Aircraft Operations. Volume II - Construction of Visual and Instrument ...

Chapter 3
UNITS OF MEASUREMENT
3.1 Units of measurement are expressed to conform with Annex 5. The conversion of the non-SI value to the SI
value has been accomplished by the use of the appropriate conversion factor listed in Annex 5 and by rounding
normally to the nearest integer in SI-units.
3.2 Where a critical parameter is involved, rounding is done to obtain an accuracy of the same order. Where a
parameter directly affects the flight crew in its control of the aircraft, rounding is normally to the nearest multiple of
five. In addition, slope gradients are expressed in percentages. For slope gradients in other units, see Instrument Flight
Procedures Construction Manual (Doc 9368).
3.3 The units of measurement for precision approach are stated in metres only. If these basic dimensions are
converted to feet and rounded according to the normal ICAO practice before they are scaled to obtain OAS boundaries
and heights, then significant anomalies will arise.
3.4 To prevent such anomalies, there are two alternatives. Either the boundaries and heights must be calculated in
metres, converted to feet (× 3.2808) and then rounded up/down as necessary, or all tabulated dimensions must be
multiplied by 3.2808, after which all subsequent calculations are in feet.
3.5 Calculations of area dimensions not related to ILS or MLS should be rounded up to 0.01 km (0.01 NM).
Dimensions of areas related to ILS or MLS should be calculated and then rounded up to 1.0 m (1.0 ft).
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I-1-3-1 23/11/06