Traffic patterns may vary somewhat at different soaring sites. Generally the pattern includes an entry leg at a 45-degree angle to the downwind leg, which is followed by a base leg and final approach. Notice that the base leg usually requires that the pilot crab into the wind. The pilot should anticipate that the turn from downwind to base will be more than 90o, and the turn from base to final less than 90o. However, the base leg can be adjusted as necessary to correct for altitude or avoid traffic.
Although the pattern and landing require a lot of attention, the pilot must not fail to maintain constant vigilance for other traffic. Pattern entry should be about 1,000 feet agl and a pre-landing checklist should be completed by the time the glider is on the downwind leg. One popular pre-landing checklist is USTALL:
|U||UNDERCARRIAGE||Check landing gear|
|S||SPEED||Establish pattern speed|
|T||TRIM||Set for landing|
|A||AIRBRAKES||Check dive brakes, spoilers, flaps|
|L||LOOKOUT||Check wind, traffic, landing area|
|L||LAND||Concentrate on landing|
|Although it is generally true that the glider sees only its relative wind and is not affected aerodynamically by motion of the air mass through which it is flying, that is not so when the glider is moving across an area of wind shear. As an analogy consider stepping on to or off of a moving sidewalk. Once you are on or off the sidewalk, motion relative to the surface you are walking on is pretty much the same, but during the transition you have to be careful.|
|Two kinds of wind shear that a pilot may encounter in landing that can cause significant airspeed fluctuations are wind velocity gradient and wind gusts. In the case of wind velocity gradient, the glider is descending into a decreasing headwind caused by friction between the wind and the surface. The result can be a decrease in airspeed and unless the pilot anticipates this effect by adding a safety factor the glider sink rate may increase significantly or in an extreme case the glider may stall. Wind gusts refer to sudden changes in wind speed or direction. They occur in both directions and can cause airspeed increases or decreases. If surface winds are reported to be 20 KTS gusting to 30 KTS that means you could expect airspeed changes of plus or minus 10 KTS.|
Recommendations for adjustments to airspeed for wind vary. Some advocate adding one half the wind speed to the calm wind speed; some say add half the gust factor; some say add half the wind speed plus a gust factor; and one says add the total wind speed. Adding half the wind speed to the calm wind approach speed will most likely be the answer the FAA is looking for. Finally, remember that performance speeds are all based on indicated airspeed so use the same value for any density altitude, but anticipate higher ground speeds at high density altitudes.
The turn to base leg is normally started when the anticipated touchdown spot is behind the glider at about a 45-degree angle to the downwind leg. Under standard conditions this will occur at an altitude about 500-600 feet agl, but the pilot should be flying primarily by outside visual references at this point instead of by reference to the altimeter. The base leg provides an opportunity to make necessary pattern adjustments, primarily altitude corrections, so that final approach can be flown as consistently as possible. Speed control from this point on is especially critical because of the low altitude. The turn to final approach is potentially dangerous. Skidding this turn can result in a spin at an altitude that precludes recovery. Slipping in the turn to lose altitude is perfectly acceptable; otherwise the yaw string should be centered and pattern speed maintained.
The skill task most frequently failed on glider pilot practical tests is 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 over-fly 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.)
|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.|
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.|
|Crosswinds are reported in terms of their speed and direction. Since the direction rarely is 90o to the runway, it is helpful to break the total wind into its crosswind and headwind components. A chart similar to the one shown here appears in the Computer Testing Supplement for that purpose. The first step is to determine the wind angle relative to the runway. This is accomplished by subtracting the runway heading from the wind direction. Lets assume that is 30o in this example. Then find the point where this angle intersects the wind speed arc, assume 40 kts. The headwind component can be read by moving horizontally from this point to the left, where we read 35 kts, and the crosswind component is found directly below the point, to be 20 kts.|
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 sideslip for crosswind correction some time during final approach.
There is often confusion about the difference between a sideslip 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 sideslip. 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 sideslip, 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.|
|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 centerline and the horizon. If the glider is over the centerline, 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 sideslip 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.|
|From the pilot's perspective the only difference between a sideslip 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.|
It is also true, though not intuitively obvious, that the amount of wing down decreases as airspeed decreases when executing a side slip. The horizontal component of wing lift depends only on bank angle. Changes in speed require offsetting changes in angle of attack to preserve vertical lift equal to the glider weight, so the horizontal component of wing lift is independent of speed. The horizontal component of fuselage lift, however, varies with both the wind correction angle (its angle of attack) and the square of airspeed. The wind correction angle increases as speed decreases, assuming a constant wind speed. At small angles it is approximately a linear relationship (i.e. a 10% reduction in speed results in approximately a 10% increase in wind correction angle.) If that were the only factor, the horizontal component of fuselage lift would also increase about 10% and an increase in bank angle would be required to equal it. However, the horizontal component of fuselage lift varies as the square of speed so a 10% decrease in speed results in an approximate 20% decrease in fuselage lift. A 10% increase in the horizontal component of fuselage lift due to wind correction angle minus a 20% decrease due to speed reduction results in a net decrease in the horizontal component of fuselage lift. Since the horizontal component of wing lift depends only on bank angle, it is necessary to reduce the bank angle to match the reduced fuselage effect of reduced speed.
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. Schweizer recommends slipping the SGS 2-33 at 45 to 50 mph.
|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 is 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.|
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. If the approach has been flown correctly the landing should be easy. The accuracy of the landing has been largely determined already. Trying to stretch it out or force it on has limited value. The essential difference between landing and crashing is the rate of descent when you touch the ground. As the glider nears the ground the objective is to reduce its rate of descent to zero gradually. If you fail to do so, it will reach zero anyway, abruptly upon contact. The glider pilot usually has two ways to reduce the descent rate. Spoilers or dive brakes, if deployed, can be retracted. The other is to trade airspeed for a reduction in descent rate, which is accomplished through a gradual increase in pitch attitude, but not so much that an increase in altitude results. If the glider does balloon, close the spoilers/dive brakes and initiate a new landing. Raising the nose just before touchdown is a good idea since touching down at minimum speed is desirable for passenger and aircraft comfort. Forcing the glider on at an excessive speed may make stopping gracefully impossible.
If the pilot recognizes that the accuracy of the landing has been jeopardized it is probably best to strive for a good touchdown anyway. Here is some advice from the "Airplane Flying Handbook" on where to look when trying to judge height above the ground. "If the pilot attempts to focus on a reference that is too close or looks directly down, the reference will become blurred, and the reaction will be either too abrupt or too late. In this case, the pilot's tendency will be to overcontrol, roundout high, and make full-stall drop-in landings. When the pilot focuses too far ahead, accuracy in judging the closeness of the ground is lost and the consequent reaction will be too slow since there will not appear to be a necessity for action, this will result in the airplane flying into the ground, nose first." (9)
|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. It is always surprising to see an applicant stop more than 200 feet (100 for commercial applicants) from the specified point, when closing the dive brakes and keeping the skid off the ground would have extended the roll out enough to meet the requirement.|
Although never desirable, downwind landings are sometimes necessary. Maintaining a safe airspeed is essential in any landing. The only concession a pilot could make here is to eliminate any allowance for wind velocity gradient since it is acting favorably on a downwind landing. In any case, ground speed will be higher than when landing into the wind, affecting the pilot's judgment as well as the space needed to land. The higher groundspeed requires that the aim point be moved farther back to compensate for a longer float. A successful touchdown may be only the beginning of the problems faced during a downwind landing. Directional control in gliders is achieved only aerodynamically (no steerable wheels) so everyone becomes a passenger while the glider is still moving relatively fast over the ground. In fact the control functions reverse when air starts flowing backward over them so instinctive control inputs will only make things worse. "The Joy of Soaring" recommends, "Once a swerve is underway, locking the wheel will help, assuming the brake is in good enough condition to do so; anyway, use the brakes hard." (12)
Despite the best of planning, the pilot may still encounter surprises late in the landing sequence. One is to find a dust devil right on the runway ahead. "The Joy of Soaring" suggests, "When the pilot sees swirling dust, leaves or debris ahead, he has warning and should take instant action. He should close the spoilers and dive close to the ground to pick up airspeed. As soon as he is over the fence he should touch down at whatever airspeed, and while rolling try to stay out of the thermal." (12)
|Another landing hazard is wake turbulence from other aircraft. This is the same disturbance we found behind the tow plane except that it can be much greater behind large aircraft, where the rolling moments may exceed the roll control authority of the glider. Wake turbulence occurs when the aircraft wing is producing lift, so expect it to begin where a departing aircraft leaves the ground and end beyond where a landing aircraft touches down.|
|Avoid flight below and behind a large aircraft's path. It is strongest when the aircraft is heavy, clean, and slow. Wake turbulence sinks behind the generating aircraft and spreads out at 2 to 3 KTS when it reaches the ground. A light crosswind could move a vortex from an adjacent runway to the one you are landing on or it could cause one to remain on your runway. Helicopters in forward flight produce wake turbulence vortices similar to fixed wing aircraft.|
The "Aeronautical Information Manual" warns of some illusions that may alter a pilot's perception of the landing environment:
© 2005 Jim D. Burch 602-942-2734 email@example.com
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