1.7 Carriage and Operation of Pressure-Altitude reporting SSR Transponders 1.7.1 All aircraft operating as IFR flights in the NAT Region shall be equipped with a pressure- altitude reporting SSR transponder. Pilots shall operate their aircraft transponders continuously on Mode A. The last assigned SSR code shall be retained for 30 minutes after entry into NAT oceanic airspace, unless otherwise instructed by ATC. Thereafter Mode A shall be selected to Code 2000, except for departing aircraft that shall have their last assigned code retained for 30 minutes. This procedure does not affect use of the special purpose emergency codes 7500, 7600 or 7700, when required. 1.8
