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Flight Performance - Level 3

Airplane Performance-Section 8

WIND SHEAR

One of the basic principles of flight is, that airspeed is not affected by the movement of the air body itself. This is true except in the situation of wind shear which involves an abrupt or sudden change in wind velocity. Since the inertia of the airplane is far greater than that of the surrounding body of air, there is an inevitable lag in the airplane's response to the sudden increase or decrease of wind resulting in a temporary gain or reduction in airflow over the wings and therefore in airspeed. The change in airspeed may not last more than a few seconds. In cruising flight, there would be no serious problem but under some landing or take-off conditions, it could be critical, since it can cause stalls (in landings and take-offs), and undershoots or overshoots (during landings).

If, for example, the wind changes suddenly from a no wind condition to a 20 knot tailwind or if there is a sudden 20 knot decrease in the headwind (negative wind shear), inertia causes a several second delay before the airplane reacts to the change in wind during which the airspeed will fall by almost 20 knots. If the airplane is approaching to land at an airspeed near the stall, the approach path could steepen or a stall could occur since any loss in airspeed means a reduction in lift.

Conversely, if a positive wind shear (increase in the velocity of the headwind or a decrease in the velocity of the tailwind) of 20 knots takes place, again inertia causes a delay in the reaction of the airplane and the airspeed briefly builds up by about 20 knots. During this brief lag, there is an increase in lift as a result of the increased airspeed, the rate of descent decreases and there is a tendency to overshoot.

Cross wind shears and vertical shears also have an adverse effect on the airplane. An abrupt cross wind shear will make it weathercock into the new wind. An abrupt downdraft causes a brief decrease in the angle of attack of the wing with a resultant loss of lift. An abrupt updraft causes an increase in the angle of attack that, if the airplane is already near the stall speed, may push the angle of attack beyond the stall angle.

The important parameter is the rate of change of the wind's velocity with respect to time. This determines the pilot's and the airplane's ability to cope with the wind shear. If the airplane could be instantaneously accelerated or decelerated to respond to changes in wind speed, there would be no problem. However, there is an inevitable lag in pilot reaction time and in airplane response. Large airplanes with larger inertia factors adjust less quickly than do smaller general aviation airplanes. The outcome of a critical wind shear encounter depends, therefore, upon quick and correct action on the part of the pilot. Even a slight delay in initiating corrective action can result in the loss of crucial performance capability needed for an airplane to recover from a severe wind shear encounter at low altitude.

If a pilot is aware of a wind shear condition, he can compensate for the expected value of the shear by varying normal approach airspeeds. This is a solution only up to a point. Wind shear is complex and unpredictable. Wind changes can be gradual or abrupt. Strong headwinds or tailwinds can suddenly become weak or vice versa. There may be turbulence. There may be a lateral component that introduces drift and heading problems.

There are certain clues that, by alerting a pilot to its possible presence, can help him to avoid a wind shear encounter. They include pilot reports (PIREPS) from other pilots who have encountered it, low level wind shear alerting system warnings, the presence of thunderstorms and virga. During any landing or take-off, a pilot should be especially alert to any fluctuation in airspeed, vertical speed and attitude. Any excessive variation should be taken as an indication of wind shear and corrective action instantly initiated.

GROUND EFFECT

Every pilot has encountered the term ground effect. What exactly is it?

The total drag of an airplane is divided into two components, parasite drag arid induced drag. Induced drag is the result of the wing's work in sustaining the airplane. The wing lifts the airplane simply by accelerating a mass of air downward. It is perfectly true that reduced pressure on top of an airfoil is essential to lift, but still that is but one of the things that contribute to the overall effect of rushing an air mass downward. The amount of downwash is directly related to the work of the wing in pushing the mass of air down and therefore to the amount of induced drag produced. At high angles of attack, induced drag is high. As this corresponds to lower airspeeds in actual flight, it can be said that induced drag predominates at low speed.

When a wing is flown very near the ground, there is a substantial reduction in the induced drag. Downwash is significantly reduced; the air flowing from the trailing edge of the wing is forced to parallel the ground. The wing tip vortices that also contribute to Induced drag are substantially reduced; the ground interferes with the formation of a large vortex.

Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface.

Ground effect exerts an influence only when the airplane is flown at an altitude no greater than its wing span, which for most light airplanes is fairly low. A typical light airplane has a wing span of perhaps 35 feet and will experience the effect of ground effect only when it is flown at or below 35 feet above the surface (ground or water).

A low wing airplane is generally more affected by ground effect than a high wing airplane because the wing is closer to the ground. High wing airplanes are, however, also influenced by this phenomenon.

Pilots get into trouble because of ground effect when they precipitate take-off before the airplane has reached flying speed. Take the scenario of a pilot trying a take-off from a poor field. He uses full power and holds the airplane in a nose high position. Ground effect reduces induced drag and the airplane is able to reach a speed where it can stagger off. As altitude is gained, induced drag increases as the effect of the ground effect diminishes. Twenty or thirty feet up, ground effect vanishes, the wing encounters the full effect of induced drag and the struggling airplane which got off the ground on the ragged edge of a stall becomes fully stalled and drops to earth.

Ground effect is also influential in landing. As the airplane flies down from free air into ground effect, the reduction of induced drag as it nears the runway comes into, effect to make the airplane float past the point of intended touchdown. In the common case of an airplane coming in with excessive speed, the usable portion of the runway may slip by with the airplane refusing to settle down to land. A go around will probably be necessary. On short fields, approach as slowly as is consistent with safety.

An airplane also tends to, be more longitudinally stable in ground effect. It is slightly nose heavy. The downwash from the wing normally passes over the tail at an angle that produces a download on the tail. Ground effect deflects the path of the downwash and causes it to pass over the tailplane at a decreased angle. The tailplane produces more lift than usual and the nose of the airplane tends to drop. To counteract this tendency, more up elevator is required near the ground. During take-off as the airplane climbs out of ground effect, the download on the tailplane increases and the nose tends to pitch up.

.CRITICAL SURFACE CONTAMINATION

An accumulation of frost, snow or ice on the wings or other horizontal surfaces will substantially alter the lifting characteristics of the airfoil. Even a very light layer of frost spoils the smooth flow of air over the airfoil by separating the vital boundary layer air, producing an increase in stall speed and a decrease in stall angle of attack. It has been proven that a few millimeters of ice will increase the stall speed by as much as 20%. Any substantial accumulation of snow or ice, in addition to adding significantly to the weight of the airplane, so drastically disrupts the airflow over the wing, that the wing may not be able to develop lift at all.

There are a number of major factors that contribute to critical surface contamination and a knowledgeable pilot will recognize them as indicators of an icing condition.

Ambient temperature provides a good indication of the potential for icing conditions.

Aircraft surface temperature indicates the susceptibility of the aircraft to icing. Aircraft surface temperature is affected by solar radiation. An aircraft will have a warmer surface temperature on a sunny day than on an overcast day with identical ambient temperatures. When the fuel in a wing fuel tank is very cold, the cold fuel in the tanks can so chill the aluminum. wing surface that moisture in humid air or rain will turn to, frost or ice over the fuel tank.

Be alert to the conditions that cause icing even before going out to, your aircraft. Get a thorough weather briefing and the most up-to-date forecast so that you are aware of temperatures and precipitation at your stops and enroute.

Examine your aircraft very carefully prior to flight. Use your eyes and hands to examine the surfaces to ensure that your aircraft is "clean" before departing on a flight. Have the aircraft de-iced by ground crews if there is any contamination. Be sure that the de-icing fluid is used evenly on both sides of the aircraft and on the under as well as the upper surfaces. Use wing covers to protect your aircraft when it is parked.

GUST CONDITIONS

A gust or bump increases the load on the wings. The speed of the airplane should therefore be reduced when flying in gusty air. In approaching to land, on the other hand, a little higher speed should be maintained to assure positive control.

LOW FLYING

Reduce speed because of the limited visibility but keep a safe margin in case it is necessary to turn quickly to avoid a collision. Always keep one hand on the throttle, ready for any sudden emergency.

Keep more than a customary sharp look-out both ahead and on either side.

Remember that it is easy to overestimate actual airspeed when flying low downwind, because of the apparent high groundspeed, and a pilot has therefore a tendency to stall after turning downwind.

Flying low in poor weather should only be resorted to, of course, in an emergency, or when authorized to proceed with conditions below VFR minima by an air traffic controller.

SCANNING

The eye is a miraculous organ but it does have certain limitations. It is vulnerable to dust, fatigue, age, optical illusions, emotion, germs. In flight, vision is affected by atmospheric conditions, hypoxia, acceleration, glare, aircraft design, windshield distortion, etc.

More than this, however, the eye only sees what the mind lets it see. A day dreaming pilot sees nothing.

The eye is subject to focusing problems. It takes time to adjust the focus from near to far objects. In hazy conditions with no distinct horizon, there is nothing to focus on and a pilot will experience empty field myopia, not seeing even opposing traffic when it does enter his field of vision. Another problem is that of narrow field of vision or tunnel vision. The eyes are limited to a relatively narrow field of vision in which they can actually focus and classify an object. The mind cannot identify targets in the periphery.

Glare on a sunny day makes objects hard to see, especially during flight directly into the sun. Contrast creates another problem. An airplane over a cluttered landscape blends into the background and can be almost impossible to see.

Since visual perception is affected by many factors, it follows that pilots must learn to use their eyes in the most efficient and effective manner in an external scan.

Learn how to scan properly, knowing how to concentrate on the areas most critical at any given time. In normal flight, the critical area is about 60 to the left and to the right of the centre of your visual area and about 10 up and down from your flight path. The slower your airplane. the greater your vulnerability and therefore the greater the scan area required.

COLLISION AVOIDANCE

Collision avoidance involves more than proper scanning techniques. There are other important factors in the see and avoid principle.

Plan your flight ahead of time. Have charts folded and in sequence. Prepare a flight log with all the information that might be required during the flight so that you need to spend as little time as possible with your head clown in the cockpit.

To do a competent scan, the windshield and windows must be clean and free of obstructions, such as solid sun visors and window curtains.

Encourage your passengers to look for other aircraft and bring aircraft sightings to your attention.

All airplanes have blind spots because of their inherent design: a window frame, the wing or wing strut, the forward fuselage, etc. These blind spots are inevitable but can be compensated for by the pilot.

Never let down, turn or climb into a blind spot. When letting down, turning, or climbing, it is advisable to make a slight S turn to have a look before initiating the maneuver. During prolonged climbs or descents, outside positive control areas, execute gentle left and right banks every few thousand feet in order to broaden your field of vision and also to increase the likelihood of being seen as a result of motion and light reflection.

Sustained periods of straight and level flight outside positive controlled airspace should be broken at intervals by gently banking the airplane in each direction in order to broaden your field of vision.

Be especially mindful of the fact that pilots of high wing and low wing airplanes can be in each other's blind spot. Collisions of this type happen most frequently at uncontrolled airports, when the low wing airplane descends on top of the high wing airplane, especially on final approach or just before touchdown, although it can happen anywhere in the circuit.

With the introduction of mandatory communication procedures at uncontrolled airports, the chance of a collision of this type is substantially reduced but not eliminated. There have been incidents of pilots who were talking to each other, but unable to see each other, colliding. They were each other's blind spot. In such situation, it would be advisable for the pilot, caught in a situation in which he knows another airplane is in the same landing pattern as he, but is unable to see him, to assume that the other airplane is in his blind spot and to make a gentle level turn to the left. At the same time he should report his intentions on the mandatory frequency. Any turn, no matter how slight, will increase the separation between the two airplanes and decrease the risk of collision.

While flying at circuit height, your airplane will cast a shadow on the ground on a sunny day. So will other airplanes. Glance at your shadow occasionally and scan a wide circle around it. Two converging shadows could foretell a collision.

When another aircraft is approaching and it has movement. left, right, up or down, there is no danger of colliding. The rate of movement governs the margin of separation. If, however, the other airplane is approaching your track and there is no apparent change in the relative position at which you first saw it, you are on a collision course and should take immediate evasive action. Any turn, climb or descent will provide a margin of separation.

When flying cross-country, avoid high density areas unless landing.

The use of landing lights greatly enhances the probability of an airplane being seen arid thus is an excellent technique to avoid mid-air collisions. Therefore, turn on your landing lights both during day and night, while landing or taking-off, when flying below 2000 feet AGL within terminal areas and in aerodrome traffic circuits, while operating under Special VFR conditions, or in conditions of reduced visibility such as haze or at dusk.

High intensity strobe and anti-collision lights should be on at any time the airplane is in the air. However, they should not be used on the ground as they are distracting to pilots taxiing, awaiting take-off or on final approach to landing. Strobe lights should be activated only immediately prior to take-off and extinguished after landing.

A Mode C transponder will permit air traffic control (ATC) to use radar to see you, know your altitude and provide you and other aircraft with timely traffic advisories.


The material for this section is reproduced from the publication, FROM THE GROUND UP, with the permission of its copyright owner, Aviation Publishers Co. Ltd. No further reproduction is authorized, in any print, electronic or other form of media, without the prior consent of the publisher at http://www.aviationpublishers.com . Any questions regarding this portion of the website should be directed to Dr. Claudius Carnegie. Questions regarding the publication, FROM THE GROUND UP, should be directed to the publisher at info@aviationpublishers.com.

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Updated: May 03, 2008