How aircraft fly
Flight occurs through a combination of the aircraft's forward speed and the lift generated by its wings. If the air moving over the top of an aircraft’s wing is faster than the air passing beneath it lift is generated as the air pressure on top of the wing is less than the pressure below.
By taking off into the wind aircraft can increase the lift generated by the wing. As the prevailing wind in the UK is east / west most runways at UK airports face in those directions.
Once speed and lift are generated the pilot then controls an aircraft through the use of ailerons and elevators. Ailerons are fitted to the back edge of each wing and turn the aircraft in flight. If a pilot wants to turn left they move the control column in the cockpit to the left. This makes the aileron on the left wing go up and the aileron on the right wing to go down. This changes the airflow over the wings and turns the aircraft to the left.
Elevators make an aircraft climb or descend and are fitted to the tail wings of the aircraft. If a pilot pushes the control column forward this drops the elevator, the airflow change pushes the tail of the aircraft up and the nose down.
Before a new type of aircraft can enter service it has to be approved by regulatory authorities. This is called certification and is normally undertaken by the aviation authority in the country that the aircraft is manufactured.
Boeing aircraft are American-built and therefore certificated by the US Federal Aviation Administration; Airbus aircraft are European-built and certificated by the European Union Aviation Safety Agency (EASA), which undertakes certification work for EU nations.
To pass the certification process aircraft must comply with a set of design requirements. These cover all aspects of the aircraft: its engines; and the materials and components they are made from.
Once an aircraft is certificated it can be sold to airlines and enter service. The original certification approval is normally recognised automatically by other countries.
The certification is granted for the type of aircraft. Each individual aircraft then has a certificate of airworthiness, the equivalent of a car’s MOT, which is renewed every few years.
Manufacturers of aircraft and engines retain responsibility for the certification and ongoing airworthiness. During an aircraft or engines lifetime issues may develop or modifications may be required. The manufacturer will take the lead on making these happen by working directly with airlines. If the change is important the certificating regulator may also make it a requirement by issuing an airworthiness notice. This will detail the change or inspection required and can indicate a mandatory completion time.
Once an aircraft enters service with an airline then the responsibility for its day to day safety is with the airline itself. This will be overseen by its safety regulator (for UK airlines that is the UK CAA). The safety regulator will licence and inspect an airline to check that it is complying with safety requirements including the operation and maintenance of its aircraft.