Lowrance electronic AirMap 1000 - Manual (Page 61)

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The solution to our vertical navigation problem shows at the top of the
screen. Our current altitude is 4500 feet and at our current airspeed,
we should begin our descent in 44 minutes and 20 seconds. If we main-
tain a 450 fpm descent starting in about 44 minutes, then we should
reach pattern altitude 5 nautical miles before we reach the airport.
You can watch the clock count down your approach. If you increase
your speed, the clock will decrease more quickly. If you slow down, it
will take longer. Either way, when you reach the point at which you
should begin your descent, the box at the top of the screen changes.
Now in place of the timer, it indicates that you should be descending.
Above that, it shows the altitude you should be at.
Vertical Navigation Calculator