Aside from landing, flying S-turns across a road is the most demanding ground reference maneuver for many student pilots. In this maneuver, the pilot flies a series of constant-radius S-turns along a reference line such as a straight section of road or railroad track. Many students have trouble mastering the proper entry for S-turns and may have misconceptions about the relationship between wind drift and bank angles. Once these concepts are understood, learning to fly the maneuver is much easier.
The Entry
Before entering any ground reference maneuver, we need to be prepared. This means completing any required checklists, properly configuring the aircraft and establishing the proper altitude, and making those all-important clearing turns. We are all taught to enter ground reference maneuvers heading downwind. Even the most common ground reference maneuver, the landing pattern, is entered on the downwind leg. If we don't begin our S-turns headed downwind, we'll most assuredly have problems with the rest of the maneuver.
The reason for entering the maneuver on the downwind is that it allows us to more readily establish the proper maximum bank. The trick is to know which way the wind is blowing.
A Common Misconception
Even if we get the entry right, we can foul up miserably if we don't understand the mechanics of the maneuver. The problem that plagues many students is that they do not know where to increase and decrease the bank angle to maintain the proper ground track.
It seems intuitive that when we are flying the crosswind leg on the downwind side of the road, the wind is blowing us away from the road, and thus we would need the greatest bank angle to pull us back toward the road. We could also conclude that when we are flying crosswind on the upwind side of the road, the wind is blowing us onto the road, and a steep bank angle would help us complete that portion of the turn more quickly. In fact, these seemingly intuitive approaches to flying S-turns across a road are based on a common misconception-our attempts to fly the maneuver based on these concepts are doomed to failure.
Key Elements
The key to performing most ground reference maneuvers is to understand the relationships between groundspeed, rate of turn, and bank angle. To understand S-turns, we begin with the maneuver's less-complex cousin-turns about a point. The objective is to fly a constant-radius, constant-altitude turn around a reference point on the ground.
To understand how this maneuver works, imagine a race car going around a circular track that is exactly two miles in circumference. If the race car is traveling 60 mph, the car will circle the track in two minutes. Since the car makes a complete circle in two minutes, the rate of turn for the car is 360 degrees every two minutes, or 3 degrees per second. If that same race car on that same track accelerates to 120 mph, it can circle the track in 60 seconds. The rate of turn will then be 360 degrees per 60 seconds, or 6 degrees per second. Therefore, the rate of turn required to complete a circle over the ground is dependent on speed. The faster the speed, the greater the rate of turn.
Now, if we fly our airplane directly over that same race track, let's say at an altitude somewhere between 600 and 1,000 feet above ground level (agl), and we fly at exactly 60 mph on a perfectly calm fall morning, we would need a rate of turn of 3 degrees per second (a standard rate turn as shown on our turn and slip indicator) to stay directly over the track. Increasing our speed to 120 mph translates to a rate of turn of 6 degrees per second. When selecting the appropriate airspeed, remember that you should not exceed the maneuvering speed (VA). An airspeed typically used when flying the traffic pattern is appropriate to this maneuver. Desired airspeed must be maintained plus or minus 10 kt.
The angle of bank that we select controls our rate of turn. The steeper the bank angle, the greater our rate of turn. But the amount of bank required for any particular rate of turn is dependent on our speed. The faster we fly, the greater the bank angle needed to produce the same rate of turn. The maximum bank should never exceed 45 degrees.
Now let's look at a more challenging example - a windy fall afternoon. Again we plan to fly over our circular racetrack. Since our groundspeed will be changing constantly because of the wind, we will need to adjust our rate of turn. We will do that by changing our bank angle as we fly the maneuver. As our groundspeed increases, we will increase our bank angle. As our groundspeed decreases, we will decrease our bank angle. Our groundspeed will be the highest when we are directly downwind, and it is at that point that the greatest bank angle is required. This is why we begin the maneuver flying downwind. We roll into our bank when abeam the reference point (45-degree-bank maximum), and we know that at every other point in the maneuver, our bank angle will be less. When the nose is pointed directly into the wind, our groundspeed is the lowest, and at that point the angle of bank is the least.
S-turns across a road can be thought of as a series of turns about a point, split in half roughly upwind and downwind, then linked together end to end. As with turns about a point, we enter on the downwind and establish our maximum bank angle as the reference point (this time the road) passes beneath the wing. The bank angle decreases to a minimum when we recross the road headed upwind. As we cross the road, we momentarily (one to two seconds) roll wings level, then establish the bank in the opposite direction. The bank angle slowly increases as we complete the next 180 degrees of turn, again reaching the maximum bank angle as we cross the road. This time we also roll wings level momentarily, and then roll into our maximum bank in the opposite direction and continue the maneuver.
Other Common Problems
Sometimes we can fool ourselves into thinking that we fly better than we really do, and that can happen when flying S-turns across a road. In light or calm winds, flying the maneuver appears to be easy, but that's because we can't readily see our mistakes. To make our mistakes more evident, it's best to practice the maneuver with a 10- to 15-knot wind blowing virtually perpendicular to the reference line. With this amount of wind, you'll need to make proper corrections, and any mistakes will be readily visible as deviations in your ground track.
If flying S-turns can be likened to a series of turns about a point, then we need to be picking points along the way as we fly the maneuver. If we pick points that are too close, we'll be tempted to use excessive bank angles and rush the maneuver. The result is a sloppy exercise at best. If the points are too far away, the turns get too big and the banks too shallow. In general, the reference points selected along the reference line should be separated by about three-quarters of a mile-a little more if we're flying faster, and a little less if we're flying slower.
Finally, look out for diversions and distractions while flying the maneuver. Some in- structors will attempt to distract you by asking innocuous questions about features on the ground (Hey, is that a new McDonald's going in over there? What kind of cows are those in that field? Is that a T63 Corvette going down the highway there?) or about the aircraft systems (Gee, is it normal for the ammeter to be reading five amps? How come we've got more fuel in the left tank than the right? Is that the right altimeter setting?) in order to distract you. Some instructors (and pilot examiners) will initiate simulated engine failures during this maneuver, just to see if you're really on top of things. The bottom line is to be prepared, don't forget to properly divide your attention, and don't get distracted.
Putting It All Together
Now that we understand the key elements, let's review the maneuver in its entirety. Remember, there's much more to this maneuver than just maintaining the proper altitude and ground track. To properly perform the maneuver, you must maintain situational awareness as you fly. Among other things, situational awareness involves watching out for other traffic, monitoring the aircraft, and being prepared for emergencies.
The maneuver begins by determining the prevailing surface wind direction. Next, select a suitable ground reference that forms a relatively straight line perpendicular to the wind. You can use a road, power line, river, shoreline, railroad track, or any other available reference. Position yourself about a mile upwind of the reference, slow the aircraft to the desired speed, and complete your prelanding checklist. Make some clearing turns to clear the area and, if you haven't already, descend to the desired altitude, preferably from 600 to 1,000 feet agl.
Once you're ready, head downwind straight for the reference line and cross it at a 90-degree angle. As you cross the line, pick a point along the road to use as a reference for the first 180-degree turn, and roll into your maximum bank of 30 to 40 degrees. (Never exceed a 45-degree bank during the maneuver.) Watch the reference point as you slowly roll out the bank to maintain a constant radius from the reference point.
As you fly the maneuver, crosscheck your speed, altitude, and bank angle, and occasionally monitor your engine instruments. Keep an eye out for traffic and note potential emergency landing sites.
As you complete the first 180-degree upwind portion of the maneuver, you should have the minimum bank angle, and you should cross the reference line at a 90-degree angle. Before you roll level to cross the reference line, be sure to check again for traffic in the direction in which you'll be turning.
Roll wings level momentarily as you cross the reference line, pick out your next reference point, and then roll into the next turn. Keep those crosschecks going as you increase the bank slowly to maintain the constant radius from the reference point. You should again reach your maximum bank (30 to 40 degrees) just before you cross the reference line the second time. Roll level as you cross the line, pick the next point, then roll in the maximum bank in the opposite direction.
Sound easy? Well, with practice and a clear understanding of the principles involved, flying S-turns can be almost as easy as crossing the street.
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