Cold temp correction problem

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Gannet167
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Re: Cold temp correction problem

Post by Gannet167 »

LPV approaches use GPS derived 3D position in space. They don't rely on barometry and therefore aren't susceptible to temperature error. Although GPS may have position error, including GPS altitude, the extra equipment required for an LPV, (WAAS, SBAS etc), provides the additional level of accuracy and continuity required. When you're looking at your GPS altitude at FL350 and it shows something different - it's showing absolute altitude. If the altimeter setting at 350 isn't 29.92, then of course it'll be different than GPS altitude. In addition, since whatever local altimeter setting source you're using 35,000 feet beneath you has some inherent error due to the altitude above the point of measurement, they'll always read differently. On the ground, with an accurate altimeter setting on the baro, GPS altitude is usually very accurate even without a WAAS.
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AuxBatOn
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Re: Cold temp correction problem

Post by AuxBatOn »

GPS and barometric altitude use a different datum. If they show the same, that’s a coincidence that at that position, both models intersect.
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Rockie
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Re: Cold temp correction problem

Post by Rockie »

AuxBatOn wrote: Thu Aug 13, 2020 11:34 am GPS and barometric altitude use a different datum. If they show the same, that’s a coincidence that at that position, both models intersect.
Explanation here:

https://www.esri.com/news/arcuser/0703/geoid1of3.html
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stephen6ix
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Re: Cold temp correction problem

Post by stephen6ix »

valleyboy wrote: Wed Aug 12, 2020 6:42 am I assume this is for exam purposes because operationally you will need to do most of it in your head. Remember that in an approach all altitudes are minimum altitudes and there is nothing about any altitude above that. So if you refer to the charts just add fat numbers for the procedure and quadrant altitudes and round up for the MDA or DH - Examples it's -40 and correction gives you a correction for procedure turn corrected to 2700 feet -- just use 3000 ft and if the MDA says 230 set 250 you know it's all about safety and not churning out numbers to 5 decimal points. You need to do this is seconds operationally with all the distractions going on as well. Oh ya -- no correction on LPV MDA

I'm impressed that you got the information you were looking for and this thread was as all should be and the posters fielded good information. This site needs more of this-- cheers
Yes this is for exam purposes and yes agreed there should be more of this kind of help
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iflyforpie
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Re: Cold temp correction problem

Post by iflyforpie »

Yes you correct an LPV DH. Even though the slope is based on GPS WAAS and unaffected by temperature unlike baro VNAV, the DH is baro. You HAVE to correct it for temperatures. If it wasn’t baro.. you wouldn’t need an altimeter setting, but you do.

And yes, don’t use the GPS altimeter. It’s based on the WGS 84 reference geoid which only roughly corresponds with sea level. Fly over the Indian Ocean and your ASL and GPS altitude will be wildly different.
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photofly
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Re: Cold temp correction problem

Post by photofly »

Is that difference because of the geoid, or because of a non-ideal atmosphere?
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iflyforpie
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Re: Cold temp correction problem

Post by 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
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photofly
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Re: Cold temp correction problem

Post by 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.
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2R
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Re: Cold temp correction problem

Post by 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 :)
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AuxBatOn
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Re: Cold temp correction problem

Post by 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.
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valleyboy
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Re: Cold temp correction problem

Post by 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.
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iflyforpie
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Re: Cold temp correction problem

Post by iflyforpie »

valleyboy wrote: Mon Aug 17, 2020 6:26 am 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.
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AuxBatOn
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Re: Cold temp correction problem

Post by AuxBatOn »

valleyboy wrote: Mon Aug 17, 2020 6:26 am 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.
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valleyboy
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Re: Cold temp correction problem

Post by 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
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doiwannabeapilot
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Re: Cold temp correction problem

Post by 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
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Rockie
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Re: Cold temp correction problem

Post by Rockie »

AuxBatOn wrote: Mon Aug 17, 2020 9:16 am
valleyboy wrote: Mon Aug 17, 2020 6:26 am 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.
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ChiefTraining
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Re: Cold temp correction problem

Post by ChiefTraining »

My 2 cents...get the APP on your phone or iPad. No one does it this way anymore. Unless you're just practicing.
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Eric Janson
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Re: Cold temp correction problem

Post by Eric Janson »

2R wrote: Thu Aug 13, 2020 7:08 am 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.
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Rockie
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Re: Cold temp correction problem

Post by Rockie »

Eric Janson wrote: Thu Aug 20, 2020 4:40 am 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.
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Eric Janson
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Re: Cold temp correction problem

Post by Eric Janson »

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