Life exists at the bottom of the ocean of the air called the atmosphere. This ocean ex-tends upward from the earth's surface for many miles, gradually thinning as it nears the top. The exact upper limit has never been de-termined. Near the surface, the air is relatively warm from contact with the earth. As altitude increases, the temperature decreases by about 2° C. (3.5° F.) for every 1,000 ft. (normal lapse rate) until air temperature reaches about - 55° C. (-67° F.) at 7 miles above the earth.
For flight purposes, the atmosphere is di-vided into two layers: the upper layer, where temperature remains practically constant, is the "stratosphere;" the lower layer, where the tem-perature changes, is the "troposphere". Al-though jets routinely fly in the stratosphere, the private pilot usually has no occasion to go that high, but usually remains in the lower layer— the troposphere. It is in this region that all weather occurs and practically all light airplane flying is done. The top of the troposphere lies 5 to 10 miles above the earth's surface.
Obviously, a body of air as deep as the atmos-phere has tremendous weight. It is difficult to realize that the normal sea-level pressure upon the body is about 20 tons on the average person. The body does not collapse because this pres-sure is equalized by an equal pressure within the body. In fact, if the pressure were suddenly re-leased, the human body would explode. As alti-tude is gained,
the temperature of the air not only decreases but the air becomes thinner; therefore there is less pressure. At first, pressure is rapidly reduced to about 6,000 m. where the pressure is only half as great as at sea level.
Wind. The pressure and temperature changes produce two kinds of motion in the at-mosphere – vertical movement of ascending and descending currents, and horizontal flow known as "wind." Both of these motions are of primary interest to the pilot because they affect the flight of aircraft during takeoff, landing, climbing, and cruising flight. This motion also brings about changes in weather, which may make the differ-ence between a safe flight or a disastrous one. Conditions of wind and weather occurring at any specific place and time are the result of the gener-al circulation in the atmosphere.
The atmosphere tends to maintain an equal pressure over the entire earth, just as the ocean tends to maintain a constant level. When the equi-librium is disturbed, air begins to flow from areas of higher pressure to areas of lower pressure. It has been thought that wind cannot affect an aircraft once it is flying except for drift and groundspeed. This is true with steady winds or winds that change gradually. It isn't true, how-ever, if the wind changes faster than the aircraft mass can be accelerated or decelerated. It is es-pecially true when wind changes direction and speed near the earth’s surface.
Surface wind is very important to pilots because of the effect it has on take-offs and landings. Surface wind is measured at 10 meters above level and open ground, i.e. where wind socks and other wind indicators are generally placed. Wind is usually less strong near the surface than at higher levels.
When the wind flows around an obstruction, it breaks into eddies – gusts with sudden changes in speed and direction – which may be carried along some distance from the obstruction. A pilot flying through such turbulence should anticipate the bumpy and unsteady flight that may be en-countered. This turbulence – the intensity of which depends upon the size of the obstacle and the velocity of the wind – can present a serious hazard during takeoffs and landings. For exam-ple, during landings it can cause a pilot to "drop in"; during takeoffs it could cause the aircraft to fail to gain enough altitude to clear low objects in its path. Any landings or takeoffs
© НИЛ НОТ НИО УВАУ ГА(и), 2009 г
Aviation English Fundamentals | 65
attempted under gusty conditions should be made at higher speeds, to maintain adequate control during such conditions.
This same condition is more noticeable where larger obstructions such as bluffs or mountains are involved. The wind blowing up the slope on the windward side is relatively smooth and its upward current helps to carry the aircraft over the peak. The wind on the leeward side, following the terrain contour, flows definitely downward with considerable turbulence and would tend to force an air-craft into the mountain side. Large moun-tains or mountain ranges cause an effect on the wind that may extend well above ground level, resulting in mountain waves. The stronger the wind, the greater the downward pressure and the accompanying turbulence. Consequently, in approaching a hill or moun-tain from the leeward side, a pilot should gain enough altitude well in advance. Be-cause of these downdrafts, it is recommend-ed that mountain ridges and peaks be cleared by at least 700 m. If there is any doubt about having adequate clearance, the pilot should turn away at once and gain more altitude. Be-tween hills or mountains, where there is a canyon or narrow valley, the wind will gener-ally veer from its normal course and flow through the passage with increased
velocity and turbulence. A pilot flying over such terrain needs to be alert for wind shifts and par-ticularly cautious if making a landing.
Wind Shear. Wind shear is best described as a change in wind direction and/or speed within a very short distance in the atmosphere. Under certain conditions, the atmosphere is capable of producing some dramatic shears very close to the ground. This, however, is not some-thing encountered every day. In fact, it is un-usual, which makes it more of a problem
The most prominent meteorological phe-nomena that cause significant low-level wind shear problems are thunderstorms and certain frontal systems at or near an airport.
Basically, there are two potentially hazard-ous shear situations. First, a tailwind may shear to either a calm or headwind component. In this in-stance, initially the airspeed increases, the aircraft pitches up and the altitude increases. Second, a headwind may shear to a calm or tailwind com-ponent. In this situation, initially the airspeed de-creases, the aircraft pitches down, and the altitude decreases. Aircraft speed, aerodynamic characte-ristics, power/ weight ratio, powerplant response time, and pilot reactions along with other factors have a bearing on wind shear effects. It is impor-tant, however, to remember that shear can cause problems for ANY aircraft and ANY pilot.