Annex 5  Units of Measurement to be Used in Air and Ground Operations
Developed by ICAO, the International Standards and Recommended Practices (SARPS) contained in the nineteen Technical Annexes to the Convention on International Civil Aviation (also called Chicago Convention) are applied universally and produce a high degree of technical uniformity which has enabled international civil aviation to develop in a safe, orderly and efficient manner.
Subject to some provisions specific to aviation, the International System of Units developed and maintained by the General Conference of Weights and Measures (CGPM) shall be used as the standard system of units of measurement for all aspects of international civil aviation air and ground operations worldwide.

Logo of the BIPM showing the Pavillon de Breteuil 
It is interesting to recall the history of the CGPM. One of the first natural measures was the meter; the decimal metric system was introduced in France on 7 April 1795 by the law "On weights and measures", causing a major change in the everyday life of ordinary people.
A Diplomatic Conference on the Meter met in Paris in 1875 and during its final session on 20 May 1875, the international treaty known as the Convention du Mètre (Meter Convention) was signed by 12 European states to conduct international activities relating to a uniform system for measurements, and to coordinate international metrology and the development of the metric system.
This treaty established the following three bodies:
1. Conférence générale des poids et mesures (CGPM, General Conference on Weights and Measures), an intergovernmental conference made up of delegates of the governments of the Member States and observers from the Associates of the CGPM; it is the supreme authority for all actions;
2. Comité international des poids et mesures (CIPM, International Committee for Weights and Measures), consisting of selected scientists and metrologists, which prepares and executes the decisions of the CGPM and is responsible for the supervision of the International Bureau of Weights and Measures; and
3. Bureau international des poids et mesures (BIPM, International Bureau of Weights and Measures), a permanent laboratory and world center of scientific metrology, the activities of which include the establishment of the basic standards and scales of the principal physical quantities and maintenance of the international prototype standards.
In view of the extension of work entrusted to the BIPM, the CIPM has set up since 1927, under the name of Consultative Committees, bodies designed to provide it with information on matters which it refers to them for study and advice.
The General Conference receives the report of the International Committee for Weights and Measures on work accomplished; it discusses and examines the arrangements required to ensure the propagation and improvement of the International System of Units (SI); it endorses the results of new fundamental metrological determinations and various scientific resolutions of international scope; and it decides all major issues concerning the organization and development of the International Bureau of Weights and Measures. The conference meets in Sèvres (Pavillon de Breteuil, southwest of Paris, where the BIPM is situated) usually once every four years. Initially it was only concerned with the kilogram and the meter, but in 1921, the scope of the treaty was extended to accommodate all physical measurements and hence all aspects of the metric system. In 1960, the 11^{th} CGPM named the system the International System of Units, abbreviated SI from the French name, Le Système international d'unités, with six base units; the seventh base unit, the mole, was added in 1971 by the 14^{th} CGPM. To celebrate the signing of the Meter Convention on 20 May 1875, 20 May is known as World Metrology Day.


SI BASE UNITS 
The 20^{th} CGPM, which met from 9 to 12 October 1995, decided to eliminate the class of supplementary units as a separate unit class in the SI. Thus, the SI now consists of only two classes of units: base units and derived units, which together form what is called The Coherent System of SI Units. The SI also includes the prefixes to form decimal multiples and submultiples of SI units.
The Système International d’Unités (SI) defines seven units of measure (see table on the leftside) as a basic set from which all other SI units are derived. The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology; however in a given realization, these units may well be interdependent, i.e. defined in terms of each other. Derived units are expressed algebraically in terms of base units or other derived units.
The International Commission for Air Navigation (ICAN) had already adopted the metric system as a standard for air navigation. The importance of a common system of measurements was realized in 1944 at the Chicago Conference on International Civil Aviation. The PICAO Air Navigation Committee established a Dimensional Practices Committee to address the issue of an universal adoption of a standard method of measurements in civil aviation used between aircraft in flight and ground stations in the exchange of air traffic control instructions and information required for the safe operations of aircraft. Resolution 27 (on the Unification of Numbering and Systems of Dimensioning in connection with International Civil Aviation) of the Interim Assembly of the Provisional International Civil Aviation Organization (PICAO) requested to determine the nature and importance of the handicaps imposed on civil aviation by lack of unification of units of measurement and to recommend the best means of overcoming such handicaps.

Annex 5 – Front Page 
By Resolution A135, the First ICAO Assembly held in May 1947 recommended that the Council adopt, as an ICAO Standard, as rapidly as practicable, for use in airground communications and relevant publications on international air navigation, the first ICAO Table of Units (based on Nautical Miles and Tens, Meters, Knots, Millibars, Degrees Centigrade, Kilograms and Metric Tons, 24hours for the time, etc.). Recognizing the possibility that some States would find it impossible to employ these units in all cases, the Assembly further recommended that the Council should incorporate into the Standards suitable alternatives to be adopted by those States obliged to continue using the pound, foot or statute mile as basic units, in order to reduce the hazards resulting from the absence of uniformity.
The Annex 5 named Dimensional Units to be used in AirGround Communications was first adopted by the ICAO Council on 16 April 1948 and became applicable on 1 January 1949. The primary purpose of Annex 5 was to reduce the number of different combinations of units used in airground communications and to provide for a progressive elimination of tables until the ICAO Table only is used throughout the world. Besides the ICAO Table, four additional interim tables of units were contained in the first edition of Annex 5, for use by those States unable to use the primary table. Annex 5 was initially applicable only to those units used in communications between aircraft and ground stations.
Every effort was developed by the Organization to encourage States to move progressively towards the ICAO Table and further amendments to Annex 5 were adopted in 1951 to reduce the number of alternatives to the ICAO Table, thus affirm the longterm policy of achieving the unification of the Units of Measurement. By 1961, the number of tables of units in Annex 5 had been reduced to two.
Assembly Resolutions A1813 Appendix J (18^{th} Session of ICAO Assembly, Vienna, 1971), A2121 Appendix F (21^{st} Session of ICAO Assembly, Montréal, 1974) and A2218 Appendix F (22^{nd} Session of ICAO Assembly, Montréal, 1977) resolved that the units of measurements to be used in civil aviation shall be achieved on the basis of the International System of Units (SI), except in those cases where nonSI units are permitted for use together with the SI units, in the interest of safety of international air transport. On 23 March 1979, the Council adopted the 4^{th} edition of Annex 5 changing the title of this Annex to Units of Measurement to be used in Air and Ground Operations, which increased the scope to cover all the aspects of the air and ground operations, made provision of a standardized system of units based on the SI, identified nonSI units for use in international civil aviation, and provided for the termination of the use of certain nonSI units. Provisions of Annex 5 are regularly amended to reflect the changing environment, such as the definition of units or technical improvements towards a safer aviation.
The latest amendments of Annex 5 deleted the references to temporary nonSI units.
ICAO maintains liaison with the International Organization for Standardization (ISO) regarding the standardized application of SI units in aviation.


France – 4 October 1950  Maximum Card 10^{th} Conference of the CGPM (Conférence générale des poids et mesures). From 1793 until 1960, the meter was defined as 1/10000000 of the meridian through Paris between the North Pole and the Equator.



Romania – 10 September 1966 100^{th} Anniversary of the introduction of the metric system in Romania. It is interesting to note that this stamp displays the 6 base units further to the recommendations of the 10^{th} CGPM held in 1954: the meter, kilogram, second, ampere, degree Kelvin (later renamed Kelvin), and candela. The 7^{th} base unit, the mole, was added in 1971 by the 14^{th} CGPM.



France – 31 May 1975  First Day Cover 100^{th} Anniversary of the Convention du Mètre and the establishment of the Bureau International des Poids et Mesures (BIPM). The stamp and cachet list the names of the seven SI Base Units. The stamp shows: 1. At the leftside: signatures and the seal of the first Convention of the definition of the meter on 20 May 1875; 2. At the rightside: the Krypton86 (Kr 86) atom, with a reminder of the international system of measurements (SI) based on the MKSA (meterkgsecondampere) system (with 7 base units); and 3. At the lowerside: the multiplier 1,650,765.73, which was used for the new definition of the meter on 14 October 1960 (at the 11^{th} CGPM), as follows: The meter is defined in terms of 1,650,765.73 wavelengths of Krypton86 atom in a vacuum. Note that the definition of the meter was revised in 1983 and was approved as the distance travelled by light through a vacuum in 1/299792458 second.




Poland – 1979 – Postal Stationery  60^{th} Anniversary of the Polish Metric System, as noted by the text: 60  lecie POLSKIEJ SŁUŻBY MIAR. The stamp in the upperright side lists the names of the seven SI Base Units. The text in the upperright side makes reference to the 1565 Constitution, whereby Sigismund II Augustus (15201572), King of Poland and Grand Duke of Lithuania from 1548 to 1572, established the Act of Weights and Measures (Ustawa na Wagi y na Miary), which partially standardized measures in some provinces (the national Diet was sitting at Piotrkow). The first actual attempt at standardization of the units of measurement came by a royal decree in 1764 with the introduction of the socalled Old Polish measurement system. The system was later replaced by the socalled New Polish Measurement System introduced on 1 January 1819. The traditional Polish systems of weights and measures were later replaced with those of surrounding nations (due to the invasions and partitions of Poland) and with the metric system by the end of the 19^{th} century. Regaining independence in 1919 at the treaty of Versailles, the Polish government reintroduced the metric system. Since 1966, the country adheres to the International System of Units (SI). 
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