Standard Application of Units of Measure in Civil Aviation

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Standard Application of Units of Measure in Civil Aviation

39TH ANNUAL CONFERENCE, Marrakech, Morocco, 6-10 March 2000

WP No. 100

Standard Application of Units of Measure in Civil Aviation


The IFALPA ATS Committee has debated over 3 years the feet/metric problem and has come to the conclusion that, despite ICAO recommendation that Civil aviation should adopt the International System of Units, (SI) (i.e. metric), the best chances for aviation to achieve a unified global system, would be to declare the NM, Knot and Feet the units of Measure for Distance, Speed and Height.


There are presently 3 systems for measuring Height, speed and distance in wide use in the world. The non- ICAO standard US system based on Feet, Nautical Mile and Knot, the Russian system, used widely in the CIS, using Meters, Km/hour and Km (but not ICAO compliant ) and the Chinese system, also using the metric system but with a different basis for height calculation (also non-ICAO compliant).

With the current globalization of transport, and the opening of Siberian and Trans-Chinese routes, the combination of the 3 systems is becoming a safety issue. ( It must be noted, that the preliminary report of the collision between a Saudia B747 and Kazak IL76 near New Delhi was partially attributed to confusion between Metric Altitude conversion and Feet FL ATC clearance)

There is an urgent need to strive for a Global Unit of Measure system, and IFALPA believes, after some research that it will be quicker and makes more sense now, to have the US Feet system declared the Global system, and to request that ICAO review the position they took back in 1944. (see text in references below)

In addition, recently the European Union (EU DG7) reminded European Member States and the JAA that after Dec 31, 2009, they had committed themselves to change all units of measure into the SI (read metric) and that included Civil Aviation. (It must be noted that the EU Commissioners no longer support this position)

To arrive at their conclusion IFALPA used the following arguments : (quote from their Draft paper to their next Conference, and of their Annex5 Chapter 3 Policy manual):

“IFALPA believes that it is inappropriate to consider using SI units of measurement for all aspects of International Civil aviation and ground Ops. The most important objective is to achieve a rational common global standard of units of measurements. It is therefore not necessary that all units be derived from the SI. As the use of different altimetry units seriously affects flight safety, only one unit should be used for altimetry. The metre does not lend itself to a simple rational system of cruising levels and the foot is by far the most commonly used unit for reporting vertical positions and vertical intervals. Therefore the worldwide use of the foot should be implemented as the basic unit of measurement for vertical distances and the feet per minute for vertical speed.

In addition the requirements of global navigation create the need for the retention of the nautical mile.

This creates an additional requirement for the retention of the Knot as the measurement for horizontal speed.

Whenever a State or region contemplates changing its current system of units, sufficient lead-time will be essential to allow for the necessary conversion of the equipment, procedures and documentation concerned, and for personnel familiarisation to be accomplished, without increase in operational complexity and without detriment to flight safety.

Cockpit instrumentation should support pilot operations with the units of measure in use, in all airspaces where the aircraft operates .”

To achieve any chances for this proposal to be accepted within ICAO it should receive the widest possible support. While recognising that both ICAO and the EU were probably in the right track when promoting the SI (metric) standard, we have to recognise that, in the year 2000, the current number of States and number of registered civil aircraft flying in the world today are using the US (feet) standard.


If it is in the interest of Safety to have a single global unit of measurement standard, then we must recognise that the IFALPA approach is the most pragmatic one, and that IFATCA should support IFALPA initiative.

IFATCA should take the view that not necessarily SI units are providing a solution for Civil aviation, and that, in order to obtain a common global standard, which is necessary for safety reasons, then the Feet, NM and Knot should be retained for the measurement of Altitude, Distance and Speed.

It is recommended that:

In view of achieving a rational common global standard of units of measurements, it is not important that ALL the units are derived from the SI.

That for the measurement of Altitude, Speed and Distance in Civil Aviation, the Feet, Knot and Nautical Mile should be the global standard used.


ICAO Annex 5, 4 th edition, (July 1979), amended Nov 1997:

para 3.1.1.: “The international System of Units (SI) developed and maintained by the General conference of Weights and Measures (CGPM) shall, subject to the provisions of para 3.. 2. and 3.3., be used as the standard system of units of measurement for all aspects of international civil aviation and ground operations.”

para 3.2.2 : “The non-SI units listed in table 3.3. [Editorial Note: basically the feet/NM/Knot, etc.] Shall be permitted for temporary use as alternative units of measurement, but only for those specific quantities listed in table 3.4. [Also ft, NM and Kt]

Note: It is intended that the use of the non-SI alternative units listed in table 3.3 and applied as indicated in table 3.4. will eventually be discontinued in accordance with individual unit termination dates, when established, will be given in Chapter 4.”

Chapter 4 , table 4.1. Termination dates for non-SI alternative units:

“Knot and Nautical Mile: Termination date: not established.

Note: For planning purposes any termination date for these units will not be before 31 December 1990.”

“Foot: Termination date: Not Established.

Note: No termination date has yet been established for the use of the foot.”

Last Update: September 28, 2020  

December 21, 2019   456   Jean-Francois Lepage    2000    

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