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minimum. Practical experience has shown that when two or more turbojet aircraft, operating along the same
route at the same flight level, maintain the same Mach Number, they are more likely to maintain a constant
time interval between each other than when using other methods. This is due to the fact that the aircraft
concerned are normally subject to approximately the same wind and air temperature conditions, and minor
variations in ground speed, which might increase and decrease the spacing between them, tend to be
neutralised 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 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 2008
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.
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 2008
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
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