Mach Number Technique
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.
Mach Number Technique Defined
[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, ¶22.214.171.124.1.] 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.]
- The Oceanic Clearance includes the assigned (True) 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 coordination 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 minimum 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 recognise 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 utilise 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.
[ICAO Annex 2, ¶126.96.36.199. 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:
- Fly the assigned Mach Number if at all possible, if your ETA varies by 3 minutes or more, inform ATC.
- If you cannot maintain the assigned Mach Number for any reason (performance, turbulence, etc.), inform ATC before making an adjustment. If you cannot get clearance first, consider broadcasting your actions on 123.45 and 121.5 to ensure aircraft ahead and behind you are aware.
Mach Number Tolerances
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:
- THE MACH NUMBER AT FIR ENTRY; OR
- ANY SUBSEQUENT SPEED CHANGE NOTIFIED TO ATC IN FLIGHT
[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, ¶7.3.5.] 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.
Procedures After Leaving Oceanic Airspace
[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.
Where Mach Number Technique is Used
The Mach Number Technique is required in various parts of the world:
- Africa (Canarias, Dakar, Recife, Sal Oceanic FIRs, and designated RNP-10 routes): [ICAO Doc 7030, ¶AFI 6.2.2.]
- Caribbean (specified areas in Houston Oceanic, Merida and Monterrey CTAs, Miami Oceanic, San Juan CTA, and New York Oceanic): [ICAO Doc 7030, ¶CAR 6.2.2.]
- Middle East/Asia (specified routes): [ICAO Doc 7030, ¶MID/ASIA 2.1.11.]
- North America (Anchorage Arctic CTA): [ICAO Doc 7030, ¶NAM 6.2.2.]
- North Atlantic: [ICAO Doc 7030, ¶NAT 6.2.2.]
- Pacific (Anchorage and Oakland Oceanic FIRs): [ICAO Doc 7030, ¶PAC 6.1.1.]
- South America (Dakar, Recife, and Sal Oceanic FIRs): [ICAO Doc 7030, ¶SAM 6.2.2.]
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.
Resume Normal Speed
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.
Portions of this page can be found in the book International Operations Flight Manual, 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 7030, Amendment 1, International Civil Aviation Organization, 8 January 2009
IFALPA Safety Bulletin 2016-10-12, "Oakland Oceanic Mach Number Variations"
ICAO NAT Doc 007, North Atlantic Operations and Airspace Manual, v 2018-1