Inside the FMS
Inside the FMS
RNAV instrument approach procedures use barometric altitudes and barometric vertical navigation (Baro VNAV) in the approach design. As a result, true altitude can differ greatly when temperatures become extremely hotter or colder than standard. In cold weather the uncompensated indicated altitude reads higher than the true altitude and conversely true altitude is higher than indicated during hot temperatures. Approaches that rely on Baro VNAV for guidance at extremely cold temperatures could diminish the required obstacle clearance (ROC) buffers built into the procedure design.
When Do I Need to Use Temp Comp?
For several years now the U.S. and other countries have required pilots to apply temperature corrections for the approach segment altitudes when temperatures are reported below the published limit and the ROC will be exceeded. Affected approaches are identified by a chart note on Jeppesen approach charts and a snowflake symbol on FAA charts (Figure 1).
How It works
Honeywell NZ6.1, EPIC, APEX/ACE, and NG FMS all have a temperature compensation (temp comp) capability. This function provides temperature compensated VNAV altitudes during operations in extremely hot or cold temperatures. Temp comp raises the waypoint altitude constraint values in cold weather and lowers the waypoint altitude constraint values in hot weather. The FMS adjusts the altitudes for all the waypoints in the approach and missed approach segments of the flight plan, ensuring the aircraft is at the published true altitudes.
Accessing Temp Comp
The temp comp configuration page (Figure 2) allows the crew to configure the FMS for:
· OFF (assumes standard day temperature)
· COLD applies temperature compensation from 0 to -50 degrees DISA (deviation ISA)
· HOT/COLD applies temperature compensation from -50 to +70 degrees DISA
Once the TEMP COMP CONFIG is set, the landing temperature is input into the LANDING INIT page, and the temp comp button is selected (Figure 2).
The uncompensated procedure altitudes are displayed along with the compensated altitudes in inverse video. Once the waypoints are reviewed the temperature compensation can be applied to the active flight plan as shown in Figure 3.
A separate input is necessary to adjust the DA / MDA, since it is not automatically compensated for temperature. The published altitude for the LNAV or LNAV / VNAV minima is input into the last page of the temp comp menu and the compensated altitude will be displayed and applied to the flight plan as shown in figure 4. It should be noted that approach procedures using LPV do not need to be temperature compensated, since LPV uses GPS altitudes with SBAS.
Note: As a reminder, crews must coordinate with ATC prior to using compensated altitudes. Not all countries authorize hot temperature compensation, so operators are encouraged to check local AIP’s for restrictions or specific procedures.
A list of Cold Temperature Restricted Airports along with guidance for performing temperature compensation manually can also be found on the FAA website if needed for reference or planning.
Derek Fiedler supports RNP, Datalink, and Forge Flight Services for Honeywell Flight Technical Services. He can be reached via email at Derek.Fiedler@Honeywell.com.