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04/08—Ground Reference Maneuvers, Part 1

By Steve Krog

Steve Krog is a flight instructor and EAA member.

Ground reference maneuvers are practiced and performed to develop the pilot’s ability to control the airplane and to recognize and correct for the effect of wind. They aid the pilot in analyzing wind’s effects while simultaneously dividing the pilot’s attention in preparation for accurate and safe maneuvering of the airplane.

Over the past six issues of Reach for the Sky we have provided an overview and understanding of techniques and maneuvers required to make you a safe and competent pilot. We’ve covered climbs; descents; shallow, medium, and steep turns; slow flight; and stalls. These maneuvers, commonly called “upper air” maneuvers, are practiced to learn the flight characteristics of the airplane and how to control the airplane to make it perform as we want. Now that we know and understand the airplane and these maneuvers, we’re going to be moving into the “low level” phase—ground reference maneuvers—commonly referred to as rectangular course, S-turns across a road, and turns around a point. Today, we’re going to concentrate on the rectangular course.

Ground reference maneuvers are practiced for several reasons:

1. To recognize the effect of the wind and how to correct for it.
2. To fly the airplane safely and at a constant altitude while dividing pilot attention.
3. To prepare for flight in the airport traffic pattern in preparation for practicing takeoffs and landings.

The rotation of the earth combined with the flow of high and low pressure air masses creates the wind. The airspeed of the airplane is not affected by the wind, but the wind does influence the groundspeed and the ground track of the airplane.

To better understand the groundspeed and ground track phenomenon, we’ll use the analogy of paddling a canoe on a river that has a current of 5 mph. If we were to paddle at a rate of 8 mph downriver with the current, we would be traveling the surface of the river at 13 mph (5 + 8 mph). Now, let’s turn around and paddle upriver. Our speed on the surface of the river would be 3 mph (8 - 5 mph).

This phenomenon also occurs when flying. The moving mass of air (the wind) in which we are flying has no effect on our airspeed, but it can drastically affect our groundspeed. If we are flying an airplane at an indicated airspeed of 80 mph and we are flying with the wind (we call this a tail wind) that is moving at 20 mph, we will have a groundspeed of 100 mph (80 + 20 mph). Conversely, if we are flying into the wind (head wind), our groundspeed is just 60 mph (80 - 20 mph).

Referring back to our river and canoe analogy, let’s look at the affect of the current as we attempt to paddle across the river. If we point the bow of the canoe at a spot on the opposite bank and begin paddling, we’ll soon find that we’re moving downstream as we make our way across the river. The current is carrying us at 5 mph. If we are to reach the opposite bank at the spot we selected, we’ll have to compensate or correct for the current to offset its effect on our desired straight-line path across the river. We do the same thing in an airplane by establishing what is referred to as either a wind correction angle or crab angle.

Now that we understand the effect of the wind not only on our ground track but also on our groundspeed, we can begin our flight. After taking off and leaving the airport traffic area, we will level off and set up for cruise flight at about 600 feet above ground level (AGL). For this lesson let’s assume the wind is from the north at 20 mph. Select a road or railroad track that runs east-west and position your airplane directly over the top of the road heading west. If we do nothing to compensate for the wind, we’ll soon find that the wind has drifted us well south of the road.

Let’s again position ourselves over the road, but this time turn the nose of the airplane toward the wind, say approximately 30 degrees. With minor heading corrections we can now keep the airplane directly over the road. Continue flying for another minute. Note the angle between the nose of the airplane and the road. This angle is called the wind correction angle or crab angle.

After flying this westerly course over the road for a mile or two, we’ll turn the airplane 180 degrees and repeat the maneuver heading east. This time with the 20 mph wind coming from the north, we’ll need an approximate 30 degree wind correction angle to the left toward the direction in which the wind is coming from. Once this is established we’ll also concentrate on maintaining a constant altitude of 600 feet AGL.

Remember, to help maintain the constant altitude we will focus on attitude flying, positioning the nose in relation to the horizon so as to maintain level flight. The combination of maintaining a wind correction angle while holding level flight will keep you quite busy for the first minute or two, but once you relax it will come quite easily. You will be dividing your attention between tracking directly over the road and looking over the nose with a quick glance at the altimeter from time to time to assure that you are maintaining a constant altitude of 600 feet AGL.

There are also times when a pilot needs to correct for wind drift while making turns. If we were to make a 360 degree medium bank turn with no wind, our turn would look like the illustration below. The complete turn would look like a smooth and uniform symmetric circle. The best way to visualize this is to pick a road intersection and fly around it while maintaining an equidistant radius around the intersection.

However, by adding a 20 mph wind and not making corrections to compensate for the influence of the wind, our track over the ground would look something like the illustration below. Note how the wind has drifted us well away from our preferred symmetric circle. Again, we’ll demonstrate this by flying around an intersection. Maintaining a constant medium bank, our 360 degree turn will be completed well south of the intersection.

To achieve the level of competency required for the sport or private pilot certificate, we need to be able to make turns while compensating for the wind to fly a desired track over the ground.

Remember this: The slower the groundspeed the shallower the angle of bank to maintain a constant radius, and the faster the groundspeed the steeper the angle of bank.

Rectangular Course

Knowing what a wind correction angle is, we can now apply it to the rectangular course maneuver. Select a section of land that is about 1 mile square. Our goal in performing this maneuver is to fly parallel to each of the four sides of the square (about 1/4 mile outside of the square) while maintaining a constant airspeed and altitude.

In this example the wind is from the east so we will enter the rectangular course flying west on the downwind leg of the course. Once established, we have a direct tail wind.

A wind correction angle is not required, but remember, we do have a groundspeed greater than our indicated airspeed. As we approach the boundary corner or road intersection, we’ll begin our turn to the left, completing the turn as we reach the next boundary road. We started the turn with a fast groundspeed and completed the turn with a 90 degree crosswind and a reduced groundspeed. Desiring to maintain a constant ¼ mile radius in the turn, we will need to start the turn with an approximate medium bank. About halfway through the turn we will begin reducing our angle of bank to an approximate shallow bank, rolling out of the turn to wings level as we cross the boundary road. Our goal is to maintain a constant 1/4 mile radius in the turn, thus the need for beginning the turn at a steeper angle of bank due to the faster groundspeed, and then reducing the angle of bank as our groundspeed slows. And, while doing so, we still want to maintain our constant altitude and airspeed.

As we proceed on a southerly heading, note that we now have a crosswind from our left and must establish a wind correction angle to the left, compensating for the wind to maintain our parallel track over the ground.

Approaching the next boundary road we transition from a crosswind to a head wind, or moderate groundspeed to a slower groundspeed. Consequently, we’ll begin our turn with a shallow or 15 degree bank and reduce it to about 10 degrees midway through the turn while maintaining our 1/4 mile radius. Now we are headed east directly into the wind. Our groundspeed is slower than our airspeed, and no wind correction angle is required.

Referring back to the rectangular course illustration, can you determine the bank angle for the next turn? We’ll be transitioning from a head wind to a crosswind again so our groundspeed will first be slow and then increase slightly. This will initially require a very shallow bank that will increase to an approximate normal shallow bank angle about halfway through the turn. Remember, as we complete the turn we now have a direct crosswind from the right requiring a wind correction angle to the right to parallel the road. Don’t forget to look out over the nose to help keep the altitude constant.

The final turn for completing the rectangular course will find us transitioning from a crosswind to a direct tail wind. With our groundspeed increasing as we proceed through the turn, our angle of bank will increase as well.

You’ve just finished the first lap around your rectangular course. Congratulations! Now try it one more time. Think ahead of the airplane and anticipate the changes you’ll need to deal with: wind correction angle, groundspeed and its effect on your angle of bank in the turns, and maintaining a constant altitude and airspeed. At this stage of your flight training it will seem as if you’ve got a dozen things to think about and time for only half of them. But after flying the rectangular course two or three times, you’ll find that your inputs will become relaxed and quite natural, making the maneuver quite easy to perform.

Once you’ve reached a comfort level with the left-turning rectangular course, try it while making right turns. Again, the first time or two around the course will keep you quite busy, but then it, too, will come quite naturally.

Next time we’ll finish with S-turns and turns around a point, completing our ground reference maneuver training. Then we can put this training to work for us flying the traffic pattern and beginning our takeoff and landing training.

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