vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft (note these are in geopotential foot units). The level to be flown is determined by the magnetic track of the aircraft, as follows: Magnetic track 000 to, and including, 089° – odd thousands of feet (FL070, 090, 110 etc.) Magnetic track 090 to, and including, 179° – odd thousands plus 500 ft (FL075, 095, 115 etc.) Magnetic track 180 to, and including, 269° – even thousands of feet (FL080, 100, 120 etc.) Magnetic track 270 to, and including, 359° – even thousands plus 500 ft (FL085, 105, 125 etc.) Reduced vertical separation minima (RVSM) Reduced vertical separation minima (RVSM) reduces the vertical separation above FL 290 to 1,000 ft. This allows aircraft to safely fly more optimum routes, gain fuel savings and increase airspace capacity by adding new flight levels. Only aircraft that have been certified to meet RVSM standards, with several exclusions, are allowed to fly in RVSM airspace. It was introduced into the UK in March 2001. On 20 January 2002, it entered European airspace. The United States, Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on 20 January 2005, and Africa on 25 September 2008. Track 000 to 179° – odd thousands (FL 290, 310, 330, etc.) Track 180 to 359° – even thousands (FL 300, 320, 340, etc.) At FL 410 and above, 4,000 ft intervals are resumed to separate same-direction aircraft and only odd Flight Levels are assigned, depending on the direction of flight: Track 000 to 179° – odd flight levels (FL 410, 450, 490, etc.) Track 180 to 359° – odd flight levels (FL 430, 470, 510, etc.) Metric flight levels The International Civil Aviation Organization (ICAO) has recommended a transition to using the International System of Units since 1979 with a recommendation on using metres (m) for reporting flight levels. China, Mongolia, Russia and many CIS countries have used flight levels specified in metres for years. Aircraft entering these areas normally make a slight climb or descent to adjust for this, although Russia and some CIS countries started using feet above transition altitude and introduced RVSM at the same time on 17 November 2011. Kyrgyzstan, Kazakhstan, Tajikistan, Uzbekistan, and Turkmenistan The flight levels below apply to Kyrgyzstan, Kazakhstan, Tajikistan and Uzbekistan and 6,000 m or below in Turkmenistan (where feet is used for FL210