GPS descent planning

Getting from cruise to pattern altitude

June 7, 2023 By Dave Hirschman

Following the GPS-derived magenta line keeps pilots on track to their destinations—and it can also provide a vertical path to follow all the way to the traffic pattern.

Illustration by Charles Floyd

Avoid Obstacles

A critical fact to keep in mind during GPS descents—particularly at night—is that they don’t guarantee terrain separation. There may be a mountain between you and the airport, and your calculations don’t account for it.

Mnemonics

Rule of threes

Estimating distance, no GPS required

There are some old mnemonics that pilots have used for decades to plan descents, and comparing them to the far more precise GPS guidance can be revealing.

The “rule of threes” is a time-honored formula that tells pilots to take their altitude to lose in thousands and multiply it by three to estimate the distance from their destination at which to descend. Using the previous example (of 8,000 feet to lose), the pilot would reduce 8,000 to eight, then multiply by three to determine the descent should begin 24 nautical miles from the destination.

To determine the proper rate of descent, pilots used to take their ground speed, divide it in half, then multiply that number by 10. For example, an airplane traveling 150 knots (divided in half is 75), then multiplied by 10 is 750—for a target rate of descent of 750 feet per minute. —DH

Even relatively simple, portable GPS units provide continuous readouts of time and distance to destination, and that’s more than enough information for pilots to plan a smooth and accurate descent.

Other GPS features include vertical speed required (VSR) that lets pilots know their desired rate of descent. And GPS units linked to modern autopilots are equipped with advanced vertical navigation (VNAV) features capable of guiding the airplane through a series of arrival gates, then intercepting and tracking an approach to landing.

Clouds, restricted airspace, traffic, turbulence, terrain, and other factors have a way of complicating even the best plans for smooth and steady descents—but using the GPS will keep you pretty close to your target.

[email protected]

Time your descent

During cruise, determine how much altitude you’ll have to lose to get to your destination. If you’re cruising at 8,500 feet msl, for example, and your destination airport has a 500-foot elevation, you’ve got 8,000 feet to lose.

Estimate the rate at which you’d like to descend. For most training aircraft, a 500-foot-per-minute descent is a good target.

This is all you need to know that you should begin your descent 16 minutes before your GPS-derived arrival time at a rate of 500 feet per minute.

Keep your airspeed as close to constant as possible during the descent

To do this, simply reduce engine power about five manifold inches—and don’t touch the elevator trim. If you’re flying an airplane with a fixed-pitch propeller that doesn’t have a manifold pressure gauge, try adding carburetor heat to reduce engine power. (Carb heat has the added benefit of making sure the carburetor is free of ice prior to making any throttle reductions.)

Adjust pitch and power to maintain your cruise airspeed

Wind speed and direction will vary with altitude, but keeping a constant indicated airspeed throughout the descent will keep you close to your desired descent path.

If your GPS shows VSR, watch for trends. If the VSR number is increasing, adjust your airplane’s pitch and power for a faster rate of descent. If the VSR number is decreasing, make adjustments that slow the airplane’s rate of descent.

If your airplane has a modern autopilot with VNAV, you can program it to arrive at a specific altitude at a particular distance from the airport, and it will follow its own internal guidance for the desired rate of descent.

You can also use the VNAV “direct” command to start an immediate descent on autopilot that ends at the same predetermined altitude and distance from the airport. Just don’t press it too soon or you’ll have a long, slow descent to your destination.

Dave Hirschman

AOPA Pilot Editor at Large

AOPA Pilot Editor at Large Dave Hirschman joined AOPA in 2008. He has an airline transport pilot certificate and instrument and multiengine flight instructor certificates. Dave flies vintage, historical, and Experimental airplanes and specializes in tailwheel and aerobatic instruction.

Go to Dave Hirschman's Profile