HANKE-Aviation GmbH - Flight Crew Training
CHAPTER 7 - 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 utilization 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 speed which might
increase and decrease the spacing between them tend to be neutralized over long
periods of flight.
PROCEDURES IN NAT OCEANIC AIRSPACE
The ATC clearance includes the assigned 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 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. It is, however, important to recognize 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 utilize accurate clocks and synchronise these with a standard time signal, based on UTC, prior to entering MNPS Airspace. It should be noted that some aircraft clocks can only be re-set while the aircraft is on the ground.
In the application of Mach Number Technique, pilots must adhere strictly to their assigned Mach Numbers unless a specific reclearance 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.
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.
PROCEDURE AFTER LEAVING OCEANIC AIRSPACE
After leaving oceanic airspace pilots must maintain their assigned Mach Number in domestic controlled airspace unless and until the appropriate ATC unit authorizes a change.
(C) HANKE-Aviation GmbH 2014