Boeing, Airbus, and Tupolev concentrate on wide-body and narrow-body jet airliners, while Bombardier and Embraer concentrate on regional airliners.
Until the 1970s, most major airlines were flag carriers, sponsored by their governments and heavily protected from competition. Since then, various open skies agreements have resulted in increased competition and choice for consumers, coupled with falling prices for airlines. The combination of high fuel prices, low fares, high salaries, and crises such as the September 11, 2001 attacks and the SARS epidemic have driven many older airlines to government-bailouts, bankruptcy or mergers. At the same time, low-cost carriers such as Ryanair and Southwest have flourished.
aircraft are used to transport hardware and personel, such as the C-17 or C-130 Hercules. There is a mission modifier designation in the USAF for Very Important Persons (V), not to be confused with Verticle Takeoff and landing Aircraft, such as the V-22. An example of a VIP aircraft is the VC-25 aka Air Force One.
control towers (including tower, ground control, clearance delivery, and other services), which control aircraft within a small distance (typically 10-15 km horizontal, and 1,000 m vertical) of an airport.
terminal controllers, who control aircraft in a wider area (typically 50-80 km) around busy airports
centre controllers, who control aircraft enroute between airports
ATC is especially important for aircraft flying under Instrument flight rules (IFR), where they may be in weather conditions that do not allow the pilots to see other aircraft. However, in very high-traffic areas, especially near major airports, aircraft flying under Visual flight rules (VFR) are also required to follow instructions from ATC.
In addition to separation from other aircraft, ATC may provide weather advisories, terrain separation, navigation assistance, and other services to pilots, depending on their workload.
It is important to note that ATC does not control all flights. The majority of VFR flights in North America are not required to talk to ATC at all (unless they're passing through a busy terminal area or using a major airport), and in many areas, such as northern Canada, ATC services are not available even for IFR flights at lower altitudes.
Environmental impact
Aircraft operating at high altitudes near the tropopause (mainly large jet airliners) emit aerosols and leave contrails, both of which can increase cirrus cloud formation — cloud cover may have increased by up to 0.2% since the birth of aviation.[2]
Aircraft operating at high altitudes near the tropopause can also release significant quantities of chemicals that interact with greenhouse gases at those altitudes, particularly nitrogen compounds, which interact with ozone, increasing ozone concentrations.[3][4]
Most light piston aircraft burn avgas, which contains tetra-ethyl lead (TEL), a highly-toxic substance that can cause soil contamination at airports. Some lower-compression piston engines can operate on unleaded mogas, and turbine engines and diesel engines — neither of which requires lead — are appearing on some newer light aircraft.
Notes
^ Babcock Gover, Philip (1990). Webster's Third New International Dictionary. Merriam-Webster. .
^ Lin, X.; Trainer, M. and Liu, S.C., (1988). "On the nonlinearity of the tropospheric ozone production.". Journal of Geophysical Research93: 15879–15888.
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