Aero-Tips!

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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.

1. Subtract target altitude from your current height. In our example, 8000 ft – 1200 feet = 6800 feet, or about 7000 feet to descend.
2. 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.
3. 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.

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