"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