Customs exemption for aircraft used for flight calibration requires undertaking to use only for calibration and prohibits sale. Amendment requires an undertaking to customs that aircraft imported by the Airports Authority for flight calibration will be used only for flight calibration, that duty will be paid on demand if not so used, and that the aircraft will not be sold or transferred; and it permits imports for servicing, repair or maintenance of aircraft imported or procured by the Airports Authority for flight calibration purposes.