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• Weight units The weight of an aircraft is most commonly measured in kilograms , but may sometimes be measured in pounds , especially if the fuel gauges are calibrated in pounds or gallons .
Many airlines request that weights be rounded to a multiple of 10 or 100 units . Great care is needed when rounding to ensure that physical constraints are not exceeded .
When chatting informally about a flight plan , approximate weights of fuel and / or aircraft may be referred to in tons .
This " ton " is generally either a metric tonne or a UK long ton , which differ by less than 2 %, or a short ton , which is about 10 % less .
Describing a route . A route is a description of the path followed by an aircraft when flying between airports . Most commercial flights will travel from one airport to another , but private aircraft , commercial sightseeing tours , and military aircraft may do a circular or out-and-back trip and land at the same airport from which they took off .
Components
Aircraft fly on airways under the direction of air traffic control .
An airway has no physical existence , but can be thought of as a motorway in the sky . On an ordinary motorway , cars use different lanes to avoid collisions , while on an airway , aircraft fly at different flight levels to avoid collisions .
One can often see planes passing directly above or below one ' s own . Charts showing airways are published and are usually updated every 4 weeks , coinciding with the AIRAC cycle .
AIRAC ( Aeronautical Information Regulation and Control ) occurs every fourth Thursday , when every country publishes its changes , which are usually to airways .
A - Z of Flight
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