It is called "Mach Number Technique" but what is actually is, is "Mach Number Procedure" because if you don't follow it you could lose your oceanic pilot's privileges. The procedure is required around much of the world, though the ICAO manual description is rather sparse. Much of the procedural information comes from the North Atlantic Manuals, but those procedures can be assumed to apply wherever Mach Number Technique is used.
If you have auto throttles you are pretty much set. Tune those puppies to the filed oceanic Mach and leave them there. Is there a "plus or minus" tolerance? In Oakland Oceanic airspace, if your Mach varies by 0.02 or more, you need to report it. In most parts of the world, if your ETA to the next waypoint varies by the required tolerance, 3 minutes or more for most of the world, update your ETA. Do not change your target Mach. Why? Read on . . .
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
[NAT Doc 007, ¶7.1.1.] 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.
It is critically important that all pilots realize they are flying a set Mach Number, as cleared, and do not adjust speed to achieve waypoint ETAs.
[NAT Doc 007, ¶7.2.1.] 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.
[ICAO Doc 4444, ¶184.108.40.206.1.] Longitudinal separation shall be applied so that the spacing between the estimated positions of the aircraft being separated is never less than a prescribed minimum. Longitudinal separation between aircraft following the same or diverging tracks may be maintained by application of speed control, including the Mach number technique. When applicable, use of the Mach number technique shall be prescribed on the basis of a regional air navigation agreement.
[NAT Doc 007, ¶7.3.]
[ICAO Annex 2, ¶220.127.116.11. c)] Change in time estimate: if the time estimate for the next applicable reporting point, flight information region boundary or destination aerodrome, whichever comes first, is found to be in error in excess of 3 minutes from that notified to air traffic services, or such other period of time as is prescribed by the appropriate ATS authority or on the basis of air navigation regional agreements, a revised estimated time shall be notified as soon as possible to the appropriate air traffic services unit.
Figure: Chasing ETA's, from Eddie's notes.
Imagine in the drawing you are the airplane in the middle. Your spacing is designed with the varying speeds taken into consideration. If all three airplanes are hit with less than forecast headwinds, all three airplanes will start arriving at their waypoints early. If you, in the middle, slow down to arrive at your waypoint on time while the faster airplane behind you correctly maintains his Mach number, you could have loss of separation. If the other two airplanes update their ETA's and you don't, it will be obvious to ATC who gets the violation.
The correct procedure:
Before the adoption of Mach Number Technique requirements, there was a 0.02 tolerance on Mach Number. That went away for most of the world but it has come back for at least one area . . .
[IFALPA Safety Bulletin 2016-10-12] ATTN ALL AIRCREWS -NEW PROCEDURAL REQUIREMENT FOR FLIGHTS OPERATING IN OAKLAND OCEANIC CONTROL AREA (KZAK) IN ORDER TO SUPPORT COST INDEX OR ECON SPEEDS AND MAINTAIN ATC SEPARATION SPACING AIRCREWS ARE REQUIRED TO USE THE FOL- LOWING PROCEDURES IN THE KZAK FIR. A PILOT MUST INFORM ATS EACH TIME THE CRUISING MACH NUMBER VARIES OR IS EXPECTED TO VARY BY A VALUE EQUAL TO OR GREATER THAN 0.02 MACH FROM:
[NAT Doc 007, ¶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.
[NAT Doc 007, ¶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. Pilots with experience of flying in oceanic airspaces other than the North Atlantic, may be familiar with a procedure in those areas which permits pilots to unilaterally elect to change their cruising Mach number by up to 0.02M, without prior ATC approval. This is not the case in the North Atlantic MNPS airspace.
Remember you want to fly a constant True Mach Number. If your cockpit doesn't present True Mach Number, your need to make the adjustment. If your auto throttles tend to fly slow or fast, you need to make the adjustment. In the case of the G450, for example, the cockpit instrument shows True Mach Number and the auto throttles do a pretty good job of bracketing the set number. We set the auto throttles on the correct Mach Number and leave them alone.
[NAT Doc 007, ¶7.3.4.] 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.
If you can't make the climb at your filed Mach number, you shouldn't be making the climb.
[NAT Doc 007, ¶7.4.] 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.
The Mach Number Technique is required in various parts of the world:
ICAO Doc 7030 is rarely updated and airspace around the world is getting more crowded so you can expect to see the use of Mach Number Technique expand. It might not be a bad idea to use it everywhere when not in radar contact.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 26.
ICAO Annex 2 - Rules of the Air, International Standards, Annex 2 to the Convention on International Civil Aviation, July 2005
ICAO Doc 4444 - Air Traffic Management, Fourteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2001 *
ICAO Doc 4444 - Air Traffic Management, Fifteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2007 *
* Not all of Doc 4444 seems to have been reproduced in the 15th edition, so you might need to look at the 15th edition and then then 14th edition for some sections.
ICAO Doc 4444 - Amendment No. 1, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 1, 2007
ICAO Doc 4444 - Amendment No. 2, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 2, 19/11/09
ICAO Doc 7030, Amendment 1, International Civil Aviation Organization, 8 January 2009
IFALPA Safety Bulletin 2016-10-12, "Oakland Oceanic Mach Number Variations"
NAT Doc 007, North Atlantic Operations and Airspace Manual Doc 007, Edition 2013