Landing

LANDING

The skill task most frequently failed on glider pilot practical tests is AREA IV, TASK Q, NORMAL AND CROSSWIND LANDING. It involves demonstrating the ability to touch down at a predetermined location and to stop within a specified distance from a specified point. The ability to stop where one wants to is enhanced by being able to touch down at the appropriate spot, so we will first examine touch down accuracy.

This involves selecting an aiming point some 150 to 200 feet short of the intended touch down point. In our illustrations the aiming point is shown as a red target. The point where the glider flight path intersects the ground is shown as a blue X, and the actual touch down point is shown as a green rectangle, approximately 150 feet beyond the blue X. If the aiming point is chosen correctly and the glider is flown correctly, the aiming point and the point where the flight path would intersect the ground will coincide. Since flying into the ground is considered poor practice, the pilot should flare just before reaching the aiming point and touch down farther down the landing area.

As long as the glider's flight path continues toward the aiming point, the aiming point has no relative motion within the field of view. It only appears to grow larger as it is approached. If constant pitch attitude is maintained, the point will move neither up nor down on the glider canopy. It is unlikely that the pilot will have a large red target on the ground at the aiming point, so it becomes necessary to develop the skill of selecting some ground feature to use instead. Once the aiming point is chosen, the pilot should concentrate on it rather than the touch down spot, until it is time to flare.

The absence of apparent relative motion of the point where the glider flight path intersects the ground is because its angle below the horizon is constant. This angle is shown in the illustration in magenta. Because the horizon is always at eye level, a line to the horizon is horizontal at any altitude.

If the current flight path will result in landing long, the point where the flight path intersects the ground will be beyond the aiming point. The angle of the aiming point below the horizon will increase as the glider approaches it and eventually flies over it.

From the glider, this results in the aiming point appearing to move down on the canopy or toward the glider. The glider will overfly anything that has this apparent movement. To correct for this, the pilot must steepen the glide path until the aiming point becomes stationary on the canopy or no longer appears to be moving toward the glider.

Making the glide path steeper means decreasing the glide ratio (L/D ratio) which means increasing drag. There are at least three ways to do that. The pilot can deploy spoilers/dive brakes, slip the glider, or increase speed by diving at the aiming point. (If a glider has flaps but not spoilers/dive brakes, the flap function in landing is essentially the same as any other drag device.)

Just diving at the aiming point is not a very practical solution because dissipating the excess speed, once the glider flares in ground effect, will carry the glider far beyond the intended touch down spot. However, "The Joy of Soaring" advocates pointing the glider at the aiming point and controlling speed with spoilers/dive brakes. Many instructors prefer to think of controlling air speed with pitch and use the spoilers/dive brakes to control glide path. The glider, of course, only responds to the combination of pitch and spoiler/dive brake without regard for what the pilot is thinking as he/she actuates the controls. In extreme cases the pilot may need full spoilers/dive brakes and forward slip as well to achieve the required glide path. In this case it would seem more logical to assign speed control to pitch. Since no one questions using pitch for glider speed control in inter-thermal flight, consistency suggests that we assign speed control to pitch and glide path control to spoilers/dive brakes and/or slips in landing as well.

If the current flight path will result in landing short, the point where the flight path intersects the ground will be closer than the aiming point. The angle of the aiming point below the horizon will decrease as the glider approaches it.

From the glider, this results in the aiming point appearing to move up on the canopy or away from the glider. To correct for this, the pilot must shallow the glide path until the aiming point becomes stationary on the canopy or no longer appears to be moving away from the glider. To make the glide path shallower means increasing the glide ratio (L/D ratio) which means reducing drag. Closing the spoilers/dive brakes is the only way to make a significant improvement in glide ratio, so it is good practice to plan to fly the final approach with half spoilers/dive brakes. Speed changes are not appropriate here since the pattern speeds for most gliders are very close to their best glide speeds and change in either direction will only steepen the glide path.

This is another good reason for assigning speed control to pitch and glide angle to spoilers/dive brakes. Attempting to stretch the glide by pulling back on the stick never works and can be disastrous.

The correct selection and use of an aiming point is essential in all landings. The task is further complicated when the pilot must also contend with a crosswind, shown in the following illustrations as a large blue arrow.

Remembering that the glider knows only performance relative to the air mass it is flying in, this illustration shows what would happen if the pilot encountering a crosswind maintained the heading that initially pointed the glider toward the runway. The path of the glider within the air mass is shown in red. Its path relative to the ground is shown in green.

To compensate for the drift caused by the crosswind, the pilot must alter the flight path through the air. One method for doing this is called crabbing, in which the glider is moving somewhat sideways over the ground.

It involves pointing the glider into the wind at an angle that produces the desired path over the ground. Once the glider is established on this heading (and assuming a constant wind) the controls are neutralized, and the glider continues in normal flight. The path of the glider within the air mass is again shown in red. The path over the ground, shown in green, is directly to the runway.

An obvious problem with using crab to correct for a crosswind is that the glider longitudinal axis must be aligned with the runway before touchdown. All aviation texts address this need by saying that the pilot must yaw the aircraft into alignment with the runway just as it is touching down. In one examiner's experience, few pilots do this very well because it is difficult to determine exactly when to initiate the yaw. Instead, most switch to a side slip for crosswind correction some time during final approach.

There is often confusion about the difference between a side slip and a forward slip. There is no difference aerodynamically, and many feel that the names should be reversed. Perhaps the best way to keep the terminology straight is to relate the name to application. If the slip is to correct for a crosswind it is a side slip. If it is to steepen the glide path it is a forward slip.
The flight path of the glider within the air mass during a side slip, again shown in red, is identical to that used when crabbing, and the path over the ground, shown in green, is also the same. In a slip the glider is moving through the air sideways, in the direction of the low wing.
When executing the side slip it may be helpful to think of the controls as if they were independent, even though they always do interact. In this case, the aileron controls the glider's lateral position over the runway or its extended center line. The rudder is used to keep the glider longitudinal axis aligned with the runway. A change in one will necessitate a change in the other, but the visual clues and the corresponding corrective action can be separated.

It would seem that recognizing when the glider is over the extended runway centerline would be easy, but many pilots have trouble with this task. One way to verify alignment is to note the angle between the runway center line and the horizon. If the glider is over the center line, the angle will be 90 degrees (i.e. the runway will be perpendicular to the horizon). If it is not, the pilot should correct by increasing bank in the direction the runway is pointing until the 90 degree angle is achieved. Then it may be necessary to continue with some wing down to maintain the position.

A pilot could meet the requirements of TASK Q by using spoilers/dive brakes and crabbing and never demonstrate a slip. The FAA closed that loophole with AREA IV, TASK R, SLIPS TO LANDING. The first objective in every task in the test standard requires that the applicant exhibit knowledge of the elements related to the task, so let's take a look at the elements related to slips.

In both the forward and side slip the glider is moving in a straight line at a constant speed. According to Newton that means that the forces acting on the glider must be balanced. Most pilots learn in lesson #1 that turns are caused by the "horizontal component of lift", produced by banking the glider so that its wing lift is no longer directed up. In a slip we prevent turning by yawing the glider in the opposite direction from the bank. In so doing the fuselage is now placed at an angle to the relative wind where it produces "lift" that exactly offsets the horizontal component of wing lift. Because the wing is a much more efficient airfoil than the fuselage, the fuselage yaw angle needed to do this is three to four times greater than the bank angle.

That is fortunate for glider pilots because it allows for a relatively large wind correction angle in a side slip before the glider wing tip would touch the ground in landing.

Airspeed control is important during a slip, but the airspeed indicator may not be reliable because the pitot/static sensors are not aligned with the flight path. If the pilot notes the pitch attitude that produces the desired pattern speed before the slip is initiated and maintains that pitch attitude during the slip, the airspeed likely will be close enough. During eighteen years of examining glider pilots, none ever slipped too slowly. Many slipped too fast and usually overshot their intended touch down spot because of doing so.

Pilots need to understand the difference between slips, which are useful and safe, and skids, which serve no useful purpose and can be extremely dangerous. In both cases the glider is moving somewhat sideways through the air. In the slip it is moving toward the low wing. If it moving toward the high wing or with wings level it is skidding. If the glider stalls while skidding it will almost certainly enter a spin. If this occurs at low altitude, perhaps by skidding the turn to final approach, there will be insufficient altitude to recover. In order to avoid even a momentary skid, pilots should enter slips by first lowering the appropriate wing and follow immediately with the corresponding rudder displacement. As the wing is lowered, adverse yaw will move the glider nose in the correct direction so initial rudder application is unnecessary.

From the pilot's perspective the only difference between a side slip to correct for a crosswind and a forward slip to steepen the glide path is that the nose of the glider is not aligned with the runway in the forward slip. The ailerons are still used to keep the glider over the runway centerline, but the rudder is used to control the severity of the slip, rather than runway alignment. A maximum slip requires full rudder displacement. Any further increase in the corresponding bank will result in a turning slip, which can be used safely in the turn from base to final if the amount of altitude loss needed dictates doing so. Just as when crabbing, the pilot must align the glider longitudinal axis with the runway before touching down.

As noted previously, if the pilot does not begin a flare a few feet above the ground, the glider will crash at the spot where its flight path intersects the ground. The flare is simply a maneuver to reduce the glider's vertical speed to zero just as it touches the ground. It is accomplished by increasing pitch, which results in trading airspeed for a reduction in sink rate, but not so much that the glider starts to climb. The pilot may also adjust the spoilers/dive brakes to vary the sink rate and modify the touch down point. If the approach has been flown correctly, careful coordination of pitch and spoilers/dive brakes should result in a smooth touch down at the desired point.

Stopping within the specified area should also be easy if the glider touched down at the correct point and at the correct speed. The pilot has a lot more control over the roll out than one might think for an aircraft with no power. Spoilers/dive brakes are very effective during the early part of the roll out. If the pilot landed with partial spoilers/dive brakes, closing them will have the same effect as adding power in a taxiing airplane. Opening the spoilers/dive brakes further, engaging the wheel brake, and dropping the skid if the glider has one, all can be used to shorten the roll out.

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