Three Main Systems of Flight Level (FL) Determination

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Three Main Systems of Flight Level (FL) Determination

35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996

WP No. 96

Three Main Systems of Flight Level (FL) Determination


There exists at present three main systems of flight level (FL) determination.

  1. The FL expressed in feet , mainly used in the western world.
  2. The FL expressed in Meters using 300 m. separation below 8400m., 500m separation between 8400m and 12000m, then 1000m above this. This system is used by the Russian Federation and some of the CIS States.
  3. The FL based in Meters, using 300m. separation below 6000m, 600m separation between 6000m and 12000m and 1000m above this. This system is used by the Peoples Republic of China (PRC).

This paper will highlight the inherent safety implications and propose IFATCA policy on the subject.

The 3 systems described in section 1. are not compatible with each other ( e.g. an aircraft at FL 290 = to 29,000 ft. , will have to climb or descend, to match the nearest Metric altitude in the CIS, but has to descend further to match the nearest PRC level.)

Until recently the number of aircraft affected has been negligible. However, since the opening of new routes transiting these airspace’s, the number of aircraft affected has significantly increased and is predicted to increase further in the near future. IFATCA believes that this has implications pertaining to safety in the areas of interface between the different systems of expressing Flight Level.

There are justifiable reasons to recognise that both the CIS States and the PRC , because of the size of their FIR’s and the number of aircraft and ground facilities involved, will retain the Metric system for the foreseeable future.

For the CIS, an agreement was reached to retain the metric system, at the Council of Aviation and Airspace at the CIS meeting on the 8th Sept. 1995. However, some States of the CIS have indicated they might change to the feet system in the near future.

It is equally reasonable to assume that considering the number of States and aircraft registered currently using the Feet system, a global adoption by those States to a Metric system seems very unlikely in the near future.

IFATCA is concerned that with the imminent introduction of Reduced Vertical Separation Minimum (RVSM) between FL 290 and FL 410, that there is an urgent need to harmonise the systems either into one accepted system or to be fully compatible.

In the Metric areas there is the use of a large number of digits to be transmitted on the R/T. Consideration should be given to reducing this for R/T transmission similar to that used under the flight level system.


There is clear evidence that as traffic levels increase in the interface areas between the use of FL Feet/FL Metric CIS /FL Metric PRC safety will be compromised.

With the increase of trans-global routing of aircraft, the existence of three separate systems for flight level determination is unsatisfactory.

The introduction of RVSM in the interface areas will increase the complexity of the transition from flight level feet to flight level metric. A calculated study should be initiated in order to determine the nearest metric altitudes to the current flight level system, taking into account the RVSM criteria as detailed in ICAO Doc 9574. There should be a single date of introduction of the new values for the entire region and that. consideration should be given for its introduction coincident with the introduction of RVSM in that region.

A practical solution for the short term would be for the States involved at the change from one system to another to agree on compatible metric/feet altitudes/flight levels procedures.

In conjunction with others concerned regarding the problem, a joint approach be made to ICAO.

It is recommended that:

A global solution should be developed using one system of flight level determination.

That the interface in the transition areas between FL Ft./FL Metric CIS /FL Metric PRC be properly managed with the introduction of procedures which will prevent the selection of the incorrect flight level.

That the introduction of compatible procedures in the interface areas be coincident with the introduction of RVSM.

That a simplification of the R/T phraseology be introduced in the expression of Metric Flight Levels as adopted by Feet Flight Levels.

Last Update: September 28, 2020  

February 12, 2020   264   Jean-Francois Lepage    1996    

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