Altimeter cold wx corrections vs A320 FINAL APP mode
 

Gents,

ICAO Doc 8168, Vol 1, Part II, Section 4, Chapter 1, says:
...
1.2.2 Atmospheric effects
1.2.2.1 Atmospheric errors associated with non-standard temperatures are considered in the design of the approach obstacle clearance surface. When temperatures are lower than standard, the aircraft’s true altitude will be
lower than its barometric indicated altitudes.
1.2.2.2 Most existing VNAV systems do not correct for non-standard temperatures. At temperatures below standard, these errors can be significant and increase in magnitude as altitude above the station increases. The gradient of the approach obstacle clearance surface is reduced as a function of the minimum temperature promulgated for the procedure.
...
1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s);
b) the DA/H; and
c) subsequent missed approach altitudes/heights.
Note.— The final approach path vertical path angle (VPA) is safeguarded against the effects of low temperature by the design of the procedure.
...
1.4.2 Temperatures below the promulgated minimum Baro-VNAV procedures are not permitted when the aerodrome temperature is below the promulgated minimum aerodrome temperature for the procedure, unless the flight management system (FMS) is equipped with approved cold temperature compensation for the final approach. In this case, the minimum temperature can be disregarded provided it is within the minimum certificated temperature limits for the equipment. Below this temperature, and for aircraft that do not have FMSs equipped with approved cold temperature compensation for the final approach, an LNAV procedure may still be used
...

Question 1: lets suppose I am planning to fly a RNAV (GNSS) with VNAV/LNAV minima to an airport where the minimum temp published on the approach chart is -25ºC. During approach preparation I realize the current OAT at the airport is -10ºC. Do I need to correct all published minimum altitudes/heights (the initial and intermediate segment(s), DA/H and MAP), even tough the current temp is still above -25ºC? :confused:
Note: the A320 fleet FMSs of my current company are still not equipped with approved cold temperature compensation systems.

FCOM PRO-NOR-18C says:
‐ The FMS vertical profile does not take into account the effect of low OAT. Therefore, vertical managed guidance:
• Must not be used when the actual OAT is below the minimum temperature indicated on the approach chart or defined by the Operator, or
• May not be used when temperature corrections are required (FINAL APP mode may not engage).

Question 2: FCOM says we must not use FINAL APP mode (and HDG-V/S) when the actual OAT is below the minimum temperature indicated on the approach chart. It also says we may not use the FINAL APP mode when temp corrections are required.
In this last case we still may use the FINAL APP mode but we should consider the possibility of FINAL APP mode not engagement. So, is it valid to say it is recommended to fly a RNAV approach with LNAV minima instead (using NAV/FPA) when temp corrections are required? :confused:

My understanding is

Q1. No, no corrections need to be made as the approach is valid down to -25.

Q2. Below -25 then the minima is not valid and then corrections must be made in which case VNAV cannot be used. Make all the relevant temperature corrections including LNAV minimum and use NAV/FPA as you say.

MDA and DA must be corrected for cold weather temp,boeing includes temp from 0 downwards.
For all the rest,Mr Denti will come soon and answer all accurately:8

For Q1:

The protected area in a LNAV/VNAV approach is valid from and including the FAF down to 50ft threshold height, but has nothing to do with the DH/MDA per say, other than being a bit lower than than a LNAV approach only. DH/MDA should always be corrected just like on an ILS

As the approach is designed for a temperature of -25°C including the DA for that temperature (any higher temperature simply increases the safety buffer) i would fly the approach in managed/managed with the published uncorrected DA.

If the temperature is lower than -25°C it has to be flown either in FPA/TRK or FPA/NAV with the temperature corrected LNAV mininum.

That is in line with our OM-A guidance:
@de facto: the correction-tables in both airbus and boeing documentations are exactly the same, at least on our fleets. Well, the minor difference is that the boeing documentation has the tables additionally in meters and airbus has a few more heights up to 5000ft whereas the boeing table ends at 3000ft.

Question 1 answer only:

The minimum temperature advise you of the temperature at which sufficient ground clearance for the procedure is maintained. Hence different airports use different temperatures, due obstacles in the approach path segment.

Correcting the FMC altitudes for cold weather depends on the approach type being flown:

• LNAV approaches do require cold weather corrections to be made;
• LNAV/VNAV approaches do not require cold weather correction from the FAF inbound (safeguarded by minimum use temperature), but do require cold weather corrections to be made prior to the final approach segment; this is as the final approach segment in LNAV/VNAV is a managed approach angle by procedure design.
• APV approaches also do not require any cold weather corrections to be made from FAF inbound due procedure design.

The DA however should always be set with the applicable cold weather correction applied. The DA is not connected to the FMS in this sense and indicates the aircraft altitude sensed in cold weather thus will under-read height above ground. Not applying the cold weather correction will get you lower than allowed at minimums, not setting it thus would allow a pilot to descend below procedure minima, a wrong operational technique to get below the reported cloud base.