How far from the ground is an aeroplane when flying?

It varies upon aircraft type, private or commercial flight, and where the aircraft is going and on what heading. Above 3000 ft in the UK and 10,000 ft in America aircraft will fly on what is called a flight level which works off a pressure measurement of 1013.2hps keeping a 500ft gap between other aircraft on different headings (the quadrontal rule or semicircular rule if very high). The small aircraft you see flying about will tend to be about 1000-3000 ft and helicopters tend to fly on 1000 ft. Large passenger aircraft may go to 55000 feet or higher. Small private aircraft typically fly in the lower altitudes, say 3000 to 17000 feet and high performance private aircraft 18000 to 25000 feet. Commercial aircraft fly in the 35000 to 40000 foot range, and small business jets can operate in the 45000 to 50000 feet. These are broad generalities as operating altitudes are selected on a day to day basis depending on various factors such as winds, weather, time requirements, etc. Many military aircraft have much higher capabilities depending on the specific mission.