It is called "Mach Number Technique" but what it 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 . . .
Update: If spacing permits, you may be allowed to "resume normal speed." That doesn't mean you can throw out your Mach number technique, but it does mean you have some options: Resume Normal Speed.
Everything here is from the references shown below, with a few comments in an alternate color.
[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.
[ICAO Doc 9426, Chapter 2]
2.1.1 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.
2.3.4 Adherence to assigned Mach number. Unless otherwise advised by the pilot concerned, ATC will assume that the last assigned Mach number will be maintained both in cruise and in any cleared step-climbs or step-descents made in the course of the flight.
2.4.7 The Mach number technique requires that pilots strictly adhere to the following procedures:
a) aircraft must strictly adhere to the last assigned Mach number;
b) if essential to make an immediate temporary change in Mach number (e.g. due to turbulence), the appropriate ATC unit should be notified as soon as possible of that change;
c) when required by the appropriate ATC unit, the current true Mach number should be included in routine position reports.
[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.
The Mach Number Technique is required in various parts of the world:
More about the North Atlantic, below.
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.
[NAT Doc 007, ¶7.1.1.] Mach Number Technique (MNT) is a technique whereby aircraft operating successively along suitable routes are cleared by ATC to maintain a Mach number for a portion of the enroute phase of 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.] MNT is used to improve the utilisation of airspace on long route segments where ATC has only position reports to ensure longitudinal separation between flights is maintained. When two or more aircraft are operating along the same route at the same flight level and maintaining the same Mach number, the time interval between them is more likely to remain constant than by using any other method.
[NAT Doc 007, ¶7.3.]
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 FOLLOWING 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, ¶16.4.2] In the NAT HLA the Mach number technique is used to manage longitudinal separations between aircraft following the same track. Chapter 7 above provides more detailed information. Consequently, flight plans for the NAT HLA segment of flight must define aircraft speed in terms of a Mach number. This is true even if procedures dictate that aircraft speed be defined in terms of TAS for other (continental airspace) segments of that same flight. Oceanic clearances include a True Mach number to follow and because this is used by ATC to regulate longitudinal separations, no tolerance is permissible.
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, ¶7.3.5.] 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 requirement to issue an assigned fixed Mach to all flights has been removed from NAT SUPPs (ICAO Doc7030), however, due to the technical design of the ACARS Clearance (CLX) message and NAT Air Navigation Service Providers (ANSP) application of longitudinal separation using the Mach number technique, nearly all oceanic clearances issued to turbojet aircraft in the NAT Region include an assigned Mach. If any variation to the assigned Mach is desired, flight crews must request such changes from ATC.
All aircraft, regardless of FANS equipage, will be eligible for the application of OWAFS in both ATS surveillance and non-surveillance airspace. Oceanic clearance procedures will remain unchanged. A fixed Mach will continue to be part of the oceanic clearance. ATC may remove the speed restriction via voice or CPDLC by issuing "Resume normal speed" instructions.
If the aircraft then receives RESUME NORMAL SPEED (via CPDLC or Voice), the flight crew no longer needs to comply with a previously issued Mach. However, the flight crew shall advise ATC if, as the result of the RESUME NORMAL SPEED message, they intend to adjust their speed by plus or minus Mach 0.02 or more from their last assigned speed.
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, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016
ICAO Doc 4444, Amendment 9 to the PANS-ATM, 15 June 2020
ICAO Doc 7030, Amendment 1, International Civil Aviation Organization, 8 January 2009
ICAO Doc 9426 - Air Traffic Planning Services, International Civil Aviation Organization, First (Provisional) Edition, 1984
ICAO Nat Doc 007, North Atlantic Operations and Airspace Manual, v. 2021-1, applicable from February 2021
IFALPA Safety Bulletin 2016-10-12, "Oakland Oceanic Mach Number Variations"
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