Pilot's Operating Handbook
Section 7
Description
Edition 0 - March 05, 2019
Rev. 2
Page 7.15.11
Temperature Compensation
If SBAS is unavailable or disabled, the GIFD will provide automatic temperature
compensated glidepath vertical guidance on approaches that have LNAV/VNAV
minima published, or on some approaches that are not authorized for SBAS.
No pilot action is required to receive the temperature compensated glide-path when
SBAS is not available or allowed.
Final Approach Segment (FAS)
Altimeter systems assume an ISA temperature model.
When actual atmosphere deviates from the ISA model it results in altitude errors.
For example, performing a Baro-VNAV during a hot day would result in guidance
relative to a glide path angle steeper than the published glide path angle. On the
contrary, during a cold day, a Baro-VNAV would be based on guidance relative to a
glide path smoother than the published glide path angle.
The approach plates indicate a temperature range for which the approach has been
designed.
Within this temperature range the LNAV/VNAV can be used with uncompensated
Baro-VNAV systems.
Outside of this temperature range, LNAV/VNAV minimums shall not be used with
uncompensated Baro-VNAV systems.
The GARMIN Approach Baro-VNAV system is automatically temperature
compensated to produce a glide-path position in space such that Baro-VNAV
approaches are always flown at the published glide path angle when the actual
temperature deviates from the ISA model. This produces results similar to ILS
glideslopes and LPV glide-paths that remain in the same position in space without
respect to temperature.
To produce the correct geometric glide path angle on the final approach segment,
temperature compensation is applied to the barometric altitude and used to determine
the displayed vertical deviation.
However, the altimeter continues to display uncompensated barometric altitude.
The temperature compensation required depends on the temperature profile over the
altitude range between the point at which the barometric setting is measured
(presumed to be the approach airport) and the present altitude of the aircraft.
This temperature profile is estimated by using the air data system static air
temperature (SAT) and applying the standard temperature lapse rate to determine the
temperature over the rest of the range.
PIM - DO NOT USE FOR FLIGHT OPERATIONS
