Temperature compensation on approaches with Garmin GTN

Thanks @lionel !

I try to use this in the Garmin PC trainer, but cannot find the functionality. It is easy to find on the PFD side (G500TXi), but not on the GTN650.

→ is it on the GTN trainer at all?

lionel wrote:

when I may apply that temperature compensation
when I should apply that temperature compensation
when I must apply that temperature compensation

Incidentally I´ve been researching this recently. Here are some of my findings (skybrary):

- When the aerodrome temperature is 0°C or colder, the temperature error correction must be added to:
DH/DA or MDH/MDA and step-down fixes inside the final approach fix (FAF).
All low altitude approach procedure altitudes in mountainous regions (terrain of 3000 ft AMSL or higher)

- When pilot intend to apply corrections to the FAF crossing altitude, procedure turn or missed approach altitude, they must advise ATC of their intention and the correction to be applied.

- Pilots may refuse IFR assigned altitudes if altitmeter temperature error will reduce obstacle clearance below acceptable minima.

lionel wrote:

On an approach with true vertical guidance (be it ILS, LNAV/VNAV on an SBAS GPS or LPV), the GS is not subject to the temperature effect, so the only effect I see from using the GTN’s temperature compensation is that I will intercept the GS more close to its design point. My GS will not change through the temperature compensation, it is already correct.

It needs to be considered that in WARMER temperatures the altimeter reads lower than the actual aircraft and therefore we fly higher. GS doesn´t change with temperature and as we fly higher we intercept it earlier (at the intermediate segment). If we choose to follow the GS outside of final approach segment on a hot day we might bust the step down altitudes.
This FAA Information for Operators describes it.

Malibuflyer wrote:

Does anyone here in a SEP has an GTN installation where cold weather compensation is actually enabled?

I played with it today. I don’t need to fly into airports that need cold temperature compensation. Changing it on the GTN adjusted all the altitudes in the Instrument flight plan from the IAF to the Missed approach hold. Changing it on the G500TXi, adjusted the DA/MDA. The adjusted values had the snow flake next to them in the GTN flight plan view.

I just watched a video from the PilotWorkshops newsletter. I found it very interesting.
https://pilotworkshop.com/cold-wx-challenge
It’s exactly on this topic. What I learned from it is that the cold temperature error becomes so small at the minimums that it is negligible for the pilot (about 20-30ft) and included in the approach design. Only if the allowances are exceeded, the approach (in the official FAA TERPs) have a snowflake symbol with a temperature limit. The FAA has an airport list that shows approach segments that are affected below certain temperatures. I don’t know if that’s also how it works in European procedures.

It is enabled on my G500TXi that connects to my GTN750. The GTN750 has VNV endabled.

Can just talk about my POH/installation: Cold weather compensation is not enabled.

Does anyone here in a SEP has an GTN installation where cold weather compensation is actually enabled?

The temperature adjustment is just for one of the three segment altitudes, the intermediate legs, the MDA/DA, and the missed approach. It does not affect the GS calculated. If one has a PFD such as a G500TXi, the correction affects the minimum altitudes of the relevant segment and should also be reflected in the GTN plan altitude. Of the three altitude segments, the intermediate altitude only has a ROC of 500 feet and because of the height above the airport is often the most significant. The hold has a ROC of 1000 feet and the DA is 200 to 250 feet, but because of the closeness in distance and altitude to the airport it will often not be the most hazardous.

While the most recent GTN units are approved for baro-VNAV descents, they are not (yet?) capable of baro-VNAV approaches.
In fact, I have not seen or heard of any piston GA airplane capable of baro-VNAV approaches.
Garmin installations approved for LNAV/VNAV approaches are to my knowledge all using SBAS, not baro-VNAV, for the vertical guidance.
Using GTN installations, or any other equipment in small GA aircraft, I think you can disregard the notes you have highlighted on the approach plates.