Airplane Descents

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Descents require a little advanced planning. The following discussion begins at cruise altitude and integrates a landing approach. 

For a cruise descent, begin at normal cruise speed at a given altitude, and slowly reduce power as you trim the aircraft to maintain the cruise airspeed. Watch the vertical-speed indicator as you begin to descend. When it settles on the desired rate, trim the aircraft to maintain the airspeed and use your power to control the rate of descent. It’s that simple. Sit back and monitor other traffic arriving at or departing from the airport.

Let’s assume you are arriving in the vicinity of your destination airport after a cross-country and you have to let down from 5000 feet to an airport at sea level. If a comfortable descent rate for your aircraft is 500 feet per minute (FPM), then it will take you about 10 minutes to come down. How far out will you have to begin the cruise descent in order to arrive at the destination airport at the traffic pattern altitude? If the destination airport has a 1000-foot traffic pattern, you will have to lose 4000 feet before beginning the approach. At 500 FPM, it will take about eight minutes. For simplicity, let’s say you have a ground speed of 120 MPH. Remember, it’s ground speed, not indicated airspeed, that will give you the most accurate information. However, if you do not know your ground speed, indicated airspeed will probably get you close enough unless you have a very strong wind.

You have eight minutes to lose 4000 feet at a ground speed of 120 MPH. Since 120 MPH equates to two miles per minute and you have eight minutes to get down, merely multiply the miles per minute by the minutes of descent in order to find the distance out to initiate your descent. In this case, it would be 2 x 8 = 16. You will start your 500-FPM descent 16 miles out in order to arrive at the destination airport at a 1000-foot traffic pattern altitude.

Once in the pattern, the airplane must be slowed, and the descent continues while configuring for landing. This is the basis for the landing approach. The approach descent can be accomplished in any one of many different configurations. For simplicity, consider the technique used for a Cessna 152. With only minor modifications, this method can be used for most general-aviation aircraft.

Starting at a given heading, altitude, and airspeed, slowly reduce the power to about 1500 RPM, and hold enough backpressure on the yoke to maintain level flight attitude. As the aircraft slows, begin trimming to relieve the pressure on the stick as you put down whatever flap setting you desire to practice for this particular configuration.

As the airspeed slows to approach speed, keep the power constant and trim for hands-off flight at that speed. At the reduced power setting, the nose will be lower and you will be in an approach descent. Rate of descent is usually controlled with power and airspeed with pitch. In other words, if you see you are descending too fast, you can bring the pitch up a little, add power, or both. You will have to work a little to find the right combinations, especially in rough air.

Select an appropriate airspeed for descent. In smooth air, a high indicated airspeed may be suitable. If the air is rough, you may consider slowing down, perhaps even to maneuvering speed, or Va. Begin by adjusting pitch attitude and reducing power to achieve a desired rate of descent. If our initial airspeed in the descent is too high, we need to raise the nose of the plane slightly, which will reduce the airspeed; if the airspeed is too low, increase the airspeed by dropping the nose slightly. With airspeed established, you can then set your rate of descent using the VSI as an indicator. For most descents you should aim for approximately 500 feet per minute. This descent rate prevents discomfort for your passengers. While you may need to increase the rate of descent due to various factors, a higher rate of descent causes your passenger’s ears to pop more frequently, and if they have any head
congestion it may make the situation painful.

As with climbs, begin to level off about 10 percent of the descent rate before your target altitude. Ease back on the control yoke, slowly raising the nose. You can also begin to add power to help prevent the airspeed from falling off. Correctly executed, you can smoothly transition from the descent to level flight and hit your target altitude. Once you have leveled off and set your engine power, retrim the plane for level flight.

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