Every aircraft requires regular servicing as part of flight safety and this can take place in between flights, while parked at the gate, or involve an overnight stop in a hangar for more detailed work. Furthermore, they are also subject to 'heavy checks' which, although less frequent, are far more comprehensive and are generally undertaken at a major maintenance facility. The 'C' check is usually carried out every 12-24 months - depending upon the number of hours flown - and includes an extensive inspection of the aircraft systems. The 'D' check, which is performed approximately every five years, is even more extensive and may entail stripping the airframe down to the bare metal, rewiring, fitting a new interior, taking off the engines and a repaint. These procedures can be both time consuming - the 'C check can take in excess of a week while the 'D' check, when combined with extensive modifications may take up to two months - and it is costly to airlines and, in some cases, may even require a replacement aircraft to fill the gap.
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