Aircraft Operations

I-4-5-4
Procedures — Aircraft Operations — Volume I
5.4.3.3 It is assumed that the aircraft altimeter reading on crossing the fix is correlated with the published altitude,
allowing for altitude error and altimeter tolerances. See Part III.
Note.— Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any deviation from ISA
will therefore result in an erroneous reading on the altimeter. If the temperature is higher than ISA, then the true
altitude will be higher than the figure indicated by the altimeter. Similarly, the true altitude will be lower when the
temperature is lower than ISA. The altimeter error may be significant in extremely cold temperatures.
5.4.3.4 In the event of loss of glide path/MLS elevation angle guidance during the approach, the procedure
becomes a non-precision approach. The OCA/H and associated procedure published for the glide path/MLS elevation
angle inoperative case will then apply.
5.5 DETERMINATION OF DECISION ALTITUDE (DA) OR DECISION HEIGHT (DH)
5.5.1 In addition to the physical characteristics of the ILS/MLS/GBAS installation, the procedures specialist
considers obstacles both in the approach and in the missed approach areas in the calculation of the OCA/H for a
procedure. The calculated OCA/H is the height of the highest approach obstacle or equivalent missed approach
obstacle, plus an aircraft category related allowance (see 5.5.8).
5.5.2 In assessing these obstacles, the operational variables of the aircraft category, approach coupling, category
of operation and missed approach climb performance are considered. The OCA/H values, as appropriate, are
promulgated on the instrument approach chart for those categories of aircraft for which the procedure is designed.
OCA/H values are based on the standard conditions (among others) listed in the sub-paragraphs that follow.
5.5.2.1 Aircraft dimensions: See Table I-4-5-1.
5.5.2.2 ILS:
a) Category I flown with pressure altimeter;
b) Category II flown with radio altimeter and flight director;
c) missed approach climb gradient is 2.5 per cent; and
d) glide path angle:
— minimum: 2.5°
— optimum: 3.0°
— maximum: 3.5° (3° for Category II/III operations).
5.5.2.3 MLS:
a) Category I flown with pressure altimeter;
b) Category II flown autocoupled/flight director, with radio altimeter;
23/11/06