World Air Ops | NAT | Doc 007 MNPS Guidance

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Guidance concerning Air Navigation in and above the NAT MNPSA CHAPTER 7CHAPTER 7:APPLICATION OF MACH NUMBER TECHNIQUE7.1 DESCRIPTION OF TERMS7.1.1 The term ‘Mach Number Technique’ is used to describe a technique whereby subsonicturbojet aircraft operating successively along suitable routes are cleared by ATC to maintain appropriateMach Numbers for a relevant portion of the en route phase of their flight.7.2 OBJECTIVE7.2.1 The principal objective of the use of Mach Number Technique is to achieve improvedutilisation of the airspace on long route segments where ATC has no means, other than position reports, ofensuring that the longitudinal separation between successive aircraft is not reduced below the establishedminimum. Practical experience has shown that when two or more turbojet aircraft, operating along the sameroute at the same flight level, maintain the same Mach Number, they are more likely to maintain a constanttime interval between each other than when using other methods. This is due to the fact that the aircraftconcerned are normally subject to approximately the same wind and air temperature conditions, and minorvariations in ground speed, which might increase and decrease the spacing between them, tend to beneutralised over long periods of flight.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 theTrue Mach. It is important to recognise that the maintenance of longitudinal separations depends upon theassumption that the ATC assigned Mach numbers maintained by all aircraft are True Mach numbers. Pilotsmust therefore ensure that any required corrections to indicated Mach are taken into account whencomplying with the True Mach number specified in the ATC clearance.7.3 PROCEDURES IN NAT OCEANIC AIRSPACE7.3.1 The Oceanic Clearance includes the assigned (True) Mach Number which is to bemaintained. It is therefore necessary that information on the desired Mach Number be included in the flightplan for turbojet aircraft intending to fly in NAT oceanic airspace. ATC uses Mach Number together withpilot position reports to calculate estimated times for significant points along track. These times provide thebasis 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 Numberswhen issuing Oceanic Clearances. It is rare that ATC will assign a Mach Number more than 0.01 faster or0.02 slower than that requested. The prescribed longitudinal separation between successive aircraft flying aparticular track at the same flight level is established over the oceanic entry point. Successive aircraftfollowing the same track may be assigned different Mach Numbers but these will be such as to ensure thatprescribed minimum separations are assured throughout the oceanic crossing. Intervention by ATCthereafter should normally only be necessary if an aircraft is required to change its Mach Number due toconflicting traffic or to change its flight level.7.3.3 It is, however, important to recognise that the establishment and subsequent monitoring oflongitudinal separation is totally reliant upon aircraft providing accurate waypoint passing times in positionreports. It is therefore essential that pilots conducting flights in MNPS Airspace utilise accurate clocks andsynchronise these with a standard time signal, based on UTC, prior to entering MNPS Airspace. It should beNAT Doc 007 42 Edition 2010
Guidance concerning Air Navigation in and above the NAT MNPSA CHAPTER 7noted that some aircraft clocks can only be re-set while the aircraft is on the ground. (See further commentson 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 assignedTrue Mach Numbers unless a specific re-clearance is obtained from the appropriate ATC unit. However, asthe aircraft weight reduces it may be more fuel efficient to adjust the Mach Number. Since the in-trail andcrossing track separations between individual aircraft are established on the basis of ETAs passed to, orcalculated by, ATC, it is essential that ATC approval is requested prior to effecting any change in cruiseMach Number. Such approval will be given if traffic conditions permit. Pilots must recognise thatadherence to the assigned Mach Number is essential. No tolerance is provided for. Pilots must notutilise Long Range Cruise or ECON FMC modes when transiting NAT MNPS airspace. If animmediate temporary change in the Mach Number is essential, e.g. due to turbulence, ATC must be notifiedas soon as possible. Pilots with experience of flying in oceanic airspaces other than the North Atlantic, maybe familiar with a procedure in those areas which permits pilots to unilaterally elect to change their cruisingmach number by up to 0.02M, without prior ATC approval. This is not the case in the North AtlanticMNPS airspace.7.3.5 Pilots should maintain their last assigned Mach Number during step-climbs in oceanicairspace. If due to aircraft performance this is not feasible ATC should be advised at the time of the requestfor the step climb.7.4 PROCEDURE AFTER LEAVING OCEANIC AIRSPACE7.4.1 After leaving oceanic airspace pilots must maintain their assigned Mach Number in domesticcontrolled airspace unless and until the appropriate ATC unit authorises a change.NAT Doc 007 43 Edition 2010
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