Flight Operations Briefing Notes Supplementary Techniques Altimeter Setting - Use of Radio Altimeter(4)

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 MDA(H) or DA(H)
Note 2
 
 
RNP RNAV Approach
 DA(H)
 
 
ILS CAT I
 DA(H)
 
 
ILS CAT II
ILS CAT III with DH
 
 DH
 
ILS CAT III with no DH
 
 Note 1
 

 
Table 1
Use Barometric Altimeter MDA(H)/DA(H) and Radio Altimeter DH
Note 1
DH set to “- 5 ft” for A300/A310/A300-600 families, “NO” entered on PERF APPR page for other Airbus aircraft families.
Note 2
DA(H), for constant-angle / constant-slope non-precision approaches, as allowed by operational authorities.
Radio-altimeter Callouts

Radio-altimeter callouts can be either:
Announced (verbalized) by the PNF or the Flight Engineer; or,

Automatically generated by a synthesized voice (e.g., smart callouts).

Callouts should be tailored to the airline’ operating policy and to the type of approach. 
To enhance the flight crew’s terrain awareness, a callout “Radio altimeter alive”,  should be announced by the first crewmember observing the radio altimeter activation at 2500 ft height AGL.
The radio altimeter reading should then be included in the instrument scanning for  the remainder of the approach.
Radio altimeter readings (i.e., feet’s AGL) below the Minimum Obstacle Clearance (MOC) values listed below, should alert the flight crew (sources – ICAO-PANS-OPS and  US TERPS):
Initial approach segment (i.e., from IAF to IF) : 

1000 ft;

Intermediate approach segment (i.e., from IF to FAF) : 

500 ft; and,

Final approach segment (i.e., after FAF, for non-precision approaches with a defined FAF, until visual references or reaching MAP) : 

250 ft.

Unless the airport features high close-in terrain, the radio-altimeter reading (i.e., height AGL) should reasonably agree with the height above airfield elevation (i.e., height AFE), obtained by :
Direct reading of the altimeter, if using QFE; or,

By subtracting the airport elevation from the altitude reading, if using QNH.

Low OAT Operation

In a standard atmosphere, the indicated altitude (with altimeter set to QNH) provides the true altitude above the Mean Sea Level (MSL) and, therefore, a reliable indication of terrain clearance.
Whenever, the temperature deviates significantly from the standard temperature,  the indicated altitude correspondingly deviates from the true altitude (Figure 4) :
Extreme high temperature : 

the true altitude is higher than the indicated altitude,

Extreme low temperature :

the true altitude is lower than the indicated altitude (i.e., 1520 ft true altitude for a 2000 ft indicated altitude, with a – 40° C OAT), thus resulting in  a lower-than-anticipated terrain separation and a potential obstacle-clearance hazard.
　
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