Flying in cold, some observations and questions on Cold Temp restricted airports

WannFly

Final Approach
Joined
Nov 28, 2016
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KLZU
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Display name:
Priyo
background of my flights yesterday:

  • surface temp at take off -23F, DA -5300
  • OAT at 5500 MSL (roughly 4500 AGL) -14C while going, OAT -16C at 3900 MSL (roughly 2900 AGL)
  • surface temp at landing -6F, DA -3600
Observations and confusions:
Before i took off yesterday, i read up on Cold temp restricted airports and as i understand its primarily for Instrument approaches with MDA, DA and other alphabet soups, however, i also read somewhere about computing that en-route and the logic is... your altimeter is showing you re at 5000 , in reality you are at 4600 -- that could lead to traffic separation problem and you should tell ATC so they know.

what i observed -- my altimeter was showing i was at 5500, my GPS Alt was showing i am at 4800 (usually its 200 ft off, but it was 700 ft off yesterday) and my Transponder was showing 5100. this is the first time i have seen such a discrepancy. usually the transponder is very close to the altimeter. it confused the heck out of me and i added 500 ft to my pattern altitude and ended up incredibly high on final (hello forward slip to land :p)

while coming back, i inquired approached as to what they are seeing as my altitude. they said 4200 ft, thats what my Altimeter was telling me, but my transponder was showing 3800 for Alt. I was under the impression ATC sees what my transponder tells them.. donno. may be because i was within TRSA and they have some other gizmo there...

so long story short... i am kinda confused with the temp correction rt now. if i am with Center and i tell them i was at 5500, i am thinking i better be with 100 ft of that, if nothing for traffic separation, another dude could be at 5000 cruising along looking at his iGizmo while i am happily cruising at 5100 ft in reality.


thoughts?
 
For one, the instrument stuff doesn't matter to you unless you're FIKI, in those temps in most the GA fleet it's strictly VFR.

As for operational stuff just dial in the altimeter setting and fly off your altimeter, that's what ATC expects and they do have the ability to factor in corrections. Again, just fly off the altimeter as normal.
 
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Your transponder is sending ATC a pressure altitude. The radar software is getting a feed from the local altimeter setting and automatically converts your PA to baro (altimeter) altitude.

As far as ATC losing sep, all they care about is all aircraft having the proper altimeter setting. If your transponder is 300 ft or more off from your reported altitude, they’ll just have you stop alt sqk. I believe vertical sep loss via the computer is done at 700 ft or less. That’s if the particular system has automatic reporting of separation loss (TARP).

Also, on final ATC doesn’t care about the temperature corrected altitude. If you apply a temperature corrected altitude prior to final, make sure you notify them and also the amount of correction.
 
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The system works when everyone uses the correct altimeter setting. The system won’t work so well when some folks use GPS altitude.

Of course see and avoid always applies.
 
Your transponder is sending ATC a pressure altitude. The radar software is getting a feed from the local altimeter setting and automatically converts your PA to baro (altimeter) altitude.

As far as ATC losing sep, all they care about is all aircraft having the proper altimeter setting. If your transponder is 300 ft or more off from your reported altitude, they’ll just have you stop alt sqk. I believe vertical sep loss via the computer is done at 700 ft or less. That’s if the particular system has automatic reporting of separation loss (TARP).

Also, on final ATC doesn’t care about the temperature corrected altitude. If you apply a temperature corrected altitude prior to final, make sure you notify them and also the amount of correction.
If you apply the temp correction to the missed approach altitudes, notify ATC as well.

As to the OP's specific altitudes, an altimeter setting of around 30.30 would account for the difference between what your altimeter and transponder were showing.
Edit: Looks like the pressures in Fargo yesterday were above 30.60...that would be about a 700-foot difference between indicated and pressure altitudes.

If you apply the cold weather correction chart at -20C to an altitude 5000 feet above the field results in a 710-foot temperature error...approaching the difference between your indicated and GPS (which approximates true) altitude.
 
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If you apply the temp correction to the missed approach altitudes, notify ATC as well.

As to the OP's specific altitudes, an altimeter setting of around 30.30 would account for the difference between what your altimeter and transponder were showing.

If you apply the cold weather correction chart at -20C to an altitude 5000 feet above the field results in a 710-foot temperature error...approaching the difference between your indicated and GPS (which approximates true) altitude.

If he's shooting a IAP in sub zero in a PA28, he's shooting it all the way to the deck regardless.
 
background of my flights yesterday:

  • surface temp at take off -23F, DA -5300
  • OAT at 5500 MSL (roughly 4500 AGL) -14C while going, OAT -16C at 3900 MSL (roughly 2900 AGL)
  • surface temp at landing -6F, DA -3600
Observations and confusions:
Before i took off yesterday, i read up on Cold temp restricted airports and as i understand its primarily for Instrument approaches with MDA, DA and other alphabet soups, however, i also read somewhere about computing that en-route and the logic is... your altimeter is showing you re at 5000 , in reality you are at 4600 -- that could lead to traffic separation problem and you should tell ATC so they know.

what i observed -- my altimeter was showing i was at 5500, my GPS Alt was showing i am at 4800 (usually its 200 ft off, but it was 700 ft off yesterday) and my Transponder was showing 5100. this is the first time i have seen such a discrepancy. usually the transponder is very close to the altimeter. it confused the heck out of me and i added 500 ft to my pattern altitude and ended up incredibly high on final (hello forward slip to land :p)

while coming back, i inquired approached as to what they are seeing as my altitude. they said 4200 ft, thats what my Altimeter was telling me, but my transponder was showing 3800 for Alt. I was under the impression ATC sees what my transponder tells them.. donno. may be because i was within TRSA and they have some other gizmo there...

so long story short... i am kinda confused with the temp correction rt now. if i am with Center and i tell them i was at 5500, i am thinking i better be with 100 ft of that, if nothing for traffic separation, another dude could be at 5000 cruising along looking at his iGizmo while i am happily cruising at 5100 ft in reality.


thoughts?

AIM 1-1-17...do not use GPS for operational altitudes.

Bob
 
AIM 1-1-17...do not use GPS for operational altitudes.

Bob
Yupp, not using GPS for operations, just got pretty confused with 3 diff altitude readings.

Sent from my SM-G935V using Tapatalk
 
If you apply the temp correction to the missed approach altitudes, notify ATC as well.

As to the OP's specific altitudes, an altimeter setting of around 30.30 would account for the difference between what your altimeter and transponder were showing.
Edit: Looks like the pressures in Fargo yesterday were above 30.60...that would be about a 700-foot difference between indicated and pressure altitudes.

If you apply the cold weather correction chart at -20C to an altitude 5000 feet above the field results in a 710-foot temperature error...approaching the difference between your indicated and GPS (which approximates true) altitude.
That makes a lot of sense. Yes, altimeter was 30.67

Sent from my SM-G935V using Tapatalk
 
My observation on flying in cold weather.....


IT'S COLD..!!!!!

I have had water bottles freeze while in the pocket of my parka while I am wearing it when flying....
 
Remember that the error in indicated altitude due to the cold temperature is corrected up to the altitude of the reporting station via the altimeter setting. On the surface of that airport there will be no error. As you climb above that airport's field elevation there will be error introduced in your indicated altitude. The greater your altitude above field elevation, the greater the error.

When you took the error at a higher altitude and applied it to pattern altitude you were correcting (adding) too much as the error is less at the lower altitude.

There is a table in the AIM which shows the error based on temperature and height above field elevation.
 
High to low watch out below applies to temperature as well as altimeter settings. There's no ATC seperation problems if everyone is doing what they are supposed to. When it's cold out planes will be closer to the rocks than when it's hot, but they will be a thousand feet apart from each other if assigned altitudes a thousand feet apart. If the dude taking the altimeter setting does it right then reported and true altitudes will be the same at the airport. It's when you get higher that things change. Cold makes the 'column' of air shrink. The bottom stays right there at the ground, but the top 'shrinks' downward.
 
Good info from luvflyin there. If you've ever looked at soundings/radiosondes/weather balloon data you'll notice the different pressures have different heights. 500mb level may be 5400m in Ohio but 5800m in Florida. Those levels/thicknesses between levels are used by forecasters all the time (generally speaking lower height/thickness = cold, much more complicated though). And since altimeters are pressure based (hence adjusting for altimeter/barometer settings) you're not actually flying an absolute number of feet above the ground at altitude, but a set pressure. That's why all pilots in a given area need to be set to the proper altimeter setting... To put them all on a level playing field.

GPS altitude on the other hand is an absolute number of feet above the ground. Wx/temp/pressure doesn't affect it. It is strictly triangulation with the satellites.

This really affects aerial mapping folks at times. We need to be at absolute altitudes not pressure altitudes, depending on the job / sensor that difference can make or break a job. And sometimes flying at 6500' to the GPS/sensor is actually 6200' when you're talking to ATC for example.
 
Good info from luvflyin there. If you've ever looked at soundings/radiosondes/weather balloon data you'll notice the different pressures have different heights. 500mb level may be 5400m in Ohio but 5800m in Florida. Those levels/thicknesses between levels are used by forecasters all the time (generally speaking lower height/thickness = cold, much more complicated though). And since altimeters are pressure based (hence adjusting for altimeter/barometer settings) you're not actually flying an absolute number of feet above the ground at altitude, but a set pressure. That's why all pilots in a given area need to be set to the proper altimeter setting... To put them all on a level playing field.

GPS altitude on the other hand is an absolute number of feet above the ground. Wx/temp/pressure doesn't affect it. It is strictly triangulation with the satellites.

This really affects aerial mapping folks at times. We need to be at absolute altitudes not pressure altitudes, depending on the job / sensor that difference can make or break a job. And sometimes flying at 6500' to the GPS/sensor is actually 6200' when you're talking to ATC for example.

GPS: The altitude is the altitude above the WGS84 reference ellipsoid. It is not the altitude above ground level or sea level.

Bob
 
My observation on flying in cold weather.....


IT'S COLD..!!!!!

I have had water bottles freeze while in the pocket of my parka while I am wearing it when flying....
Wow. That's a level of cold I haven't experienced. Even in Vermont, hiking in -18C weather with water bottle in my knapsack, I've never had THAT happen!
 
GPS: The altitude is the altitude above the WGS84 reference ellipsoid. It is not the altitude above ground level or sea level.

Bob
Yes! I was trying to keep it simple but I guess that does work out to roughly 100ft in alot of areas, so it's significant.
 
Yes! I was trying to keep it simple but I guess that does work out to roughly 100ft in alot of areas, so it's significant.
What's WGS84?

Sent from my SM-G935V using Tapatalk
 
What's WGS84?

Sent from my SM-G935V using Tapatalk
It is a reference ellipsoid used to define the Earth's coordinate system whose surface is used for raw altitude information. Oddly, the 0 longitude for WGS84 is about 100 meters east of the prime meridian. One could say it is an idealized model of the earth's surface.
 
It is a reference ellipsoid used to define the Earth's coordinate system whose surface is used for raw altitude information. Oddly, the 0 longitude for WGS84 is about 100 meters east of the prime meridian. One could say it is an idealized model of the earth's surface.
It would be more accurate to say that it is a reference ellipsoid which is used to provide a consistent basis for surveying and mapping the Earth. It is commonly used in translating lat-long into coordinates suitable for mapping. Altitude is just a small part of its use.
 
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