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noted that some aircraft clocks can only be re-set while the aircraft is on the ground. (See further comments
on time-keeping/longitudinal navigation in Chapter 1 and Chapter 8.)
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 7
NAT MNPS 38 Edition 2005
7.3.4 In the application of Mach Number Technique, pilots must adhere strictly to their assigned
True Mach Numbers unless a specific re-clearance is obtained from the appropriate ATC unit. However, as
the aircraft weight reduces it may be more fuel efficient to adjust the Mach Number. Since the in-trail and
crossing track separations between individual aircraft are established on the basis of ETAs passed to, or
calculated by, ATC, it is essential that ATC approval is requested prior to effecting any change in cruise
Mach Number. Such approval will be given if traffic conditions permit. If an immediate temporary change
in the Mach Number is essential, e.g. due to turbulence, ATC must be notified as soon as possible.
7.3.5 Pilots should maintain their last assigned Mach Number during step-climbs in oceanic
airspace. If due to aircraft performance this is not feasible ATC should be advised at the time of the request
for the step climb.
7.4 PROCEDURE AFTER LEAVING OCEANIC AIRSPACE
7.4.1 After leaving oceanic airspace pilots must maintain their assigned Mach Number in domestic
controlled airspace unless and until the appropriate ATC unit authorises a change.
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 8
NAT MNPS 39 Edition 2005
Chapter 8: MNPS Flight Operation & Navigation Procedures
8.1 INTRODUCTION
8.1.1 The aircraft navigation systems necessary for flying in NAT MNPS Airspace are capable of
high-performance standards. However it is essential that stringent cross-checking procedures are employed,
both to ensure that these systems perform to their full capabilities and to minimise the consequences of
equipment failures and possible human errors.
8.1.2 Navigation systems are continuously evolving and early editions of this Manual concentrated
on offering specific guidance on the use of individual systems. Rather than specifying the types of
equipment required for flying in defined airspace, current thinking is moving towards specifying a Required
Navigation Performance (RNP), in other words a track keeping capability. As an example, the navigation
performance accuracy of the aircraft population operating in airspace designated RNP X airspace would be
expected to be X nm on a 95% containment basis. The NAT Minimum Navigation Performance
Specification (MNPS) inter alia defines a requirement for the standard deviation of lateral track errors to be
less than 6.3 nms. Since two standard deviations provide for about 95 % containment, the MNPS statement
is effectively equivalent to an RNP value of 12.6. It is also perhaps interesting to note that actual
measurements of the achieved navigation performance by the entire fleet of NAT aircraft, even before GPS
came into use by a large proportion, indicated an achieved standard deviation of approximately 2 nms.
8.1.3 MNPS was devised and implemented in the NAT Region long before the RNP concept was
developed. MNPS was established primarily with the NAT OTS environment in mind. The defining
waypoints of OTS tracks are specified by whole degrees of latitude and, using an effective 60 nms lateral
separation standard, most adjacent tracks are separated by only one degree of latitude at each ten-degree
meridian. The traffic density in the OTS is higher than in any other oceanic airspace. In such a densely
populated flexible track system (one that changes twice every day), it is essential to avoid (whole degree)
waypoint insertion errors by crews. Such errors in the NAT MNPSA will inevitably result in a conflict with
traffic on an adjacent track. For this reason Minimum Navigation Performance Specifications had to include
not just the technical navigation accuracy of the Long-range Navigation Systems used on the aircraft but also
the crew navigation procedures employed. The MNPS statement thus involves both cockpit/flight deck
procedures and crew training requirements. In the early days of the RNP concept, it was these additional
requirements that separated MNPS from RNP. However, RNP has come a long way since its inception and
the development of the RNP-10 approvals for PAC operations have brought it much closer to the original
MNPS concept. The ICAO Air Navigation Plan for the North Atlantic Region states that the intention in the
future is that navigational performance is expected to be tied to a level of RNP.
(For more detailed information on RNP and MNPS see the following ICAO Documents: Doc 9613 –
‘Manual on Required Navigation Performance’ and NAT Doc 001 – ‘Consolidated Guidance and
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