Article 39 Minimum navigation performance
An aircraft registered in the United Kingdom shall not fly in airspace prescribed for the purposes of this article unless:
it is equipped with navigation systems which enable the aircraft to maintain the prescribed navigation performance capability;
the navigation systems required by paragraph (a) hereof are approved by the Authority and installed and maintained in a manner approved by the Authority;
the operating procedures for the navigation systems required by paragraph (a) hereof are approved by the Authority; and
the equipment is operated in accordance with the approved procedures while the aircraft is flying in the said airspace.
An approval granted by the Authority for the purposes of this article shall be in writing and may be subject to such conditions as the Authority thinks fit.