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7.2.2 For many aircraft types the cockpit instrument displays the True Mach being flown. However, for some types the AFM notes a correction that must be made to the Indicated Mach to provide the True Mach. It is important to recognise that the maintenance of longitudinal separations depends upon the assumption that the ATC assigned Mach numbers maintained by all aircraft are True Mach numbers. Pilots must therefore ensure that any required corrections to indicated Mach are taken into account when complying with the True Mach number specified in the ATC clearance.
7.3 PROCEDURES IN NAT OCEANIC AIRSPACE
7.3.1 The Oceanic Clearance includes the assigned (True) Mach Number which is to be maintained. It is therefore necessary that information on the desired Mach Number be included in the flight plan for turbojet aircraft intending to fly in NAT oceanic airspace. ATC uses Mach Number together with pilot position reports to calculate estimated times for significant points along track. These times provide the basis for longitudinal separation between aircraft and for co-ordination with adjacent ATC units.
7.3.2 ATC will try to accommodate pilot/dispatcher requested or flight planned Mach Numbers when issuing Oceanic Clearances. It is rare that ATC will assign a Mach Number more than 0.01 faster or 0.02 slower than that requested. The prescribed longitudinal separation between successive aircraft flying a particular track at the same flight level is established over the oceanic entry point. Successive aircraft following the same track may be assigned different Mach Numbers but these will be such as to ensure that prescribed minimum separations are assured throughout the oceanic crossing. Intervention by ATC thereafter should normally only be necessary if an aircraft is required to change its Mach Number due to conflicting traffic or to change its flight level.
7.3.3 It is, however, important to recognise that the establishment and subsequent monitoring of longitudinal separation is totally reliant upon aircraft providing accurate waypoint passing times in position reports. It is therefore essential that pilots conducting flights in MNPS Airspace utilise accurate clocks and synchronise these with a standard time signal, based on UTC, prior to entering MNPS Airspace. It should be
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 7
NAT MNPS 39 Edition 2009
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.)
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. Pilots must recognise that adherence to the assigned Mach Number is essential. No tolerance is provided for. Pilots must not utilise Long Range Cruise or ECON FMC modes when transiting NAT MNPS airspace. If an immediate temporary change in the Mach Number is essential, e.g. due to turbulence, ATC must be notified as soon as possible. Pilots with experience of flying in oceanic airspaces other than the North Atlantic, may be familiar with a procedure in those areas which permits pilots to unilaterally elect to change their cruising mach number by up to 0.02M, without prior ATC approval. This is not the case in the North Atlantic MNPS airspace.
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 40 Edition 2009
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 philosophy is to specify a Required Navigation Performance (RNP), effectively 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 better than X NM on a 95% containment basis. The NAT Minimum Navigation Performance Specifications (MNPS) inter alia define a requirement for the standard deviation of lateral track errors to be less than 6.3 NM. Since two standard deviations provide for about 95 % containment, the MNPS statement is effectively akin to an RNP value of 12.6 in a lateral sense. 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 NM.
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NORTH ATLANTIC MNPS AIRSPACE OPERATIONS MANUAL(33)
