3.6 Procedures for Aircraft Suffering Partial Loss of Navigation Capability Before Entry into MNPS Airspace 3.6.1 ICAO Annex 6, Chapter 7 of Parts I and II, states a requirement for aircraft to be sufficiently equipped navigationally to ensure that, in the event of the failure of one item of equipment at any stage of flight, the remaining equipment will enable the aircraft to proceed in accordance with MNPS where applicable. For flight in NAT MNPS Airspace, this is interpreted to mean that, while two sets of LRNS equipment have to be carried when operating in the major part of the NAT MNPS Airspace, there are routes on which only one LRNS is sufficient and where the requirement for standby equipment is satisfied by the carriage of approved short-range navigation equipment. In the event of an aircraft suffering partial loss of necessary navigation capability prior to entry into oceanic airspace (e.g. only one Inertial Navigation System (INS) or Flight Management System/Inertial Reference System (FMS/IRS) serviceable, the pilot should consider landing at a suitable aerodrome before the boundary, or returning to the departure airfield, or using one of the special routes listed in Part 1. Use of those particular routes following partial loss of navigation capability is subject to the following criteria a) that sufficient navigation capability remains to meet the MNPS, i.e. one LRNS plus short range navigation aids b) that the requirements of ICAO Annex 6, Chapter 7 of Parts I and II, with regard to the provision of navigation equipment necessary to enable the aeroplane to be navigated in accordance with its operational flight plan, plus the requirements of ATS, can be met by relying on the use of short range navigation aids in the event of failure of the remaining LRNS; ca revised flight plan is filed with the appropriate ATS unit and d) an appropriate ATC clearance is obtained. Note: A revised oceanic ATC clearance will be issued after coordination between all OACs concerned. Should the OTS at the time of the incident extend to the northern part of the NAT Region, the aircraft concerned maybe required to accept a lower than optimum flight level in its revised oceanic clearance, especially during peak traffic periods. This guidance does not relieve the pilot of the obligation to take the best possible course of action under the prevailing circumstances.