. the true altitude is higher than the indicated altitude,
.
Extreme low temperature :
. the true altitude is lower than the indicated altitude, thus creating a lower than anticipated terrain separation and a potential obstacle-clearance hazard.
For example, when performing an ILS approach with a published 2000 ft minimum glide-slope interception-altitude and a – 40 ° C OAT, the minimum glide-slope interception altitude should be increased by 480 ft.
True altitude
Standard OAT
Figure 4
Effect of OAT on True Altitude
The ICAO PANS-OPS, Volume I, provides altitude corrections to be added to the published minimum safe altitudes (heights).
The temperature correction (i.e., correction to be added to the indicated altitude) depends on the aerodrome surface temperature and on the desired true altitude (height) above the elevation of the altimeter-setting source.
Altimeter Setting – Use of Radio Altimeter
Page 4
Flying into a low temperature area has the same effect as flying into a low-pressure area; the aircraft is lower than the altimeter indicates.
These effects are summarized and illustrated in Table 2, featuring a well-known aviation golden rule:
Atmospheric Pressure High Low Look out below
OAT Warm Cold
Table 2
The pilot is responsible for performing this correction, except when under radar control in a radar vectoring area; in this case, the controller normally is responsible for terrain clearance, including accounting for the cold temperature correction.
Nevertheless, the operator and/or pilot should confirm this responsibility with the air traffic services of the country of operation.
The temperature correction on altitude affects the following published altitudes, which therefore should be increased under low OAT operation:
.
MEA, MSA;
.
Transition routes altitude;
.
Procedure turn altitude (as applicable);
.
FAF altitude;
.
Step-down altitude(s) and MDA(H) during a non-precision (non-ILS) approach;
.
OM crossing altitude during an ILS approach; and,
.
Waypoint crossing altitudes during a GPS approach flown with vertical navigation.
ICAO PANS-OPS does not provide altitude corrections for extreme high temperatures; the temperature effect on the true altitude should not be ignored when planning for a constant-angle non-precision approach (i.e., to maintain the required flight path angle and/or vertical speed).
Summary of key points
Altimeter-setting errors result in a lack of vertical situational awareness; the following key points should be emphasized to minimize altimeter-setting errors and to optimize the use of the barometric-altimeter bug and radio-altimeter DH:
.
Awareness of altimeter setting changes due to prevailing weather conditions (extreme cold or warm fronts, steep frontal surfaces, semi-permanent or seasonal low pressure areas);
.
Awareness of the altimeter setting unit in use at the destination airport;
.
Awareness of the anticipated altimeter setting, using two independent sources for cross-check (e.g., METAR and ATIS messages);
.
Effective PF/PNF crosscheck and backup;
.
Adherence to SOPs for:
. reset of barometric-altimeters in climb and descent, for example:
°. in climb : at the transition altitude; and,
°. in descent : when cleared to an altitude;
. use of standby-altimeter to crosscheck main altimeters;
. altitude callouts;
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