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How to calculate TOC and TOD?



 
 
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  #1  
Old October 21st 04, 06:37 PM
Andrea da lontano
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Default 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  
Old October 21st 04, 06:53 PM
Doodad
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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  
Old October 21st 04, 07:03 PM
Peter Duniho
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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  
Old October 21st 04, 09:24 PM
Dean Wilkinson
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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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