Menu Sign In Contact FAQ
Banner
Welcome to our forums

Temperature compensation on approaches with Garmin GTN

10 Posts

A Garmin GTN can apply temperature compensation to the “to the FAF” segment platform altitude, based on the temperature at the destination. I assume it assumes constant ISA deviation. What I’m not clear, is:

  1. when I may apply that temperature compensation
  2. when I should apply that temperature compensation
  3. when I must apply that temperature compensation

My guess is that I never must, since the approaches I fly are legal to fly without temperature compensation, and that when ATC clears me to the non-compensated altitude, going to the compensated altitude would be a deviation from the clearance, so I may not do it.

Mostly, I don’t really see the point, even if I may use it:

  1. Again, the approach is legal without temperature compensation, so its design must have taken the temperature effect into account, so safe without temperature compensation.
  2. 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.
  3. On an approach without vertical guidance (LNAV, NDB, …), in practice I’m still going to use my every-nmi check altitudes printed on my approach plate, kindly computed by Jeppesen for me. Unless I manually apply temperature compensation to those, the only effect is that I will be at the correct true altitude at the FAF, but then I will quickly dip to the “too low” profile using my plate check altitudes. One may argue one really should apply temperature compensation to the Jepp table, but I doubt people routinely do that (with the exception of Canadian / Scandinavian / … airlines).

The only scenario where I see it being useful, is an approach with true vertical guidance, in very cold temperature, below the procedure’s minimum design temperature. There, it would keep me from flying into terrain / an obstacle during the “intercept the glideslope from below flying the final approach track to the FAF” segment, as well as into the missed approach procedure, insofar as it contains climbs to altitude (not FLs). But, then, to be safe, I would still need to (manually, in my head, the GTN will not do it for me) apply temperature compensation to the DH…

I also see it as being useful in an airplane that uses Baro-VNAV on an LNAV/VNAV approach. There, it will actually correct the GS, and will make the approach possible/legal in temperatures colder than what the approach plate forbids “for uncompensated Baro-VNAV systems”.

On that subject, I wonder why some approaches are purely banned for all Baro-VNAV below a certain temperature, while others are banned only for uncompensated Baro-VNAV? Compare, for LFJL, RNP04 and RNP22.


ELLX

I don’t have answers to a lot of your questions, but this is also specific to the installation. This is the POH supplement for a GTN750:

I don’t think the minimums would be corrected only in your head. I would enter a higher minimum in the system in cold temperatures.

On the ATC question, you should tell them if you are applying a temperature correction on a segment other than the final approach.

This is what the FAA has to say about the topic: https://www.faa.gov/air_traffic/publications/atpubs/ntap_feb_18/part4_GEN17000.html

The Jepessen manual produces a low temperature correction table. It should be used whenever the destination airport reports 0 o C or below.

The temperature corrected minima is the new legal minima.

Using the temperature correction feature in warm conditions (ISA plus) creates a perverse below minima alert.

Enstone (EGTN), Oxford (EGTK)

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.

Last Edited by huv at 14 Jan 09:26
huv
EKRK, Denmark

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.

KUZA

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?

Germany

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

KUZA

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.

Last Edited by ArcticChiller at 17 Jan 16:08

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.

KUZA

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.

Last Edited by Destinatus at 18 Jan 12:26
Prague
10 Posts
Sign in to add your message

Back to Top