Climb and Top o Descent points? Thanks in advance for any useful hint. Ciao Andrea |
#2
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At 500 ft/min climb, use 2 min per 1,000 feet and figure out how long it
will take to get from airport elevation to cruise alt. Then knowning that time and your climb groundspeed (or airspeed if that's all you have), you can use your noggin or E-6B to calculate distance downrange at which you will reach TOC. Same thing for TOD. I like to use 3 minutes per 1000 feet because I like to start my descent farther out and use 300-400 ft/min. Figure out how long that will take, then figure out how far that is based on your descent groundspeed. Add a couple of miles for reaching pattern alt just prior to the airport area. Quick example is 7,000 ft descent to pattern alt will take 21 minutes which at 120 knots groundspeed in descent is 42 miles + 2 = 44 miles from destination as TOD. "Andrea da lontano" wrote in message ... Hi everybody, just a simple question: does anybody of you know a simple way of calculating the Top of Climb and Top o Descent points? Thanks in advance for any useful hint. Ciao Andrea |
#3
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"Andrea da lontano" wrote in message
... just a simple question: does anybody of you know a simple way of calculating the Top of Climb and Top o Descent points? Not really sure what you mean. The math isn't all that hard in the first place. Is there something specific that's giving you trouble? Most of the light GA airplanes we fly cruise in the 120 knot neighborhood, even in a descent. That's 2 miles per minute. Assuming a descent of 500' per minute, you get 4 miles per 1000'. So you simply start descending at a distance that is four times the number of thousands of feet from where your starting the descent. Depending on the airplane, you might be faster and have to adjust the 4x factor to 5x (for an airplane descending at 150 knots) or 6x (for an airplane descending at 180 knots). Note that in reality, it rarely matters that you arrive at your final altitude at just the right moment, so these factors can easily be used for descend speeds varying significantly from the nominal 120, 150, and 180 knots. Note also that the factor depends specifically on groundspeed, not airspeed, but again...since you can be off by significant amounts without significantly affecting the flight itself, this isn't a big deal. Remember, of course, to add whatever margin you feel is appropriate so that you're at your final altitude far enough out from the airport to make a proper pattern entry and approach. As for "top of climb", that will depend on your aircraft manual. The "recent" Cessnas (built in the last 20-30 years or so) include a climb chart as part of the Performance section of the manual, and this makes it very easy to figure out. Other airplanes have very little information on climb rate in the manual, but in a few flights you should be able to gain enough experience regarding climb rate to get a rough estimate of total time to a particular altitude. Combining this time with your climb speed (again, groundspeed is what matters, but airspeed is generally going to be close enough for basic flight planning), you get total time to altitude. Always, keep in mind WHY you are calculating these things. I use my climb time to incorporate fuel burn and trip time for the climb in my total flight planning. I ignore the descent for the purpose of fuel planning, assuming instead that I simply cruise all the way to my destination (i.e. my total fuel burn estimate winds up a little high). Depending on the airplane (power, Vy or cruise climb speed, level cruise speed, etc.), you may find that the climb and descent nearly balance each other out, allowing you to simply use a point to point calculation to make a reasonable fuel and time estimate for a flight. For climbing, if you have a different way to get valid fuel and time estimates, then you just get to your cruise altitude when you get there...no need to worry too much about the exact time you'll arrive. You should, of course, have at least a basic idea of how long the climb should take, so that you can compare actual performance with expected performance. A large difference (off by 50%, for example) would be cause for concern. Planning the descent is, of course, always useful even if you already have valid fuel and time estimates, since you still need to know when to start the descent. But as I pointed out at the beginning of this post, calculating the descent point is relatively simple, once you know in what speed range is the aircraft you're flying. Also, do not forget to account for terrain! Just because you need to be at 1000' AGL 5 miles from the airport and your average descent rate is 500 fpm, that doesn't mean you can disobey the laws of physics and descent through a 5000' AGL obstruction between you and the airport. On many flights, especially if you're in mountainous terrain, you may find that calculating a point at which to start a descent is not the right things to do; rather, you'll know where you want to start the descent (after passing an obstruction, for example) and you'll want to calculate a descent rate to get you where you want to be. Hope that helps... Pete |
#4
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Hi Andrea,
I have a freeware flight planner called AirPlanLite that calculates TOC and TOD points for you if you want to use it for reference. http://www.razorsedgesoft.com/airplanlite.zip Go to the preferences menue and select the segmented leg option to turn on the TOC and TOD feature. Best Regards, Dean Wilkinson http://www.razorsedgesoft.com "Andrea da lontano" wrote in message ... Hi everybody, just a simple question: does anybody of you know a simple way of calculating the Top of Climb and Top o Descent points? Thanks in advance for any useful hint. Ciao Andrea |
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