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Mach Number Technique for Aircraft Separation

Key Takeaways

  • The two ways to implement the separation between aircraft are horizontal separation and vertical separation.

  • The Mach number technique is one method employed in air navigation to maintain the separation between aircraft.

  • The Mach number technique is only applicable to subsonic turbojet aircraft. 

Mach number technique

“Separation” is a common term in air traffic control. This term describes the actions taken by air traffic control to prevent collisions between aircraft flying in the same region. The separation can be either horizontal or vertical. Mach number technique is a procedure that pilots use to maintain longitudinal separation between aircraft flying on identical tracks or those changing tracks. This technique is also used to maintain a specific speed. 

Separation of Aircraft

It is important to keep some distance between aircraft flying in the same area. Changes in altitude help maintain a separation between aircraft flying adjacent to each other. This separation is essential for preventing collisions.

Separation between aircraft can be introduced in two ways:

  1. Horizontal separation 
  2. Vertical separation

Separation in the horizontal plane can be affected in two ways:

  1. Longitudinal separation: Spacing is provided between aircraft flying on the same track.
  2. Lateral separation: Aircraft are spaced side by side by introducing a specified distance between each other.

Let’s explore the spacing required between aircraft.


The required spacing between aircraft is called minima and is usually specified in terms of distance. Horizontal distance is measured in nautical miles or degrees of latitude, whereas vertical distance is measured in flight levels or feet. The vertical distance can also be described in the time that the preceding aircraft takes to pass over the same point.

Maintaining Separation Through Speed Control

Longitudinal separation is when the aircraft are spaced at a distance not less than the prescribed minima. The maintenance of this separation minima is established through speed control.

The Mach number technique is one method employed in air navigation to maintain separation between the aircraft. The Mach number is a procedure followed by the pilots to maintain the minimum separation between the adjacent aircraft.

Mach Number Technique

Mach number technique is applicable to aircraft operating on the same identical track or on aircraft that continuously diverge the tracks. In such aircraft, the required longitudinal separation is introduced by maintaining specified true Mach numbers.

The main objectives of the Mach number technique are:

  1. To provide longitudinal separation between successive aircraft on a long en-route segment with minimum instructions from air traffic control.
  2. To improve the utilization of the air traffic space.
  3. To contribute to the financial aspects of the flight operations.

The Mach number technique is only applicable to subsonic turbojet aircraft. The Mach number technique is suitable for aircraft either ascending or descending to the same level. In practical cases, the Mach number technique is applied on aircraft operating along the same route at the same flight level. The speed regulation at the same Mach number in successive aircraft helps maintain a constant time interval between each other. The prescribed minima separation is achieved by sticking to the same Mach number. This minima is achieved only because both aircraft are subjected to the same wind, same air temperature, and almost the same ground speed.

Strict Procedures Are Followed in the Mach Number Technique

The following procedures are strictly followed to achieve the prescribed minima using the Mach number technique.

  1. Unless otherwise instructed by air traffic control, the aircraft must adhere to the last Mach number.
  2. If the aircraft Mach number is temporarily changed due to turbulence, air traffic control should be notified immediately.
  3. If requested by air traffic control, the true Mach number should be included in the routine position reports.
  4. The aircraft should follow the revisions made by air traffic control in the estimated time.

Airspace Utilization and the Mach Number Technique

As air traffic around the world increases, the Mach number technique will continue to be useful. The Mach number is critical, as it can improve air space utilization. Cadence CFD simulation tools can help in identifying the aerodynamic changes pertaining to each Mach number.

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