Cold temp correction problem

[photofly]

Is that difference because of the geoid, or because of a non-ideal atmosphere?

[iflyforpie]

My understanding is that it’s non-ideal gravity that makes the actual geoid different than the reference geoid. In reality most of the error would be atmospheric.. but on the rare day the atmosphere is ISA, your GPS and baro altitudes probably still won’t match.

The geoid is mathematically defined as the earth bulging out a certain percentage for each latitude between the poles, when in reality—even if it was all water and averaged out for tides, it’s more of a deformed oblate spheroid with bulges and depressions above and below the reference geoid.

And this is because earths gravity is not even. The absence of gravity over the Indian Ocean is what I was referring to. It’s the curse of many satellites that operate over it (pretty much all that aren’t geostationary).

https://eos.org/science-updates/seismol ... ssing-mass

[photofly]

I looked online and there are some charts of MSL and WGS-84 and there does indeed appear to be about a 100m difference over the Indian Ocean.

[2R]

Better be careful doing an approach “ down to minimums” @ -40C in the Indian Ocean area , especially if you hear someone pirep it is fluctuating below mins , but it was good when we got in

[AuxBatOn]

Go into rapidly rising terrain and regardless of datum difference, there will be significant errors just because of the way elevation data is stored into your GPS.

[valleyboy]

Seems that they consider the information on terrain stored in GPS data bases good enough for TAWS but I get what you are saying but under WAAS/LPV will automatically give you the missed approach at minimums based on height above the ground. You can temperature correct you minimums which should give you the barrow setting for the missed approach which if done correctly should match the minimums attained in the box. One would be stupid not to set a reference barometric altitude because first rule is more information is always better.

This is such a simple operational issue and true to av canada it has taken on a life of its own. The term pepper and fly shit come to mind. One more caveat don't forget to advise ATC if you are missing and going to a corrected altitude.

[iflyforpie]

This is such a simple operational issue and true to av canada it has taken on a life of its own. The term pepper and fly shit come to mind. One more caveat don't forget to advise ATC if you are missing and going to a corrected altitude.

You were the first person to go off on the LPV tangent. <Ring ring ring> “hello kettle?”

The airplane I fly doesn’t tell you when you are at the MAP. You either have to look at the small SEQ INHIBIT annunciation on the PFD or on the legs or progress page on the FMS.

Thus.. as per the aircraft manual and our SOPs and the CARS, you conduct a missed approach at a corrected baro altitude.

Yes.. WAAS is correcting position and altitude information in real-time based on ground receivers in known geographic positions which reduce the position delta to near zero. But you still can’t use it for determining minimums and you still have to correct your altitudes. A prudent person would also correct their glide path check altitude too.

Also.. LPV GP is only when the IF is a from waypoint and you’ve armed the approach, or if you are 2 nm from the FAF at less than 45 degrees. All other procedure altitudes including area, sector, T segments, and missed approach and any VNAV path information are all 100% based on baro. Please correct them.

[AuxBatOn]

One would be stupid not to set a reference barometric altitude because first rule is more information is always better.

Assuming it is correct information perhaps. I would rather have no information than bad information.

[valleyboy]

I'm not sure if it were a tangent but I guess people are missing the point as usual here. So enough said but I do stand by my thoughts that only on av Canada can such a simple thing get blown up to the point of ridiculous. Like I said pepper and fly shit - lmfaoooooo

[doiwannabeapilot]

Should my logbook be in decimals or actual minutes ?

Also, can I use my Apple watch to start the timer for actual IMC or do i have to use TC approved chronometer ?

TIA

[Rockie]

One would be stupid not to set a reference barometric altitude because first rule is more information is always better.

Assuming it is correct information perhaps. I would rather have no information than bad information.

Why would anybody base minimums decisions on anything but what is published? Published minimums are not “more” information, they are the ONLY minimums unless for whatever reason you choose something higher.

You’ve said a few things so far valleyboy that are flat out wrong, show a serious lack of understanding for an instrument rated pilot and are unsafe. Time to get back in the books.

[ChiefTraining]

My 2 cents...get the APP on your phone or iPad. No one does it this way anymore. Unless you're just practicing.

[Eric Janson]

One of the best tools is the GPS altitude display . Hard to find on some older gps models but well worth the effort for the added safety .

Ever wondered why GPS isn't used instead of a pressure altimeter?

The GPS altitudes are based on a model using a sphere (WGS 84).

The earth isn't a sphere so there can be a big difference between the pressure altimeter and GPS.

Do not rely on GPS altitude.

For the cold weather corrections:-

All the RNAV/GNSS approaches I've seen have a minimum published temperature for the procedure. I've been taught that you only make cold weather corrections when the temperature goes below the minimum published temperature. On the airbus we would then not be able to use FINAL APP but would need to fly NAV/FPA.

[Rockie]

All the RNAV/GNSS approaches I've seen have a minimum published temperature for the procedure. I've been taught that you only make cold weather corrections when the temperature goes below the minimum published temperature.

TLIM, or temperature limit only applies to LNAV/VNAV and RNP AR approaches because of the way they are designed. LNAV/VNAV and RNP AR can be flown below that temperature but only if your aircraft is equipped with, and you use onboard temperature compensation in the FMS. If you do not have it you cannot do LNAV/VNAV or RNP AR and are restricted to LPV where the TLIM does not apply, or LNAV with temp corrections manually added to the FAF in order to maintain obstacle clearance in the intermediate segment and to maintain a normal glidepath.

Temperature corrections are required to all DA and MDA regardless of the type of approach because they are a barometric minimum.

[Eric Janson]

Thanks for the information Rockie - my company flies LNAV/VNAV only. We are not certified for LPV.

[digits_]

One of the best tools is the GPS altitude display . Hard to find on some older gps models but well worth the effort for the added safety .

Ever wondered why GPS isn't used instead of a pressure altimeter?

The GPS altitudes are based on a model using a sphere (WGS 84).

The earth isn't a sphere so there can be a big difference between the pressure altimeter and GPS.

Do not rely on GPS altitude.

Not true. https://support.virtual-surveyor.com/en ... -is-wgs84-

WGS 84 is based on an ellipsoid, not a sphere. Assuming the GPS is linked to a charted application, it has the capability to know exactly what the error is, and correct for it. I'm sure not all GPS systems do so, but Garmin seems to do a fine job. Note that you don't need to know the error on each square meter of earth coverage, you just need to know the difference between the WGS 84 ellipsoid and mean sea level, which is much less complicated.
If there is a big difference between the pressure altimeter and the GPS, then the GPS is more likely to be correct, as the pressure altimeter shows exactly that: pressure differences, depending on temperature. The GPS will be more accurate. If you don't have a chart of errors, then you'll have a problem. But would your GPS even tell you the altitude in that case?
The garmin website describes something similar, and does not mention any WGS 84 ellipsoid errors: https://support.garmin.com/en-US/?faq=v ... 6eC2vGMpz9

There can be a significant difference during the approach phase as well, and that is the whole purpose of the cold weather correction.

But yes, don't use the GPS altitude as a means to determine approach crossing altitudes or minima.

[stephen6ix]

I’m all for healthy dialogue but damn didn’t expect all this lol but anyway I passed my inrat yesterday and I got the cold weather temp question correct because it didn’t show up on the back on my sheet

[rxl]

It’s amazing how threads on here take on a life of their own.
Here’s my last 2 cents for this discussion ... who cares about GPS altitude? The science behind it is interesting for sure, but for civil aviation, THE defacto altitude reference (for now anyway) is the properly calibrated and correctly set good old fashioned pressure altimeter.
It has worked pretty darn good for 100 plus years.

stephen6ix - Congratulations on passing the exam!
All the best with your training.

[stephen6ix]

It’s amazing how threads on here take on a life of their own.
Here’s my last 2 cents for this discussion ... who cares about GPS altitude? The science behind it is interesting for sure, but for civil aviation, THE defacto altitude reference (for now anyway) is the properly calibrated and correctly set good old fashioned pressure altimeter.
It has worked pretty darn good for 100 plus years.

stephen6ix - Congratulations on passing the exam!
All the best with your training.

Thanks so much!!