6.9 The Mach Number technique

6.9.1 The term “Mach number technique” is used to describe the technique of clearing turbo-jet aircraft operating along the same route to maintain specified Mach numbers in order to maintain adequate longitudinal separation between successive aircraft at, or climbing or descending to, the same level.

OBJECTIVES OF USE OF THE MACH THE NUMBER TECHNIQUE

6.9.2 The principal objectives of use of the Mach the number technique are:

  1. To ensure continued longitudinal separation between successive aircraft on long route segments with a minimum of ATC intervention;
  2. To obtain improved utilisation of such routes, thus contributing to the economy of flight operations of traffic concerned.

6.9.3 To achieve these objectives the speeds of aircraft operating along the same track at the same level or climbing or descending to operate at the same level are stabilised. This stability permits reasonably accurate projections of the expected longitudinal separation between aircraft to points well beyond the point where separation is first confirmed, which reduces the need for frequent ATC intervention.

6.9.4 Minor variations in speed, which might temporarily increase or decrease the spacing between aircraft, tend to be neutralised over prolonged periods of flight. For this reason, longitudinal separation between succeeding aircraft at the same level, with Mach numbers matched, should be at least 20nm. This applies to South African airspace, other countries’ local procedures could allow for more or less.

GENERAL PROCEDURES

6.9.5 The ATC clearance must include the assigned Mach number that is to be maintained. It is therefore necessary that information on the desired Mach number be included in the flight plans by pilots intending to operate along routes in the area concerned.

6.9.6 ATC has a requirement to calculate estimated times at which aircraft will pass significant points along their track. These calculations are necessary both for;

  1. The provision of longitudinal separation between aircraft on crossing tracks, and
  2. For co-ordination with adjacent ATC units. Therefore ATC must be provided with necessary data to do this.

6.9.7 Thereafter, provided that aircraft maintain their last assigned Mach numbers, intervention by ATC for the portion of flight where the Mach number technique is used, should normally only be necessary if an aircraft, for some reason, is obliged to change its number or if there is conflicting traffic on crossing tracks or a flight level change is intended.

6.9.8 The Mach number technique requires that pilots strictly adhere to the following procedures:

  1. Aircraft must strictly adhere to the last assigned Mach number;
  2. If essential to make an immediate temporary change in Mach number (e.g. due to turbulence), the appropriate ATC should be notified as soon as possible of that change;

EN-ROUTE STEP-CLIMBS AND STEP-DESCENTS

6.9.9 The Mach number technique may be used as a means of applying longitudinal separation between aircraft carrying out step-climbs or step-descents and other en-route traffic on the same track provided;

  1. That the prescribed minimum longitudinal separation between the climbing/ descending aircraft and other affected en-route traffic exists at the time a climb/descent clearance is issued; and
  2. Will exist during climb/descent, as well as at each further significant point along track and at the exit point.

6.9.10 Application of this procedure is based on the assumption that the last assigned Mach number will be maintained during step-climbs and step-descents, and that in the event it is not feasible, ATC is advised at the time of the climb/descent request.