The Rules of the Air Regulations 1996
Quadrantal rule and semi-circular rule
30.—(1) Subject to paragraph (2), in order to comply with the Instrument Flight Rules, an aircraft when in level flight above 3000 feet above mean sea level or above the appropriate transition altitude, whichever is the higher, shall be flown at a level appropriate to its magnetic track, in accordance with the appropriate Table set forth in this rule. The level of flight shall be measured by an altimeter set:
(a) in the case of a flight over the United Kingdom, to a pressure setting of 1013.2 hectopascals; or
(b) in the case of any other flight, according to the system published by the competent authority in relation to the area over which the aircraft is flying.
(2) An aircraft may be flown at a level other than the level required by paragraph (1) if it is flying in conformity with instructions given by an air traffic control unit or in accordance with notified en-route holding patterns or in accordance with holding procedures notified in relation to an aerodrome.
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