Letting Down Easy


Proper flight planning is extremely important and a vital component for your safety and the safety of your passengers. Just as we plan the takeoff, climb, cruise and landing phases of flight, we also should be thinking about and planning our letdown.

Among the variables to consider are power settings to accommodate our approach profile, airspace or known ATC restrictions, aircraft operating limitations and any weather averse to a smooth, efficient descent. As our letdown continues, we monitor our progress, calculate for time and distance, adjust our plan by jockeying power, pitch and speed. The idea is to complete the flight with a textbook landing so we can score another victory for proper flight planning.

The Descent Gradient

What goes up, must come down,” goes the old saying. Those of us involved with aviation spend a lot of time trying to keep even the number of times we go up and the number of times we come down. But we often dont think too much about the best ways to descend. Which is a shame, because-no matter what kind of aircraft is involved-weve expended a good amount of energy to get to altitude.

Some of that energy is stored, in the form of height about ground, allowing us to trade it for airspeed and groundspeed, reduced fuel consumption, or some of both. Like any other flight mode, of course, theres a right and a wrong way to accomplish our goal. Descending to a landing from cruise flight is just one more mode to consider.

Another benefit to enhancing our letdown planning is helping minimize costly engine repairs. During the descent and approach phases of flight, our engine(s) operate at various power settings. Applying proper power management will help minimize any related issues, like shock cooling and uneven engine temperatures from a reduced power setting, or carburetor ice.

Todays technology provides us with aerodynamically cleaner and faster aircraft than ever before. While we enjoy greater efficiency as a result, we also have greater responsibility, and often more effort is required to ensure a smooth letdown.

As one result, poor approach planning in a slippery aircraft can leave us too high and too close-in to our destination, a point from which its more difficult to accomplish a smooth landing. In extreme cases, well have to go around. Of course, ATC has a role to play in our descent planning, also. When things dont work out as we planned, we can always blame the controllers.

But one objective is to prevent overshooting the airfield because we couldnt slow down and get down at the same time. Over the years, engineers have evolved a variety of tools and methods to dirty-up an aircraft and to slow it down. In addition to reduced power settings, we can extend landing gear and lower wing flaps to their approach setting, if any. In some airplanes, we also have spoilers or air brakes we can deploy.

This articles objective, then, is to provide the tools allowing you to plan and realize a near-perfect letdown. The formulae offered will help provide some additional guidelines in selecting a suitable descent gradient. With the appropriate gradient, we can approach the intended target altitude with a greater degree of precision.

Setting The Stage

Calculating the desired descent gradient requires some basic data like the aircrafts gross weight, weather conditions and terrain. We also need to know target altitudes, including any intermediate targets, starting descent points, descent rates and the time we have available or want to use before reaching our targets.

Once these basic data points are identified, understood and agreed to, we can choose a gradient based on necessity, comfort, power management and terrain. In the real world, not only will the basic data we use to calculate our descent change during our letdown, but so may the priority we assign each variable. For example, on a given flight with mountainous terrain near our destination, we may be forced to wait until were much closer to our intended destination before beginning a descent or continuing one.

On other flights over flatter terrain, we may be able to start down farther out, using a shallower descent gradient. Since our pitch attitude wont be the same, a shallower gradient wont give us the same groundspeed as a steep descent at an identical power setting. But well be at a higher groundspeed longer than if still cruising, with a net gain over the slam-dunk were forced to use in the mountains. Of course, ATC may ruin the plan from time to time, so theres no free lunch.

Picking Some Numbers

In order to provide our approach and descent with greater precision, we need to know progressive target altitudes, a start-descent range from our destination and the appropriate gradient. Why use any gradients? Why pick one particular glideslope over another? Is the gradient choice related only to performance and terrain?

Using the formulae presented on page 7 will help us calculate the parameters we need to ensure a precision descent and landing as well as for good altitude management.

As always when calculating aircraft performance, plan for normal conditions in preliminary calculations conducted during your pre-flight preparations. Use the results as part of your overall flight planning and in the flight plan you file with the FAA, and put these values into the body of your plan when filing.

Figure in book speeds, altitude, weather and terrain, then determine a suitable letdown point for the intended descent gradient. Once thats accomplished, plot the planned descent into the flight plan. Of course, planning and implementing your letdown gradient is not specifically for the final approach or landing descents alone; its intended as a tool used during the en route flight phase for cruise and altitude management. Determining the descent gradient you want to use, however, certainly can be used in the terminal area and on approach, as the sidebar above demonstrates.

Running The Numbers

Now that we have the gradient chart found on page 5 and understand we can customize it for our specific needs, we only need to crunch a few numbers to nail down our planned descents parameters.

For example, an aircraft is flying at an altitude of 10,000 feet and is 15 miles from a crossing fix. We need to lose 3000 feet in 15 miles to cross the fix at 7000 feet Heres the math:

3000 (feet to lose) 15 (miles to fix) = 2.0, or a two-degree gradient.

To check your answer: A two-degree gradient will result in a loss of 200 ft. of altitude per mile. Therefore, in 15 miles the aircraft will descend 3000 feet.

If our groundspeed is 150 knots, well cover those 15 miles in six minutes. To descend 3000 feet in six minutes, well need to establish and maintain a 500-fpm descent.

150 (groundspeed) 15 (miles) = 10 (percent of an hour).

60 (minutes) * .10 = six minutes.

For another example, lets ignore descent gradient. We need to determine when to begin our descent and what rate to maintain to our target altitude. In this instance, were cruising at 10,000 feet and need to be at 5000 feet in seven minutes. This math is rather simple, given the other calculations weve made:

5000 7 = 714.3 fpm.

Little of this is rocket science. Instead, its just basic math, readily adapted to our needs once we understand the various relationships between variables like speed, distance, time and angles of descent. Most of us can easily use the basic E6-B “whizwheel” to do these calculations, but an electronic calculator makes it effortless. The basic formulae and examples of how to use them are summarized above.

Nowadays, the black boxes shoehorned into our instrument panels do a lot of this work for us, at least in the en route environment. The ability to do these calculations “on the fly” will help perfect operations like non-precision approaches, planning for an ILS approach and a variety of other airborne tasks. The idea-at least for me-is to establish an attitude, descent gradient and/or rate of descent at altitude and maintain it all the way to pattern altitude, an intermediate altitude or the runway, as appropriate.

The smoother the flight-whether in a climb, in cruise, in a descent or, especially, when landing-the more impressed will be our passengers. They, in turn, will be more likely to return for subsequent flights with you and your airplane will see greater utilization. Everyone wins.

Randy Merager lives in Van Nuys, Calif., and has earned his Instrument and Multi-engine ratings in almost 30 years of flying.


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