Civil aviation regulations
CONTENTS
PART 20
UNITS OF MEASUREMENT TO BE USED IN AIR AND GROUND OPERATIONS
20.1 General
20.1.1 Applicability
20.1.2 Definitions
20.2 Standard application of units of measurement
20.2.1 SI Units
20.2.2 prefixes
20.2.3 nonSI Units
20.2.3.1 nonSI Units for permanent use with the SI
20.2.3.2 nonSI alternative units permitted for temporary use with the SI
20.2.4 application of specific units
20.2.4.1 the application of the units of measurement for certain qantities
20.2.4.2 whenever applicable, means and provisions
20.3 Termination of use of noonSI alternative units
20.3.1 the use of the alternative nonSI units
attachment 1: guidance on the application of the SI
attachment 2: guidance on the application of the SI
attachment 3: conversion factors
attachment 4: coordinated universal time
attachment 5: presentation of date and time in allnumeric form
20.1. General
20.1.1. Applicability
Part 20 prescribes the requirements for the use of a standardized system of units of measurement in international civil aviation air and ground operations which shall be applicable to all aspects of international civil aviation air and ground operations.
20.1.2. Definitions
For the purpose of CARNA Part 20, the following definitions shall apply:
1. Ampere (A): The ampere is that constant electric current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10^{7} per metre of length.
2. Becquerel (Bq): The activity of a radionuclide having one spontaneous nuclear transition per second.
3. Candela (cd): The luminous intensity, in the perpendicular direction, of a surface of 1/600000 square metre of black body at the temperature of freezing platinum under a pressure of 101 325 per square metre.
4. Celsius temperature (t °C): The Celsius temperature is equal to the difference t °C = T – Tobetween two thermodynamic temperatures T and To where To equals 273.15 Kelvin.
5. Coulomb (C): The quantity of electricity transported in 1 second by a current of 1 ampere.
6. Degree Celsius (°C): The special name for the unit Kelvin for use in stating values of Celsiustemperature.
7. Farad (F): The capacitance of a capacitor between the plates of which there appears difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb.
8. Foot (ft): The length equal to 0.3048 metre exactly.
9. Gray (Gy): The energy imparted by ionizing radiation to a mass of matter corresponding to joule per kilogram.
10. Henry (H): The inductance of a closed circuit in which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at a rate of 1 ampere per second.
11. Hertz (Hz): The frequency of a periodic phenomenon of which the period is 1 second.
12. Human performance: Human capabilities and limitations which have an impact on the safety, security and efficiency of aeronautical operations.
13. Joule (J): The work done when the point of application of a force of 1 is displaced adistance of 1 metre in the direction of the force.
14. Kelvin (K): A unit of thermodynamic temperature which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
15. Kilogram (kg): The unit of mass equal to the mass of the international prototype of the kilogram.
16. Knot (kt): The speed equal to 1 nautical mile per hour.
17. Litre (L): A unit of volume restricted to the measurement of liquids and gases which is equal to 1 cubic decimetre.
18. Lumen (lm): The luminous flux emitted in a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela.
19. Lux (Lx): The illuminance produced by a luminous flux of 1 lumen uniformly distributed over a surface of 1 square metre.
20. Metre (m): The distance travelled by light in a vacuum during 1/299 792 458 of a second.
21. Mole (mol): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon12.
Note: When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles
22. Nautical mile (NM): The length equal to 1 852 metres exactly.
23. (N): The force which when applied to a body having a mass of 1 kilogram gives it an acceleration of 1 metre per second squared.
24. Ohm: The electric resistance between two points of a conductor when a constant difference of potential of 1 volt, applied between these two points, produces in this conductor a current of 1 ampere, this conductor not being the source of any electromotive force.
25. Pascal (Pa): The pressure or stress of 1 per square metre.
26. Radian (rad): The plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.
27. Second (s): The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
28. Siemens (S): The electric conductance of a conductor in which a current of 1 ampere is produced by an electric potential difference of 1 volt.
29. Sievert (Sv): The unit of radiation dose equivalent corresponding to 1 joule per kilogram.
30. Steradian (sr): The solid angle which, having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
31. Tesla (T): The magnetic flux density given by a magnetic flux of 1 Weber per square metre.
32. Tonne (1): The mass equal to 1 000 kilograms.
33. Volt (V): The unit of electric potential difference and electromotive force which is the difference of electric potential between two points of a conductor carrying a constant current of 1 ampere, when the power dissipated between these points is equal to 1 watt.
34. Watt (W): The power which gives rise to the production of energy at the rate of 1 joule per second.
35. Weber (Wb): The magnetic flux which, linking a circuit of one turn, produces in it an electromotive force of 1 volt as it is reduced to zero at a uniform rate in 1 second.
20.2. Standard application of units of measurement
20.2.1. Si units
The International System of Units developed and maintained by the General Conference of Weights and Measures (CGPM) shall, subject to the provisions of 20.2.1.2 and 20.2.2, be used as the standard system of units of measurement for all aspects of international civil aviation air and ground operations.
Note 1: As used herein the term Sf unit is meant to include base units and derived units as well as their multiples and submultiples.
Note 2: See Attachment 1 for guidance on the general application of prefixes.
20.2.2. Prefixes
The prefixes and symbols listed in Table 201 shall be used to form names and symbols of the decimal multiples and submultiples of SI units.
Table 201 SI unit prefixes
Multiplication factor

Prefix

Symbol

1000 000 000 000 000 000 = 10^{18}

exa

E

1 000 000 000 000 000 = 10^{15}

peta

P

1000 000 000 000 = 10^{12}

tera

T

1 000 000 000 = 10^{9}

giga

G

1 000 000 = 10^{6}

mega

M

1 000 =10^{3}

kilo

K

100 = 10^{2}

hecto

H

10 = 10^{1}

deca

da

0.1 = 10^{1}

deci

d

0.01 = 10^{2}

centi

c

0.001 = 10^{3}

milli

m

0.000 001 = 10^{6}

micro

µ

0.000 000 001 = 10^{9}

nano

n

0.000 000 000 001 = 10^{12}

pico

p

0.000 000 000 000 001 = 10^{15}

femto

f

0.000 000 000 000 000 001 = 10^{18}

atto

a

20.2.3. NonSI Units
20.2.3.1
NONSI units for permanent use with the SI. The nonSI units listed in Table 202 shall be used either in lieu of, or in addition to, SI units as primary units of measurement but only as specified in Table 204.
Table 202. NonSI units for use with the SI
\Specific quantities in Table 204 related to

Unit

Symbol

Definition
(in terms of SI units)

mass

tonne

t

1 t = 103 kg

plane angle

degree

o

1o = (p/180) rad


minute

’

1’ = (1/60)o = (p/10 800) rad


second

’’

1’’ = (1/60)’ = (p/648 000) rad

temperature

degree Celsius

oC

1 unit oC = 1 unit Ka)

time

minute

min

1 min = 60 s


hour

h

1 h = 60 min = 3 600 s


day

d

1d = 24 h = 86 400 s


Week, month, year

–


volume

litre

L

1L = 1 dm^{3}= 10^{3} m^{3}

a) See Attachment 2 for conversion factor




20.2.3.2
NonSI alternative units permitted for temporary use with the SI. The nonSI units listed in Table 203 shall be permitted for temporary use as alternative units of measurement but only for those specific quantities listed in Table 204.
Note: It is intended that the use of the nonSI alternative units listed in Table 203 andapplied as indicated in Table 204 will eventually be discontinued in accordance with individual unit termination dates established by ICAO. Termination dates, when established, will be given in an amendment of Chapter 20.3 of this part.
Table 203 Non SI alternative units permitted for temporary use with the SI
Specific quantities in Table 204 related to

Unit

Symbol

Definition (in terms of SI units)

distance (horizontal)

nautical mile

NM

1 NM = 1 852 m

distance (vertical)a)

foot

ft

1 ft = 0.304 8 m

Horizontal speed
vertical speed

knot
feet per minute

kt
ft/min

1 kt = 0.514 444 m/s

a) altitude, elevation, height.




20.2.4. Application of specific units.
20.2.4.1
The application of units of measurement for certain quantities used in international civil aviation air and ground operations shall be in accordance with Table 204.
Note: Table 204 is intended to provide standardization of units (including prefixes) for those quantities commonly used in air and ground operations. Basic provisions herein apply for units to be used for quantities not listed.
20.2.4.2
Whenever applicable, means and provisions for design, procedures and training should be established for operations in environments involving the use of standard and nonSI alternatives of specific units of measurement, or the transition between environments using different units, with due consideration to human performance.
Note: Guidance material on human performance can be found in the 0 Human Factors Training Manual (Doc 9683) and Circular 238 (Human Factors Digest No.6 – Ergonomics).
Table 204. Standard application of specific units of measurement
Ref.No.

Quantity

Primary unit (symbol)

NonSI Alternative Unit(symbol)

1. Direction/Space/Time




1.1

altitude

m

ft

1.2

area

m^{2}


1.3

distance (long)a)

km

NM

1.4

distance (short)

m


1.5

elevation

m

ft

1.6

endurance

h and min


1.7

height

m


1.8

latitude

^{o}’ ’’


1.9

length

m


1.10

longitude

^{o}’ ’’


1.11

plane angle (when required, decimal subdivisions of the degree shall be used)

o


1.12

runway length

m


1.13

runway visual range

m


1.14

tank capacities (aircraft)^{b)}

L


1.15

time

s



min




h




d




week




month




year



1.16

visibility (note: visibility of less than 5 km may be given in meters)

km


1.17

volume

m^{3}


1.18

wind direction (wind directions other than for a landing and takeoff shall be expressed in degrees true; for landing and takeoff wind directions shall be expressed in degrees magnetic)

o


2. Massrelated
2.1

air density

kg/m^{3}

2.2

area density

kg/m^{2}

2.3

cargo capacity

kg

2.4

cargo density

kg/m^{3}

2.5

density (mass density)

kg/m^{3}

2.6

fuel capacity (gravimetric)

kg

2.7

gas density

kg/m^{3}

2.8

gross mass or payload

kg


t


2.9

hoisting provisions

kg

2.10

linear density

kg/m

2.11

liquid density

kg/m^{3}

2.12

mass

kg

2.13

moment of inertia

kg/m^{2}

2.14

moment of momentum

kg/m^{2}/s

2.15

momentum

kg/m/s

3. Forcerelated
3.1

airpressure(general)

kPa

3.2

altimeter setting

hPa

3.3

atmosphericpressure

hPa

3.4

bending moment

kN.m

3.5

force

N

3.6

fuel supply pressure

kPa

3.7

hydraulic pressure

kPa

3.8

modulus of elasticity

MPa

3.9

pressure

kPa

3.10

stress

MPa

3.11

surface tension

mN/m

3.12

thrust

kN

3.13

torque

N.m

3.14

vacuum

Pa




4. Mechanics
4.1

airspeed

km/h

kt

4.2

angular acceleration

rad/s^{2}


4.3

angular velocity

rad/s


4.4

energy or work

J


4.5

equivalent shaft power

kW


4.6

frequency

Hz


4.7

ground speed

km/h

kt

4.8

impact

J/m^{2}


4.9

kinetic energy absorbed by brakes

MJ


4.10

linear acceleration

m/s^{2}


4.11

power

kW


4.12

rate of trim

^{0}/s


4.13

shaft power

kW


4.14

velocity

m/s


4.15

vertical speed

m/s

ft/min

4.16

wind speed

km/h

kt

5. Flow
5.1

engine airflow

kg/s

5.2

engine waterflow

kg/h

5.3

fuel consumption (specific)



piston engines

kg/(kW.h)


turboshaft engines

kg/(kW.h)


jet engines

kg/(kN.h)

5.4

fuel flow

Kg/h

5.5

fuel tank filling rate (gravimetric)

kg/min

5.6

gas flow

kg/s

5.7

liquid flow (gravimetric)

g/s

5.8

liquid flow (volumetric)

L/s

5.9

mass flow

kg/s

5.10

oil consumption



gas turbine

Kg/h


piston engines (specific)

g/(kW.h)

5.11

oil flow

g/s

5.12

pump capacity

L /min

5.13

ventilation airflow

m^{3}/min

5.14

viscosity (dynamic)

Pa.s

5.15

viscosity (kinematic)

m^{2}/s

6. Thermodynamics
6.1

coefficient of heat transfer

W/ (m^{2}.K)

6.2

heat flow per unit area

J/m^{2}

6.3

heat flow rate

W

6.4

humidity (absolute)

g/kg

6.5

coefficient of linear expansion

^{0}C^{1}

6.6

quantity of heat

J

6.7

temperature

°C

7. Electricity and magnetism
7.1

capacitance

F

7.2

conductance

S

7.3

conductivity

S/m

7.4

current density

A/m^{2}

7.5

electric current

A

7.6

electric field strength

C/m^{2}

7.7

electric potential

V

7.8

electromotive force

V

7.9

magnetic field strength

A/m

7.10

magnetic flux

Wb

7.11

magnetic flux density

T

7.12

power

W

7.13

quantity of electricity

C

7.14

resistance

?

8. Light and related electromagnetic radiations
8.1

illuminance

Ix

8.2

luminance

Cd/m^{2}

8.3

luminous exitance

Im/m^{2}

8.4

luminous flux

lm

8.5

luminous intensity

cd

8.6

quantity of light

lm.s

8.7

radiant energy

J

8.8

wavelength

m

9. Acoustics
9.1

frequency

Hz

9.2

mass density

Kg/m^{3}

9.3

noise level

dB^{c)}

9.4

period, periodic time

s

9.5

sound intensity

W/m^{2}

9.6

sound power

W

9.7

sound pressure

Pa

9.8

sound level

dB^{c)}

9.9

static pressure (instantaneous)

Pa

9.10

velocity of sound

m/s

9.11

volume velocity (instantaneous)

m^{3}/s

9.12

wavelength

m

10. Nuclear physics and ionizing radiation
10.1

absorbed dose

Gy

10.2

absorbed dose rate

Gy/s

10.3

activity of radio nuclides

Bq

10.4

dose equivalent

Sv

10.5

radiation exposure

C/kg

10.6

exposure rate

C/kg.s

a) As used in navigation, generally in excess of 4 000 m.






b) Such as aircraft fuel, hydraulic fluids, water, oil and high pressure oxygen vessels.






c) Visibility of less than 5 km may be given in m.



d) Airspeed is sometimes reported in flight operations in terms of the ratio MACH number.






e) The decibel (dB) is a ratio which may be used as a unit for expressing sound pressure level and sound power level. When used, the reference level must be specified.



20.3. Termination of use of nonsi alternative units
Introductory Note: The nonSI units listed in Table 203 have been retained temporarily for use as alternative units because of their widespread use and to avoid potential safety problems which could result from the lack of international coordination concerning the termination of their use. As termination dates are established by the council, they will be reflected as Standards contained in chapter 4 of Annex 5 and as amendments to this chapter. It is expected that the establishment of such dates will be well in advance of actual termination. Any special procedures associated with specific unit termination will be made available by the Minister..
20.3.1
The use in international civil aviation operations of the alternative nonSI units listed in Table 203 shall be terminated on the dates that will be established by the council and which at that time will be listed in Table 201.
Table 201. Termination dates for nonSI alternative units
NonSI alternative unit

Termination date


Knot

}

Not established at this time

Nautical mile

Foot

Not established at this time


a) No termination date has yet been established for use of nautical mile and knot.
b) No termination date has yet been established for use of the foot.



Attachment 2
Guidance on the application of the si
Introduction
1.1
The International system of units is a complete coherent system which includes three classes of units:
1.2
The SI is based on seven units which are dimensionally independent and are listed in Table BI.
1.3
The supplementary units of the SI are listed in Table B2 and may be regarded either as base units or as derived as units.
1.4
Derived units of the SI are formed by combining base units, supplementary units and other derived units according to the algebraic relations linking the corresponding quantities.The symbols for derived units are obtained by means of the mathematical signs for multiplication, division and the use of exponents. Those derived SI units which have special names and symbols are listed in Table B3.
Note: The specific application of the derived units listed in Table B3 and other units common to international civil aviation operations is given in Table 204
1.5
The SI is a rationalized selection of units from the metric system which individually are not new. The great advantage of SI is that there is only one unit for each physical quantity – the metre for length, kilogram (instead of gram) for mass, second for time, etc. From these elemental or base units, units for all other mechanical quantities are derived.
These derived units are defined by simple relationships such as velocity equals rate of change of distance, acceleration equals rate of change of velocity, force is the product of mass and acceleration, work or energy is the product of force and distance, power is work done per unit time, etc. Some of these units have only generic names such as metre per second for velocity; others have special names such as newton (N) for force, joule (J) for work or energy, watt (W) for power. The SI units for force, energy and power are the same regardless of whether the process is mechanical, electrical, chemical or nuclear. A force of 1 newton applied for a distance of 1 metre can produce 1 joule of heat, which is identical with what 1 watt of electric power can produce in 1 second.
1.6
Corresponding to the advantages of SI, which result from the use of a unique unit for each physical quantity, are the advantages which result from the use of a unique and well defined set of symbols and abbreviations. Such symbols and abbreviations eliminate the confusion that can arise from current practices in different disciplines such as the use of ‘b’ for both the bar (a unit of pressure) and barn (a unit of area).
1.7
Another advantage of SI is its retention of the decimal relation between multiples and submultiples of the base units for each physical quantity. Prefixes are established for designating multiple and submultiple units from ‘exa’down to ‘atto’ for convenience in writing and speaking.
1.8
Another major advantage of SI is its coherence. Units might be chosen arbitrarily, but making an independent choice of a unit for each category of mutually comparable quantities would lead in general to the appearance of several additional numerical factors in the equations between the numerical values. It is possible, however, and in practice more convenient, to choose a system of units in such a way that the equations between numerical values, including the numerical factors, have exactly the same form as the corresponding equations between the quantities. A unit system defined in this way is called coherent with respect to the system of quantities and equations in question. Equations between units of a coherent unit system contain as numerical factors only the number 1. In a coherent system the product or quotient of any two units quantities is the unit of the resulting quantity. For example, in any coherent system, unit area results when unit length is multiplied by unit length, unit velocity when unit length is divided by unit time, and unit force when unit mass is multiplied by unit acceleration.
Note: Figure B1 illustrates the relationship of the units of the SI.
2. mass, force and weight
2.1
The principal departure of SI from the gravimetric system of metric engineering units is the use of explicitly distinct units from mass and force. In SI, the name kilogram is restricted to the unit of mass, and the kilogramforce (from which the suffix force was in practice often erroneously dropped) is not to be used. In its place the SI unit of force, the newton is used. Likewise, the newton rather than the kilogramforce is used to form derived units which include force, for example, pressure or stress (N/m^{2} = Pa), energy (N . m = J), and power (N . m/s = W).
2.2
Considerable confusion exists in the use of the term weight as a quantity to mean either force or mass. In common use, the term weight nearly always means mass; thus, when one speaks of a person's weight, the quantity referred to is mass. In science and technology, the term weight of a body has usually meant the force that, if applied to the body, would give it an acceleration equal to the local acceleration of free fall. The adjective ‘local’ in the phrase ‘local acceleration of free fall’ has usually meant a location on the surface of the earth; in this context the ‘local acceleration of free fall’ has the symbol g (sometimes referred to as ‘acceleration of gravity’) with observed values of g differing by over 0.5 per cent at various points on the earth's surface and decreasing as distance from the earth is increased. Thus, because weight is a force = mass x acceleration due to gravity, a person's weight is conditional on his location, but mass is not. A person with a mass of 70 kg might experience a force (weight) on earth of 686 newtons ( 155 lbf) and a force (weight) of only 113 newtons ( 22 lbf) on the moon. Because of the dual use of the term weight as a quantity, the term weight should be avoided in technical practice except under circumstances in which its meaning is completely clear. When the term is used, it is important to know whether mass or force is intended and to use S1 units properly by using kilograms for mass or newtons for force.
2.3
Gravity is involved in determining mass with a balance or scale. When a standard mass is used to balance the measured mass, the direct effect of gravity on the two masses is cancelled, but the indirect effect through the buoyancy of air or other fluid is generally not cancelled. In using a spring scale, mass is measured indirectly, since the instrument responds to the force of gravity. Such scales may be calibrated in mass units if the variation in acceleration of gravity and buoyancy corrections are not significant in their use.
3. Energy and torque
3.1
The vector product of force and moment arm is widely designated by the unit newton metre. This unit for bending moment or torque results in confusion with the unit for energy, which is also newton metre. If torque is expressed as newton metre per radian, the relationship to energy is clarified, since the product of torque and angular rotation is energy:
3.2
If vectors were shown, the distinction between energy and torque would be obvious, since the orientation of force and length is different in the two cases. It is important to recognize this difference in using torque and energy, and the joule should never be used for torque
4. Si prefixes
4.1. Selection of prefixes
4.1.1
In general the SI prefixes should be used to indicate orders of magnitude, thus eliminating nonsignificant digits and leading zeros in decimal fractions, and providing a convenient alternative to the powersoften notation preferred in computation. For example:
4.1.2
When expressing a quantity by a numerical value and a unit, prefixes should preferably be chosen so that the numerical value lies between 0.1 and 1 000. To minimize variety, it is recommended that prefixes representing powers of 1 000 be used. However, in the following cases, deviation from the above may be indicated:
a) in expressing area and volume, the prefixes hecto, deca, deci and centi may be required: for example, square hectometre, cubic centimetre;
b) in tables of values of the same quantity, or in a discussion of such values within a given context, it is generally preferable to use the same unit multiple throughout; and
c) for certain quantities in particular applications, one particular multiple is customarily used. For example, the hectopascal is used for altimeter settings and the millimetre is used for linear dimensions in mechanical engineering drawings even when the values lie outside the range 0.1 to 1 000.
4.2. Prefixes in compound units
It is recommended that only one prefix be used in forming a multiple of a compound unit. Normally the prefix should be attached to a unit in the numerator. One exception to this occurs when the kilogram is one of the units. For example: V/m, not mV/mm; MJ/kg, not kJ/g
4.3. Compound prefixes
Compound prefixes, formed by the juxtaposition of two or more SI prefixes, are not to be used. For example: 1 nm not Imµm; 1 pF not 1µµF
If values are required outside the range covered by the prefixes, they should be expressed using powers of ten applied to the base unit.
4.4. Powers of units
An exponent attached to a symbol containing a prefix indicates that the multiple or submultiple of the unit (the unit with its prefix) is raised to the power expressed by the exponent.
For example:
5. style and usage
5.1. rules for writing unit symbols
5.1.1
Unit symbols should be printed in Roman (upright) type regardless of the type style used in the surrounding text.
5.1.2
Unit symbols are unaltered in the plural.
5.1.3
Unit symbols are not followed by a period except when used at the end of a sentence.
5.1.4
Letter unit symbols are written in lower case (cd) unless the unit name has been derived from a proper name, in which case the first letter of the symbol is capitalized (W, Pa).
Prefix and unit symbols retain their prescribed form regardless of the surrounding typography.
5.1.5
In the complete expression for a quantity, a space should be left between the numerical value and the unit symbol. For example, write 35 mm not 35mm, and 2.37 lm, not 2.371m. When the quantity is used in an adjectival sense, a hyphen is often used, for example, 35mm film.
Exception: No space is left between the numerical value and the symbols for degree,minute and second of plane angle, and degree Celsius.
5.1.6
No space is used between the prefix and unit symbols.
5.1.7
Symbols not abbreviations should be used for units. For example, use ‘A’, not ‘amp’, for ampere.
5.2. Rules for writing unit names
5.2.1
Spelledout unit names are treated as common nouns in English. Thus, the first letter of a unit name is not capitalized except at the beginning of a sentence or in capitalized material such as a title, even though the unit name may be derived from a proper name and therefore be represented as a symbol by a capital letter (see 5.1.4). For example, normally write ‘new ton’ not ‘‘ even though the symbol is N.
5.2.2
Plurals are used when required by the rules of grammar and are normally formed regularly, for example, henries for the plural of henry. The following irregular plurals are recommended:
Singular Plural
lux lux
hertz hertz
siemens siemens
5.2.3
No space or hyphen is used between the prefix and the unit name.
5.3
Units formed by multiplication and division
5.3.1
With unit names:
Product, use a space (preferred) or hyphen: newton metre or newtonmetre
in the case of the watt hour the space may be omitted, thus: watthour.
Quotient, use the word per and not a solidus: metre per second not metre/second.
Powers, use the modifier squared or cubed placed after the unit name: metre per secondsquared.
In the case of area or volume, a modifier may be placed before the unit name: square millimetre, cubic metre.
This exception also applies to derived units using area or volume: watt per square metre.
Note: To avoid ambiguity in complicated expressions, symbols are preferred to words.
5.3.2
With unit symbols:
Product may be indicated in either of the following ways:
Note: When using for a prefix a symbol which coincides with the symbol for the unit, special care should be taken to avoid confusion.
The unit newton metre for torque should be written, for example,
Nm or to avoid confusion with mN, the millinewton.
An exception to this practice is made for computer printouts, automatic typewriter work, etc., where the dot half high is not possible, and a dot on the line may be used.
Quotient, use one of the following forms:
In no case should more than one solidus be used in the same expression unless parentheses are inserted to avoid ambiguity.
For example, write:
5.3.3
Symbols and unit names should not be mixed in the same expression. Write:
5.4. Numbers
5.4.1
The preferred decimal marker is a point on the line (period); however, the comma is also acceptable. When writing numbers less than one, a zero should be written before the decimal marker.
5.4.2
The comma is not to be used to separate digits. Instead, digits should be separated into groups of three, counting from the decimal point towards the left and the right, and using a small space to separate the groups. For example:
73 655 7 281 2.567 321 0.133 47
The space between groups should be approximately the width of the letter ‘ i ‘and the width of the space should be constant even if, as is often the case in printing, variablewidth spacingis used between the words.
5.4.3
The sign for multiplication of numbers is a cross ( x ) or a dot half high. However, if the dot half high is used as the multiplication sign, a point on the line must not be used as a decimal marker in the same expression.
5.4.4
Attachment of letters to a unit symbol as a means of giving information about the nature of the quantity under consideration is incorrect. Thus MWe for ‘megawatts electrical (power)’, Vac for ‘volts ac’ and kJt for ‘kilojoules thermal (energy)’ are not acceptable. For this reason, no attempt should be made to construct SI equivalents of the abbreviations ‘psia’ and ‘psig’, so often used to distinguish between absolute and gauge pressure. If the context leaves any doubt as to which is meant, the vord pressure must be qualified appropriately.
For example:
”. . . at a gauge pressure of 13 kPa’.
Or ?. . . at an absolute pressure of 13 kPa’.
Attachment 4
Coordinated universal time
1.Coordinated Universal Time (UTC) has now replaced Greenwich Mean Time (GMT) as the accepted international standard for clock time.
It is the basis for civil time in many States and is also the time used in the worldwide time signal broadcasts used in aviation. The use of UTC is recommended by such bodies as the General Conference on Weights and Measures (CGPM), the International Radio Consultative Committee (CCIR) and the World Administration Radio Conference (WARC).
2.The basis for all clock time is the time of apparent rotation of the sun. This is, however, avariable quantity which depends, among other things, on where it is measured on earth. A mean value of this time, based upon measurements in a number of places on the earth, is known as Universal Time. A different time scale, based upon the definition of the second, is known as International Atomic Time (TAI). A combination of these two scales results in Coordinated Universal Time. This consists of TAI adjusted as necessary by the use of leap seconds to obtain a close approximation (always within 0.5 seconds) of Universal Time.