This page was created by Jonas Pfeiffer.
As you can probably guess, air and its movement are elemental to aviation. As lift is proportional to air density and the square of the velocity
relative to the lifting surface, variations in the air, its speed or direction impact lift a lot.
This is especially true in a take off or landing scenario, as slow flight situations are generally more
dangerous and lift reserves are smaller (due to high angles of attack and low speeds).
As a small introduction to the whole air complex, the following section will demonstrate the differences
between airspeed (VA),
ground speed (VK) and wind speed
(VW).
To that purpose, I have decided to include some STOL examples.
As aircraft which are built to to take off and land in short distances are able to achieve very low
airspeeds, they make it possible to visualize the differences between air relative and ground relative
speeds.
As you can see in the following video, given a suitably large headwind, STOL capable planes can fly (almost) backwards.
After talking about almost flying backwards, the second video shows a plane actually flying backwards
(albeit very slowly). As the aircraft type (Zenith STOL CH 701) is able to fly very
slowly, the headwind in this scenario was strong enough to propel the aircraft backwards.
Another type of plane which is by nature of its existence able to fly very slowly is the glider. As the
following video shows, it is possible to fly backwards in a glider as well.
Air can and does move in any conceivable direction, but the air motion most of you are likely to think of
first is a predominantly horizontal movement, also known as wind. The origins of wind are incredibly diverse and
mostly boring (to me at least), but in essence wind is the result of atmospheric pressure differences. These
can have a
multitude of reasons, but are mostly down to temperature differences. From an aviation standpoint it is
important to understand, that local weather conditions often influence winds and are intricately connected
to them.
It is therefore essential, to check weather forecasts prior to departure. Some of the dangers related to
wind include winds from inopportune directions.
In common practice, aircraft take off and land directly into he wind, allowing a reduced ground speed in
this critical flight phases. For a variety of reasons (noise, geography) some airports
restrict flight operations to one direction, meaning that in unfavorable wind conditions, aircraft are flown
with tailwinds. This can mean that payload must be
reduced, danger for passengers is increased and extra precautions may be necessary. In this scenario ground
speed is higher than airspeed.
Crosswinds are even more dangerous. A crosswind is the
portion of wind blowing rectangularly to the intended direction. Operating aircraft in crosswinds
necessitates special flying
techniques to counter a drift sideways and make sure the aircraft touches down without to much yaw.
Landing with minimal yaw is important, as otherwise the landing gear can easily be overstressed. The following video video shows one of the very few exceptions to the no yaw rule, as the landing gear of the massive Boeing B-52 bomber is able to rotate to make landing in crosswinds possible (otherwise wing strikes would be likely because of the long wings and short landing gear legs).
Wind shear is one of the most dangerous occurrences in this section. A wind shear is a difference in wind speed and/or direction over a short distance. They most often emerge in the vicinity of jet streams, weather fronts or inversions. Changes in windspeeds may cause loss of lift, sudden unexpected crosswinds or crosswind inversions and more.
Apart from the covered weather induced winds, systems of near constant winds, such as the jet streams, exist. They are used to boost speed and save fuel by using them as permanent tailwind.
As mentioned above, air can also move vertically. The terms turbulence or air pockets are familiar to most
people, but what are they?
Most of what's known colloquially as turbulence is called clean air turbulence in meteorology. CAT
is characterized by turbulent movement of air masses in absence of visual clues. Detecting zones of CAT is
difficult, as they aren't visible to the naked eye and tricky to detect by radar. CAT often occurs near the
jet streams, but temperature gradients (horizontal or vertical) are also beneficial factors. Another one is
wind shear, a condition in which to adjoining masses
of air move in differing directions or at different speeds.
Wind shear can cause vortices if the difference is sufficient, which in turn cause CAT.
Mountain ranges can also induce turbulence.
Clear air turbulence can cause aircraft to suddenly "jump" or "fall". This is dangerous for occupants not
wearing seatbelts and may in excessive cases even be dangerous to the aircraft structure.
In contrast to CAT, thermals occur much closer to the ground. They are caused by air rising after being heated near the ground. As the air density decreases with increasing temperature, the air begins to rise and does so, until it cools to the temperature of the surrounding air. As such, thermals often occur under cumulus clouds or over fields, woods and the like.
Downbursts are one of the deadliest phenomena yet. In a downburst (or a microburst, a smaller downburst), rain cooled air falls until it collides with the ground, then spreading outwards. This creates a vertical wind shear in proximity to the ground, a grave danger for landing planes. Another very dangerous characteristic is that the air colliding with the ground is thereby accelerated outward at high speeds. If an aircraft encounters this, the pilots will note a spike in airspeed. If corrected, the airflow over the wings will then suddenly decrease after passing the center of the microburst and entering the tailwind area. A catastrophic stall is then very likely.
As was briefly mentioned in the lift section, lift is a function of the air density. It decreases with temperature and altitude, meaning lift does so too. As a consequence, when taking off or landing at hot and high airports, a longer ground roll or lighter aircraft is required. This condition, known as hot and high, is made even more dangerous by the fact that low air density also reduces engine thrust. Famous hot and high airports include Mexico City International Airport, Edwards Air Force Base and O. R. Tambo International Airport in South Africa.
Another important characteristic of air is its moisture. Given the right circumstances, moisture in the air can condensate and create fog or worse lead to the build up of ice on an aircraft. The difference between dew point and temperature is called spread. In a low spread low temperature scenario, it is possible for water to condensate on an aircraft and freeze. The resulting ice may then negatively impact aircraft performance (see icing).
This image shows a sunset over the Atlantic Ocean above the wing of a Boeing 757.