ANNEX 5 to the Convention on International Civil Aviation

Units of Measurement to be Used in Air and Ground Operations

The question of the units of measurement to be used in international civil aviation goes back as far as the origin of ICAO itself. At the International Civil Aviation Conference held at Chicago in 1944, the importance of a common system of measurements was realized and a resolution was adopted calling on States to make use of the metric system as the primary international standard.

A special committee was established to look into the question and as a result the First Assembly of ICAO in 1947 adopted a resolution (A1-35) recommending a system of units to be issued as an ICAO Standard as soon as possible. Stemming from this resolution, the first edition of Annex 5 was adopted in 1948. This contained an ICAO table of units based essentially on the metric system, but it also contained four additional interim tables of units for use by those States unable to use the primary table. It was evident from the beginning that the achievement of standardization in units of measurement would not be easy, and Annex 5 was initially applicable only to those units used in communications between aircraft and ground stations.

Many attempts to improve the level of standardization were made in the following years and a number of amendments to Annex 5 were introduced. By 1961 the number of tables of units in the Annex had been reduced to two, which remained until Amendment 13 was adopted in March 1979. Amendment 13 extended considerably the scope of ICAO's role in standardizing units of measurements to cover all aspects of air and ground operations and not just air-ground communications. It also introduced the International System of Units, known as SI from the "Système International d’Unités", as the basic standardized system to be used in civil aviation.

In addition to the SI units the amendment recognized a number of non-SI units which may be used permanently in conjunction with SI units in aviation. These include the litre, the degree Celsius, the degree for measuring plane angle, etc. The amendment also recognized, as do the relevant ICAO Assembly Resolutions, that there are some non-SI units which have a special place in aviation and which will have to be retained, at least temporarily. These are the nautical mile and the knot, as well as the foot when it is used in the measurement of altitude, elevation or height only. Some practical problems arise in the termination of the use of these units and it has not yet been possible to fix a termination date.

Amendment 13 to Annex 5 represented a major step forward in the difficult process of standardizing units of measurement in international civil aviation. Although complete standardization is still some time away, the foundation has been laid for resolving a problem which has been recognized by ICAO since its inception. With this amendment a very large degree of standardization has been achieved between civil aviation and other scientific and engineering communities.

Amendments 14 and 15 to Annex 5 introduced a new definition of the metre, and references to temporary non-SI units were deleted.

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