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Aero-Tips 03.20.06
Most current-generation GPS devices can calculate a descent
profile to take you to a planned altitude by a certain location.
Until you have such a device in your airplane, and as a cross-check
against a possible computer error, it’s easy to ball-park the
time to begin descent if you have a good idea of your current
ground speed and the amount of altitude you need to lose.
Descent by the Numbers
Say you’re
cruising at 8000 feet and approaching an airport with a pattern
altitude at 1200 feet Mean Sea Level (MSL). When should you begin
your descent, to arrive at pattern altitude just before reaching
the airport?
First, decide on your planned descent rate. Optimally a 500 feet
per minute (fpm) descent provides cabin comfort while descending at
an acceptable rate.
- Subtract target altitude from your current
height. In our example, 8000 ft – 1200 feet = 6800
feet, or about 7000 feet to descend.
- Twice the descent required (in thousands of feet) =
minutes to descend. In our example it’ll take
roughly 14 minutes to descend to pattern altitude.
- Begin descent about one minute before the computed time
to destination. As a fudge factor add another minute to
the descent time. This compensates for typically lower wind speeds
at lower altitudes (if you have a tailwind aloft) and any initial
hesitance in setting up the descent. In our example, begin descent
at the point you (or your equipment) compute you’re 15
minutes from destination.
You can do the same thing with a 1000-fpm descent, perhaps
necessary for Air Traffic Control or if low altitude heat,
turbulence or other adverse factors make it desirable to stay high
as long as possible. In our example, it’ll take about seven
minutes to descend at 1000 fpm; begin descent when you’re
eight minutes form destination.
Aero-tip of the day: Use these easy
calculations to plan descents to pattern, instrument approach or
crossing-restriction altitudes. It’s good to cross-check GPS
descent solutions also, in case of computer error.