It happens time and again. A pilot puts the airplane on the runway in a reasonable position for a safe landing, then the airplane takes a trip off the side of the runway into whatever doom may be waiting.
While there occasionally are mechanical reasons for departing the side of the runway – such as flat tires, binding brakes or problems with some of the landing gears hardware, the vast majority boil down to seemingly simple mistakes in technique on the part of the pilot.
One of the most common is failure to land in the center of the runway with the aircraft moving in the same direction as the runway. As obvious as this sounds, you can convince yourself its true by sitting near the approach end of the runway and watching airplanes land. Pilot after pilot will use the entire width of a 100-foot-wide runway, despite the fact that they have only a 35-foot wingspan and about 10 feet of main wheel separation.
On the other end of the spectrum lie pilots such as one featured in an aviation-oriented television program. The pilot is an Alaska Airlines captain who flies his own Cessna 180 all around the Alaskan bush, including a landing on a – well, runway is too strong a word for it. The strip is about 1,500 feet long, and the cleared portion is only a couple of feet wider than the wheels of the plane; the wings hung out over tall brush. And, for what its worth, this captains regular run includes taking a 737 into a 3,500-foot runway.
The difference between this captain and the folks you normally see is that he controls the flight path of his aircraft all the way down final. He lines up early and does not let the airplane drift laterally off the extended centerline.
How Much Bank?
There are several elements involved in keeping runway alignment, along with some good control coordination. Student pilots learning crosswind landings often ask, How do you know how much bank angle to use in a crosswind landing? This question evinces a failure to understand both the changing wind conditions as the airplane proceeds down final both longitudinally and vertically, and the changes in aircraft dynamics as the airplane decelerates from approach speed to touchdown speed.
The first part of the problem is that whatever the wind is as youre descending through 200 feet on final, it is almost certainly different at flare height. Therefore, the amount of crosswind component changes as the plane descends – and that means the amount of horizontal lift component needed to compensate for drift also changes.
That would be true even if you were landing on a strip in the middle of a flat desert. When you add in the effects of all the trees, buildings and hills around the runway, the wind changes significantly every inch of the way. Further, whatever the wind is at the anemometer, its surely different at the end of the runway.
Therefore, you cannot accurately determine what the wind is at any particular point on the approach. Thus, there is no way to compute the amount of compensation youll need at any point.
The second part is that the airplane is decelerating as it approaches the flare and continues to decelerate as it touches down. This deceleration is accompanied by changes in the angle of attack to compensate for the changed speed. Unless you want a carrier-type landing, you must increase the amount of lift to reduce the aircrafts sink rate.
Oddly enough, while the amount of crab angle you need to compensate for a given wind increases as the aircraft decelerates, the amount of bank angle does not. However, the amount of control deflection to maintain that bank angle increases and the pilot must be continuously changing the yoke position to achieve that.
When you add in the fact that the wind is also changing, there are now twice as many factors at work forcing the pilot to make continuous corrections to control inputs.
Another problem that occurs is the effect of adverse yaw during this time of continuously changing aileron position and bank angle. One way pilots get into trouble in this area is through the failure to use proper rudder-aileron coordination when making bank angle inputs on short final or in the flare. They simply dont use enough rudder input at the same time as aileron input to keep the nose from yawing away from the runway centerline.
The best training solution for this is a Dutch roll exercise at altitude. In this task, the pilot must rock the airplane back and forth in roll, about 15 degrees each way, while keeping the nose nailed to a single point on the horizon.
First try it with your feet flat on the floor, and it wont take long to see the way the nose slices opposite the direction of yoke movement as the aileron is shifted from one direction to the other. Dont do this too much, as its fairly nausea-inducing.
Then, repeat the exercise using firm rudder application to hold the nose stationary in yaw. It takes some practice to get hands and feet moving in synchronization, but once it happens, the pilot will discover that such maneuvering is not only more physically comfortable but it also establishes better control aerodynamically.
Down the Runway
The Dutch roll exercise really pays off when adjusting bank angle on approach. Pilots often fail to grasp the importance of keeping the nose pointed in the same direction that the airplane is moving over the ground at the point of touchdown. You need to ensure that the wheels touch down in the same direction they turn rather than skidding sideways on the runway surface. This error is often referred to as touching down in a crab.
Unless the wind is blowing directly down the runway, an airplane must touch down in a slip if the wheels are to touch down aligned with the flight path over the ground. There are several negative effects of a crabbed touchdown.
The first, and most obvious, is that it is very hard on the tires. The second is that, particularly with landing gear systems like the light Cessnas and Grummans, the gear has some lateral flexibility, and the airplane can hop a bit. If that goes too far, the airplane can wind up on its back.
The bottom line is that as you approach the flare and touchdown, it is necessary to actively fly the plane all the way. This includes using bank angle changes to control lateral drift, and rudder inputs both to prevent adverse yaw from swinging the nose away from the desired direction and to keep the nose pointed in the same direction the plane is going.
One of the sources of problems in touching down straight is that students are taught from the beginning to make nice, coordinated turns. The inputs they are taught to make will cause the nose to yaw in the same direction the plane is banked, at a rate that keeps the ball in the center. Unfortunately, on short final, nice coordinated turns are not what the doctor ordered.
What pilots often try to do is correct for lateral drift by turning the airplane. Unfortunately, on short final, this is counterproductive. If you are over the runway centerline, you do not want the nose to turn away from the centerline. As a matter of technique, I teach students to get away from the idea of trying to make coordinated turns down final.
The objective is actually to have hands and feet moving independently. The airplane is banked to control lateral drift, and yawed to control the direction the nose is pointed. The two are related, but should not be married as they are in the normal coordinated turn.
One of the most important points is that the pilot cannot accept significant deviation left or right of centerline believing he can just turn it back and straighten out later. This laziness can put a pilot in a position where he is angling across the runway when low enough that there isnt time to correct. In the following accident report from the NTSB files, note that the pilot had drifted all the way to the right edge of the runway, and then tried to correct.
In a written statement, the pilot said he departed runway 28, a 3,600 foot-long, 75 foot-wide, asphalt runway, and remained in the traffic pattern. The pilot had completed a full stop landing and a simulated go-around, and was on his third approach to the runway, when he encountered a strong gust or shear, which pushed the airplane up and to the right. The pilot further stated, I had almost completed the correction from the right edge [of the runway] to the center line when the bottom dropped out, dropping me about 10 to 12 feet to the runway… The airplane bounced, began to porpoise, and the right wing and front propeller contacted the runway. The airplane then veered off the right side of the runway, and entered a ditch.
If the pilot accepts as a goal the idea of putting the airplane on the runway centerline and recognizes that the airplane needs to be pointed in the same direction its traveling, this all comes together. The bank angle becomes a way of controlling distance left and right of the centerline and the rudder is the means to control nose direction.
If the airplane is right of centerline, bank it more and more to the left until it starts moving back to centerline. Hold the bank until the airplane reaches the centerline, at which time you reduce the bank until the left-right drift stops. Use the rudder to prevent the plane from turning to the left.
As all these aileron inputs are made, rudder inputs are made to keep the nose pointed at the center of the approach end of the runway. As the airplane gets closer to the runway, the nose pointing aim point moves farther down the runway.
While it is true that this will result in an uncoordinated condition throughout the approach and touchdown, thats OK. It will also, unless the wind is right down the runway, result in a touchdown on the upwind wheel. That, too, is OK. After all, unless some bank is maintained, the crosswind component will cause the plane to drift laterally. Once the upwind wheel is on the ground, there is enough friction to prevent drift as the other wheels are lowered to the runway.
The hard work here is being demanding of yourself throughout the approach and landing. Its very easy to get a bit lazy and turn rather than slip the plane, or to allow the approach to drift a bit to one side and then approach the runway on an angle rather than right on the extended centerline. These are sloppy habits that force you to make bigger corrections later in the approach at a time when you want to have everything lined up and stabilized.
The Role of Power
Another area where a lot of planes wind up off the edge of the runway is during touch and goes, particularly right after the rotation on the go. Heres an example from the NTSBs files:
The student pilot was practicing touch-and-go landings on his second solo flight. After the first landing was completed, the pilot advanced the throttle for the next takeoff roll when the aircraft swerved to the left, impacting a snowbank along the left side of the runway. The pilot reported, I performed the first circuit correctly and placed the aircraft a little to the left of the centerline. My intention was to do a touch and go. I looked down upon touchdown to give the aircraft full throttle. As I did this I felt the nose shake a little. When I looked up, the tail simultaneously fishtailed to the right. The pilot reported, I was heading directly for the left side of the runway edge. I tried to correct the situation with right rudder, but my recovery was either not quick enough or I didnt get enough response from the rudder. The pilot stated, The nose hit the snow immediately at the edge of the runway and sunk in. The rest of the aircraft flipped over the sunken nose wheel.
The problem for pilots during events such as touch and goes is that there are four primary factors at work, all trying to pull the nose of the airplane to the left (unless youre flying something with an Eastern European engine, in which case it pulls right). Those factors are torque, P-factor, rotating slipstream, and gyroscopic force.
Torque is rotating force created by the rotation of the propeller and crankshaft. P-factor is the differential in thrust produced by the upward-moving versus downward-moving propeller blades due to their differing angles of attack when the airplane has a pitch attitude that is higher than the flight path vector – as when you are rolling on the runway with the nose pitched up. Rotating slipstream is the effect of the fact that the slipstream produced by the prop rotates around the airplane in the same direction as the prop, producing a significantly larger amount of push on one side of the vertical stabilizer (the left side for most singles).
Finally, there is a gyroscopic force produced as the airplane is rotating from level attitude on the runway to a positive takeoff attitude as the prop and crankshaft act as a gyroscope, which translates forces applied to it into a direction 90 degrees from the application of the force.
A lot of the forces described above are already acting on the plane as power is advanced, but an airplane with a steerable nosewheel on the runway has such a strong resistance to yaw that the nose doesnt move. However, as soon as the nosewheel breaks ground, that resistance is lost, and the nose pulls hard left as all four forces go to work.
Pilots must learn to anticipate this and apply right rudder simultaneously with the pitch input, much as they have learned to apply rudder simultaneously with aileron to prevent adverse yaw in flight. Instructors have to brief students to anticipate this effect so the student is not surprised by it.
For instructors dealing with student pilots, there are two good reasons for covering this in ground training and preflight briefings. First, the student may be startled by the effect – and a startled student is highly unpredictable. Second, on a narrow enough runway, the airplane may depart the edge before the instructor can make a compensating input.
In either case, an overcontrol of the rudder is likely, resulting in fishtailing. While this can usually be countered by lowering the nose wheel to the runway, it can rapidly result in a divergent oscillation and ground loop in a taildragger.
Perhaps the most insidious problem in directional control is the failure of the pilot to fly the plane all the way to the chocks. I cannot count the number of times Ive seen a pilot fly a beautiful approach all the way to the flare, and then just stop flying at the point the pilot thinks hes flared enough and touchdown is imminent.
The controls are left where they were as the airplane starts to drift, and the result is a touchdown either crabbed or drifting towards the weeds. There are many things happening at this point, so inaction is a bit puzzling.
As airspeed bleeds off, a continuously increasing back stick movement must be made just to hold the airplane in the same pitch attitude, since the reduced airflow over the elevator makes it decreasingly effective. Likewise, larger inputs of rudder and aileron are required to achieve the same roll and yaw forces.
This problem is intensified if the airplane bounces. The airplane is now at minimum flying speed or less, so very large yet very precise control inputs are needed to control the plane with very little airflow over the control surfaces. Consider this accident from the NTSB files:
The flight instructor was giving dual instruction to the pilot/airplane owner. They were practicing landings. On final approach, with a right crosswind, the pilot/owner flared high, and the airplane bounced. The airplane drifted off the left side of the runway, and an aborted landing was initiated. The instructor then determined that the altitude gained was insufficient to safely continue, so he retarded the throttle to effect a landing next to the runway. During the landing, the airplane rolled into muddy terrain, and the nose landing gear collapsed.
In this situation, the effects of the crosswind are compounded by the low airspeed – the slower you are, the greater the angle of drift created by a constant wind. In the particular accident above, the addition of power and rotation to climb attitude added significant left yaw to go with the crosswind from the right to create the ultimate combination of negative factors.
Preventing lateral excursions from the runway on landing requires that the pilot be pro-active from the beginning of the approach. This means anticipating wind effects in the pattern and through the turn to final so as to roll out on centerline, and aggressively controlling any deviation from centerline all the way down final. It means understanding the interrelationships between aileron, roll/bank, rudder, and yaw. It means using each control in a way that achieves the particular result desired – an airplane on centerline all the way down final, flare, and rollout, with the fuselage pointed in the direction the plane is moving.
Also With This Article
Click here to view “Tricks and Traps.”
Click here to view “By the Numbers.”
-by Ron Levy
Ron Levy is an ATP, CFI and director of the Aviation Sciences Program at the University of Maryland Eastern Shore.