Weather Charts and Frontal Wind Shear
Weather charts may give information about the possibility of shear during initial climb-out or approach.
Such information could be:
a. the position of the front relative to the runway: see examples related to fig. 2 and fig. 3;
b. the temperature difference across the front: with a 10°F/5°C or greater temperature differential significant frontal shear may be expected;
c. the speed at which the front is moving: with a frontal speed of 30 kt or more significant frontal shear may also be expected.
Frontal Shear Examples
Airport A: Surface wind, below the front, shortly after passage: 320°/20 kt. Wind in the warm sector, above the front: from 220°, arrow BC.
A take-off to the NW with heading unchanged during initial climb will cause no particular problems: a light headwind is becoming a crosswind.
A left turnout after take-off, to a heading of approximately 220° will create a performance increasing condition due to increasing headwinds.
A right turnout after take-off, to a heading of approximately 040° will bring the aircraft in a performance decreasing situation.
An approach to a runway 27 is an example of possible strong negative wind shear when the front is passed during final approach.
Airport D: Surface wind, below the warm front, approximately 080°/20 kt. Wind above the warm front, from 250°, arrow EF.
A take-off and climb-out to the North-East will bring the aircraft in the undesirable performance decreasing situation.
A right or left turnout, approximately 90° heading change, will minimize the shear effects. An approach to the North-East is an example of a possible strong tailwind on the approach becoming a headwind.
Shear Associated with Inversion
A lot of cases of strong vertical shears occur in a stable flow or within a stable atmosphere. Most of all these cases were observed at night and/or were associated with marked temperature inversions.
Examinations have confirmed the existence, in many countries, of temporary low level jetstream flows in the boundary layer, usually below a marked radiation inversion.
The aviation problem here is that on radiation nights the reported surface wind is usually light and that there are no visual cues. In case of an approach a comparison between surface temperature and wind with upper values may draw the attention to this type of shear.
Shear Caused by Terrain or Obstructions
In mountainous areas mountain wave systems may produce strong, closely spaced shears, in both horizontal and vertical plane.
Terrain irregularities or surface obstructions, such as buildings, are an important source of strong low level wind shear.
Thunderstorms
Winds around a thunderstorm are complex. Wind shear can be found on all sides of a cell. The wind shift-line or gust front associated with thunderstorms can precede the actual storm by up to 15 NM.
Consequently, if a thunderstorm is near an airport of intended landing or take-off, low level wind shear hazards may exist.
In case the flight path goes below or through the storm cell the dangerous effect of downburst may be encountered as well. (See fig. 4).
Microburst
A microburst can be defined as:
– an intense downdraft, dry or wet, with vertical speeds up to 6000 or 7000 ft per minute;
– core diameter 1200 to 2400 meter extending to 3200-4000 meter at surface level;
– maximum gust velocity of the air after diverging at the surface 60 to 80 kt;
– lifetime only a couple of minutes (2-4).
Scattered to broken high-based cumulus clouds at 9000 to 10,000 feet with very warm and dry surface conditions in combination with scattered light rain and virga (valstrepen) are precursors of an impending microburst. When precipitation falls into very dry air from high-based convective clouds a column of air much colder than the surrounding air is created as the falling moisture evaporates, thereby extracting heat from the air. This cold air rushes downward and may reach the ground as a microburst.
High surface temperatures and a large dew point spread (20°C) are usually associated with this phenomenon.
Microbursts also occur from small, rapidly building storm cells that may not yet contour on radar, especially when the antenna tilt is downward. It is suspected that large amounts of water are carried to the upper portion of a building cell by updrafts until the water can no longer be supported. Then a large downdraft is generated, which fires a pulse of high-velocity air and/or water at the ground.
As the diameter of a microburst – as the name implies – is small, they often go unnoticed by anemometers and raingauges.
Effects of Shear
Departure
Wind shear can create hazardous conditions during take-off.
a. An increasing headwind or decreasing tailwind will cause an increasing aircraft performance: increase in indicated speed, increase in lift, causing a pitch up and increasing rate of climb.
Normally, except during approaches, this condition should not cause a problem if the crew is aware of how this shear affects the aircraft.
b. A worse situation on departure occurs when the aircraft encounters a decreasing headwind or increasing tailwind. This will cause a decreasing aircraft performance: decrease in indicated airspeed, decrease in lift, causing an initial pitch down and decreasing rate of climb, possibly becoming a rate of descent.
A preventive measure is the most effective way to combat wind shear during climb-out e.g. adaptation of take-off and climb-out direction. If initial climb-out through a shear condition is suspected, early build-up of extra speed may provide the necessary airspeed cushion.
c. If a micro- or macroburst is encountered performance capabilities become critical: The airplane is subject to the following phenomena:
– an increasing performance wind shear as it enters the gustfront, associated with the microburst;
– an intense downdraft as it flies through the core area;
– a decreasing performance wind shear as it enters the outflow of air at the far side of the microburst. (See fig. 5).
Microburst Encounter Procedures
Therefore a microburst encounter should be avoided, if possible.
In case of an unexpected encounter the following actions should be taken:
– If derated thrust is used, full take-off thrust should be set immediately.
– Existing pitch attitude should be maintained or even increased. Every pilot is induced to reduce pitch attitude as soon as a speed decay is noticed; this temptation should be resisted as it will only aggravate the problem.
– If ground contact seems inevitable, pitch attitude should be further increased until reaching stick shaker actuation speed. It should be appreciated that V2 speed is at least ten knots above stick shaker actuation speed.
– Pitch attitude should be increased smoothly, not by a sudden pull as otherwise a dynamic stall will occur, rotation rate should be 1 to 2 degrees per second. Once the stick shaker starts to operate, the full force should be slightly relaxed to stop stick shaker operation. The flight director and speed command system will not give correct commands in this situation due to built-in pitch limits and margins above the stall.
– Stabilizer trim should not be used, to facilitate the subsequent recovery, so elevator control forces required may be high.
– As soon as positive climb is apparent from (barometric or) radio altimeter indications, pitch attitude shall be slightly reduced, while maintaining a positive gradient.
Lift Curve and Downburst Angle of Attack
The effect of an increase in pitch attitude and thus angle of attack on the coefficient of lift is illustrated in fig. 6. It should be noted that CLmax is obtained at an angle of attack considerably higher than the angle of attack corresponding to V2 speed.
From fig. 7 it can be seen that there is a dramatic decrease in angle of attack at the same pitch angle when the microburst is encountered.
Although the basic lift equation L = CL . S . 1/2ρV² is only valid in stabilized 1G conditions, it gives a good idea what the remedy is when encountering a downburst. As a speed increase can only be obtained at the expense of a serious altitude loss, the correct procedure is to increase CL and thus angle of attack.
Encounter with Downburst in Heavy Weight Take-off
Approach: Shear Decreasing Aircraft Performance
The most hazardous consequences occur when wind shear occurs close to the ground. Assume power set, airspeed stabilized, descent profile normal.
a. First assume the situation of a shear decreasing the performance. Consider an aircraft flying on a stabilized approach at 140 kt IAS with a 20 kt headwind, which shears away completely (fig. 8).
At shear entry several things will happen: point A in fig. 8:
– airspeed drops from 140 to 120 kt (approximately 15%);
– lift drops (approximately 27%);
– the nose pitches down;
– the aircraft drops below the glide slope.
The aircraft will then be both slow and low.
If the pilot only pulls-up the nose, up to a point even higher than before the shear in an effort to recapture the glide slope, the airspeed situation deteriorates.
Aggressive power application will be necessary to a power setting much higher than the initial setting, in combination with an increase in pitch.
If corrective pitch up and power application are insufficient the aircraft may land short, slow and hard: point B, fig. 8.
If the glide slope can be regained before touch down the so-called double reverse problem arises: point C, fig. 8.
If the airspeed has returned to 140 kt on the glide slope, the ground speed and rate of descent will be higher and thus the required power will be lower than before shear entry. So, as soon as glide slope and speed are regained the power levers must be pulled back even further than the initial setting.
If the pilot does not quickly retard the power levers, the aircraft will soon have an excess of power, resulting in being high and fast over the threshold: point D, fig. 8.
Approach: Shear Increasing Aircraft Performance
b. When on approach a shear is encountered which increases the performance, the reverse of a. is applicable. Initially IAS and pitch will increase and the aircraft will balloon above the glide slope: point A, fig. 9.
Power should initially be reduced and pitch be decreased to correct this condition or the approach may be high and fast with the danger of overshooting: point B, fig. 9.
However, after the initial power reduction is made and the aircraft is back on speed and glide slope, the “double reverse” again comes into play: point C, fig. 9.
An appropriate power and pitch increase will be necessary to restabilize in the new wind-groundspeed-rate of descent condition.
If power and pitch increases are not accomplished promptly, a high sink rate can develop and the landing may be short and hard: point D, fig. 9.
Approach: Significant Downburst or Microburst
c. In significant downburst or microburst conditions the recovery techniques are essentially the same as in the departure phase; simultaneous application of go-around thrust together with a pitch attitude increase to go-around attitude or higher. Do not worry too much about loss of airspeed until approaching stickshaker speeds.
Performance data show that airspeed changes of as much as plus 20 kt or minus 20 kt from VREF do not produce large degradations in the maximum climb capability (stickshaker speed will be approx. 25 kt below VREF).
It will be obvious that there is an overall improvement in the climb performance when a reduced landing flap setting is used.
Furthermore it should be fully appreciated that when encountering an increasing performance wind shear on approach, this could be a precursor of a microburst encounter. If in this case the above-mentioned actions are taken, the aircraft will be in a situation of low thrust setting with speed decaying the moment after microburst is entered with fatal results.