Pilot's tip of the week

Descending From Cruise Altitude

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Subscriber question:

"What is the best point to start a descent on a VFR cross-country flight? Or put otherwise, what is the best rate of descent (FPM) to fly a comfortable descent and enter the traffic pattern?" - Philip

Bob:

“500 feet per minute is a good rate to work with. Generally, passengers will not be alarmed by a decent rate of 500 ft/min. Pressure changes will occur slowly in an unpressurized aircraft.

Cruise descent rateAlso, keep in mind that ATC will expect a minimum rate of 500 ft/min on an IFR flight plan unless you advised them otherwise.

Let’s say traffic pattern is 1,000 feet AGL, and you need to lose 4,000 feet to get from your cruising altitude (5,000 ft) to pattern altitude. At 500 ft/min, it will take you 8 minutes to lose 4,000 feet. At 120 knots which is two miles per minute (120 nautical miles per hour divided by 60 minutes = 2 miles per minute), you need to begin your descent 16 miles out.

If you have a GPS with a VNAV function, it will do this calculation for you. Learning to use it is a worthwhile exercise and not difficult.

On the VNAV page you enter the destination, desired altitude that you will be descending to (AGL or MSL altitude), and distance from the airport where you want to be at this altitude. Typically if you are entering a traffic pattern at an airport, you probably want to be at that altitude 2 or 3 miles from the airport. You will then see on the screen the necessary descent rate to make that happen.”

 

You may find this video from the Air Safety Institute helpful:

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