Guidance concerning Air Navigation in and above the NAT MNPSA
APPLICATION OF MACH NUMBER TECHNIQUE
DESCRIPTION OF TERMS
The term ‘Mach Number Technique’ is used to describe a technique whereby subsonic
turbojet aircraft operating successively along suitable routes are cleared by ATC to maintain appropriate Mach Numbers for a relevant portion of the en route phase of their flight.
The principal objective of the use of Mach Number Technique is to achieve improved
utilisation of the airspace on long route segments where ATC has no means, other than position reports, of ensuring that the longitudinal separation between successive aircraft is not reduced below the established 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.
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.
PROCEDURES IN NAT OCEANIC AIRSPACE
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.
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 thereafter should normally only be necessary if an aircraft
the oceanic crossing. is required to change its
Intervention by ATC Mach Number due to
conflicting traffic or to change its flight level.
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
NAT Doc 007