Mach number: True or false

 - October 8, 2007, 5:38 AM

Business aircraft crews are being urged to ensure they are using true Mach numbers in all airspace where the Mach number technique (MNT) is applied for longitudinal separation. According to a bulletin from the International Business Aviation Council (IBAC), some pilots are unaware that their displays show uncorrected Mach numbers and will not therefore be conveying their true Mach speed to air traffic controllers.

Peter Ingleton, IBAC director and International Civil Aviation Organization (ICAO) liaison, explained that two undisclosed business aircraft manufacturers have been made aware that their products do not display true Mach speed. Both airframers are now supposed to be informing their customers of the need to apply the corrected Mach numbers, which can be found in the aircraft flight manual. IBAC is urging all operators to check that they apply true Mach numbers in MNT airspace, which includes numerous routes worldwide, including the north and south Atlantic sectors, the northern Pacific and the U.S. West Coast.

In airspace where MNT applies, crews are required to stick to the Mach number approved by controllers, with increases or decreases in speed to be requested. According to Ingleton, the difference between the true and uncorrected Mach numbers could be enough to reduce the existing 10-minute MNT longitudinal separation by two or three minutes over the course of an oceanic crossing. This discrepancy would become much more serious if–as is now being discussed in air-traffic-management circles–the MNT separation were to be reduced to seven minutes.

More information about MNT procedures can be found in ICAO’s PAN-ATM document (Doc. 4444 ATM/501-Fourteenth Edition-2001) and in the ICAO air traffic services planning manual (Doc. 9426-An 924). Ingleton said the lack of awareness among business aircraft operators about true Mach numbers became apparent during IBAC discussions with ICAO about air traffic issues in the North Atlantic.